Unconstrained Segmental Biomechanics Applied to Gait Initiation: A Conceptual Approach

Unconstrained Segmental Biomechanics Applied to Gait Initiation: A Conceptual Approach | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Unconstrained Segmental Biomechanics Applied to Gait Initiation: A Conceptual Approach Arianna Fogliata, Lorenzo Cantoni, Alessio Gambetta, Antinea Ambretti, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7749692/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Traditional biomechanical models of locomotion describe the human body as an articulated chain with constrained degrees of freedom, where movements are represented as rotations around fixed joint axes. They assume joint constraints as fixed points in space, thereby nullifying the forces acting at those points and reducing the ability of the model to represent the dynamic forces within the system, particularly during the non-cyclical phases of locomotion. This theoretical study introduces Unconstrained Segmental Biomechanics (USB), a conceptual approach that represents movement as the temporal activation of basic motor units, each composed of a pair of segments articulated by a non-fixed joint constraint. This representation enables the simultaneous analysis of dynamic and kinematic interactions and highlights the specificity of transitional phases, such as gait initiation, compared to cyclical locomotion. To examine how these models are currently transmitted in education, we investigated how gait is taught through a review of the literature and of teaching practices in leading sport science institutions. This analysis revealed that the adopted models remain exclusively kinematic, with no application of integrated dynamic representations such as USB. This work is proposed as a conceptual approach to reframe the foundations of locomotion biomechanics and to stimulate new perspectives. Biomechanics of locomotion gait initiation locomotor transitions joint constraints models dynamic and kinematic interactions human movement. Figures Figure 1 Figure 2 Figure 3 Introduction Human movement is a highly complex phenomenon, resulting from the interaction between the musculoskeletal system, neurophysiological functions, and biomechanical principles governing its kinematics and dynamics [ 1 ]. Due to this complexity, mechanical or biomechanical models used to analyse movement have often employed theoretical simplifications to describe the real interactions between muscular and joint structures. These simplifications serve to reduce the system’s variability, thereby facilitating its study and modelling [ 2 ]. Among the most common simplifications in segmental mechanics is the representation of the human body as a system of rigid segments connected by joints, which are modelled as simplified kinematic constraints. These joints are frequently treated as fixed points or as having reduced degrees of freedom to simplify the equations of motion, while still maintaining a functional description of locomotion [ 3 ]. For example, in upper limb modelling, the shoulder is often considered a proximal fixed point, the elbow an intermediate hinge, and the wrist a mobile terminal point, neglecting the actual force distribution and variability of joint connections. While this idealisation simplifies biomechanical analysis and might enhance its usability, it leads to a loss of precision in describing actual joint interactions [ 4 ]. To reduce this simplification and enhance analytical capacity, the “Grocco model” was first introduced in 2008 by De Bernardi, an Italian physiotherapist, in a technical manual written in Italian; a concept that, to date, remains unacknowledged in international literature. Building upon this theoretical insight, the present work proposes an conceptual framework of that model, here defined as unconstrained segmental biomechanics, which enables a combined analysis of dynamic and kinematic interactions between articulated segmental units without fixed constraints [ 5 ] term refers to the basic unit of movement, conceived as a system composed of two jointed segments connected by a pivot and activated by an active muscle. Grocco units and their interactions are analysed without assuming fixed points, thus allowing a more realistic depiction of joint connections and the distribution of acting forces. Although this model is still a simplification, it is less restrictive and allows for the analysis of more forces involved in the movements, thus offering a more accurate description of joint dynamics. Recent studies have shown that increasing the number of analysed variables improves the precision in describing various biomechanical phenomena, including the displacement of the body’s centre of mass [ 6 ] performance evaluation [ 7 ] and energy expenditure assessment [ 8 ]. In particular, the analysis of multiple variables could be especially useful in studying human gait, which, although widely documented, is often analysed using a limited number of parameters [ 9 ]. Gait is an essential motor function and is well-documented in the literature with considerable accuracy; however, the analysis is mainly based on kinematic parameters [ 10 ]. The integration of a broader set of biomechanical parameters could therefore enable a more detailed evaluation [11; 12]. Traditionally, locomotion has been analysed through various theoretical models describing the so-called “gait cycle”, as a sequence of defined phases, each characterised by specific biomechanical and kinematic conditions [13; 14]. These models conceptualise locomotion as a cyclical phenomenon, where the sequence of motor events regularly repeats and returns the system to its initial state after a defined period, governed by the principles of energy conservation and classical mechanics [15; 16]. One of the internationally recognised models, used to teach gait kinematics in academic motor science programmes, is that proposed by Perry and Burnfield (2010). This model divides the gait cycle into eight main phases, each with distinct kinematic and dynamic conditions: initial contact (0%), loading response (10%), mid-stance (30%), terminal stance (50%), pre-swing (60%), initial swing (73%), mid-swing (87%), and terminal swing (100%). Here, the first phase of the gait cycle is identified as initial contact, the moment when the heel of the leading limb touches the ground [ 17 ]. This simplification aims to ensure applicability in academic teaching (Stergiou, 2020); however, while widely used and effective, this simplification introduces a degree of abstraction that may limit the ability to interpret and explain the analysed phenomenon [ 18 ]. The use of less simplified models, such as the one proposed here, may provide a more accurate representation, especially in locomotion, which, despite some cyclical features, is intrinsically non-stationary, characterised by dynamic variations that can support a deeper understanding of kinematics [ 19 ]. Specifically, beginning the analysis of locomotion from the discrete temporal phase of initial contact (0%) omits the gait initiation phase, and consequently allows for its omission while teaching it. Studying this phase separately, as a transition between two distinct states (static position and dynamic movement), is often simplified to the point where initial contact appears kinematically identical whether it follows terminal swing (100%) or initiates the gait itself [13; 20]. This simplification, which considers only the foot-ground contact (phase 0%) in terms of spatial position, may be misleading if detached from the dynamic components of locomotion [21; 22]. From the perspective of gait dynamics, these two conditions are not equivalent at all: at gait initiation, the momentum has not yet been generated, whereas in initial contact after terminal swing, the system already possesses pre-existing momentum [ 23 ]. In gait initiation, the body must overcome standing inertia, requiring the generation of acceleration, which is no longer needed to the same extent in subsequent steps, in accordance with Newton’s first law of motion, which states that a body at rest remains at rest unless acted upon by an external force [ 24 ]. Therefore, if the first step is not taught as an integral yet distinct part of locomotion analysis, one risks to presenting a system as periodic merely because the initiation process that triggered it has been ignored, potentially creating comprehension difficulties for students [ 25 ]. Given the relevance of gait initiation as a transitional phase, we aimed to verify how gait is currently taught in leading sport science curricula, in order to assess whether the models used in education include dynamic representations or remain limited to purely kinematic schemes. Graduates in motor sciences will eventually need to handle movement in real-world contexts, such as rehabilitation, motor education, or sport, where the highest degree of difficulty often lies in transitions between different motor states [ 26 ]. Thus, employing a model that examines both the unobservable causes of movement (dynamics) and the observable effects (kinematics) through the study of basic units and their interactions, could prove especially beneficial. Furthermore, the inclusion of its unconstrained segmental mechanics highlights a more accurate representation of motor transitions. This methodological expansion could be particularly valuable in the teaching of biomechanics in motor science universities, as it is widely acknowledged that in-depth analysis of forces is fundamental to understand mechanical responses in human movement [ 27 ]. Materials and Methods This section presents a comparative analysis between the traditional models adopted in academic and scientific contexts and the proposed approach (USB), in order to highlight the limitations and gaps that justify the need for a new conceptual framework. To assess the academic representation of locomotion and its relevance within the context of human movement biomechanics, a systematic international analysis was conducted. Academic Representation of Locomotion The goal was to evaluate the dissemination and acceptance of segmental biomechanics and kinematic-based analytical models in academic sport and exercise sciences. For this purpose, the top 20 institutions listed in the Shanghai Ranking’s Global Ranking of Sport Science Schools and Departments 2024 (ShanghaiRanking Consultancy. Academic Ranking of World Universities 2024. Retrieved from https://www.shanghairanking.com ) were selected, owing to the methodological robustness of their academic evaluations [ 28 ]. The choice of sample size (between 10 and 30 institutions) aligns with prior research indicating this range as statistically optimal for comparative studies involving academic rankings [29;30]. Each selected university department specialising in movement science was contacted with a request to provide bibliographic references, books or articles, used in teaching human locomotion to students. Twelve institutions (60%) responded to this request. Of these, three universities (25%) stated that they did not include locomotion in their academic curricula; three universities (25%) reported that their teaching was based on the model proposed by Perry & Burnfield (2010); and the remaining six universities (50%) indicated the use of various references, amounting to seven different textbooks [ 17 ]. In order to further analyse the theoretical foundations of locomotion education, all textbooks and teaching materials recommended by the academic programs of the universities examined were incorporated into the core of bibliographic references. A total of 34 volumes were identified and studied. For each of them, the presence or absence of references to the "Grocco model" or comparable models based on intersegmental causality was systematically assessed. (see Table 1 ). Those texts were reviewed and analysed to identify possible grocco-like models used in locomotion instruction [ 31 ]. To contextualize the proposed framework within current biomechanical research, a systematic literature review was conducted. Table 1 List of the 34 manuals analysed: absence of the Grocco model or equivalent segmental causal models. Book Title APA Citation 1 Clinical disorders of balance, posture and gait Bronstein, A. M., Brandt, T., & Woollacott, M. H. (Eds.). (2004). Clinical disorders of balance, posture and gait (2nd ed.). Arnold Publishers. 2 Biomechanical basis of human movement Hamill, J., & Knutzen, K. M. (2020). Biomechanical basis of human movement (4th ed.). Wolters Kluwer. 3 Sensors for gait, posture, and health monitoring Lockhart, T. (Ed.). (2020). Sensors for gait, posture, and health monitoring. Volume 1. MDPI. 4 Conceptual biomechanics and kinesiology Hazari, A., Maiya, A. G., & Nagda, T. V. (2021). Conceptual biomechanics and kinesiology. Springer. https://doi.org/10.1007/978-981-33-4552-1 5 Fundamentals of biomechanics: Equilibrium, motion, and deformation Knudson, D. (2021). Fundamentals of biomechanics: Equilibrium, motion, and deformation (3rd ed.). Springer. 6 The comprehensive textbook of clinical biomechanics Richards, J. (2022). The comprehensive textbook of clinical biomechanics (2nd ed.). Elsevier. 7 Analysis of human movements, sport and health promotion Perry, J., & Burnfield, J. M. (2010). *Gait analysis: Normal and pathological function* (2nd ed.). SLACK Incorporated. 8 Kinesiology of the musculoskeletal system: Foundations for rehabilitation Neumann, D. A. (2016). Kinesiology of the musculoskeletal system: Foundations for rehabilitation (3rd ed.). Elsevier. & 2019 9 Running mechanics and gait analysis Ferber, R., & Macdonald, S. (2014). Running mechanics and gait analysis. Human Kinetics. 10 Whittle’s gait analysis Levine, D., Richards, J., & Whittle, M. W. (2021). Whittle’s gait analysis (6th ed.). Elsevier. 11 Gait, Balance, and Mobility Analysis Theoretical, Technical, and Clinical Applications Stuart S., Morris R. (2024) Gait, Balance and Mobility. Analysis Theoretical, Technical, and Clinical Applications. Elsevier 12 Biomechanics and gait analysis Stergiou, N. (2020). Biomechanics and gait analysis. Elsevier 13 Sensorimotor control of movement and posture Gandevia, S. C., & Proske, U. (Eds.). (2002). Sensorimotor control of movement and posture (Vol. 508). Springer 14 Adaptive gait and postural control: From physiological to pathological mechanisms, towards prevention and rehabilitation Blumen, H., Cavallari, P., Mourey, F., & Yiou, E. (Eds.). (2020). Adaptive gait and postural control: From physiological to pathological mechanisms, towards prevention and rehabilitation. Frontiers Media SA. 15 Task oriented gait training Lee, Y.-S. (2024). Task oriented gait training. Springer Nature 16 Observational gait analysis: A visual guide Adams, J., & Cerny, K. (2018). Observational gait analysis: A visual guide (1st ed.). Routledge. 17 Human walking Rose, J., & Gamble, J. G. (Eds.). (2006). Human walking (3rd ed.). Lippincott Williams & Wilkins. 18 Human walking Inman, V. T. (1989). Human walking. Edwin Mellen Press. 19 Human identification based on gait Nixon, M. S., Tan, T., & Chellappa, R. (2006). Human identification based on gait (1st ed.). Springer. & 2010. 20 Learning human gait. Kasebzadeh, P. (2019). Learning human gait. Linköping Studies in Science and Technology. 2012 21 Biomechanics of movement: The science of sports, robotics, and rehabilitation Delp, S. L., & Uchida, T. K. (2020). Biomechanics of movement: The science of sports, robotics, and rehabilitation. The MIT Press. 22 Neuromechanics of human movement Enoka, R. M. (2024). Neuromechanics of human movement (6th ed.). Human Kinetics 23 The science of walking: Investigations into locomotion in the long nineteenth century. Mayer, A. (2020). The science of walking: Investigations into locomotion in the long nineteenth century. University of Chicago Press. 24 Anatomy and human movement: Structure and function Palastanga, N., & Soames, R. W. (2019). Anatomy and human movement: Structure and function (7th ed.). Elsevier. 25 Biomechanics of human motion: Basics and beyond for the health professions LeVeau, B. F. (2010). Biomechanics of human motion: Basics and beyond for the health professions (1st ed.). Routledge. 26 Basic biomechanics Hall, S. J. (2023). Basic biomechanics (10th ed.). McGraw-Hill Education. 27 Kinesiology: Scientific basis of human motion Wells, K. F. (2001). Kinesiology: Scientific basis of human motion (10th ed.). McGraw-Hill Education - Europe. 28 3D analysis of human movement, sport, and health promotion. Petrigna, L. (Ed.). (2023). 3D analysis of human movement, sport, and health promotion. MDPI. 29 Biomechanics and motor control of human movement Winter, D. A. (2009). Biomechanics and motor control of human movement (4th ed.). Wiley 30 Human movement & biomechanics Kerr, A., & Rowe, P. (Eds.). (2019). Human movement & biomechanics (7th ed.). Elsevier. 31 Gait analysis laboratory manual Cicirelli, G., & Perri, A. G. (2018). Gait analysis laboratory manual. Springer. 32 Kinetics of human motion Zatsiorsky, V. M. (2002). Kinetics of human motion. Human Kinetics. 33 Gait analysis: Normal and pathological function Perry, J. (1992). Gait analysis: Normal and pathological function. Section 1 1–47. 34 Exercise and sport science Garrett, W. E., & Kirkendall, D. T. (Eds.). (2000). Exercise and sport science (1st ed.). Williams & Wilkins Systematic Literature Review on Gait Models Additionally, all relevant articles indexed on PubMed (2024) were consulted using the keywords: “gait”, “gait cycle”, “gait cycle analysis and “gait cycle analysis EMG”, in accordance with the PRISMA model. These terms were selected to cover a broad spectrum of gait-related research: “Gait” provided general insights into locomotor function across diverse populations and methodologies. “Gait cycle” focused the search on the biomechanical and temporal segmentation of walking, particularly the phases of stance and swing. “Gait cycle analysis” was used to retrieve studies that provided integrated descriptions of the gait cycle, often including both spatiotemporal and neuromechanical parameters. “Gait cycle analysis EMG” allowed for the inclusion of neuromuscular activation studies, essential for assessing the integration between motor control and biomechanical output. This combination of keywords ensured that both kinematic and electromyographic perspectives were included in the dataset, with the aim of identifying whether recent literature incorporates any dynamic segmental models similar to the Grocco. The first analytical phase involved the qualitative examination of 301 articles selected through a systematic search on PubMed. Starting from the keyword "gait", in 2024 it produced a total of 6,478 results, a series of progressive filters were then applied to ensure relevance and focus. The dataset was then limited to: studies with human participants only and free availability of the full-text, obtaining 2,195 results. Subsequently, to narrow the methodological field of application, the types of articles were selected: comparative study, evaluation study, observational study, randomized controlled clinical trial and validation study. The second analytical phase, focused specifically on the term “pitch cycle,” followed the same filtering procedure. The initial search yielded 442 results, reduced to 157 after applying the human and free full-text filters. However, applying the same five filters by article type, only one study matched all criteria. Consequently, this phase was integrated into the broader gait-based screening and later expanded with “gait cycle analysis and gait cycle EMG analysis,” in a desire to integrate studies on neuromuscular control and kinematic segmentation in healthy subjects. The research focused on “gait cycle analysis” in 2024 produced 227 results and, following the previous selection, 13 studies, all of which were analysed qualitatively. The fourth phase, focused on “gait cycle analysis EMG” in 2024, returned 13 articles. After applying the same filters as the previous selections, a final sample of 2 studies was obtained and analysed. Comparative analysis of these 317 articles revealed a clear predominance of approaches based on segmental biomechanics and kinematics, with a marked absence of models that explicitly integrate dynamic analysis of locomotion for kinematic interpretation; moreover, the treatment of the gait initiation phase and ground contact are treated as separate and independent phenomena. In light of these observations, we wanted to analyse “gait initiation,” to identify studies that specifically deal with the onset of ambulation and to check whether its description was as a biomechanical and neuromotor event distinct from the gait cycle. The filtered search like the previous ones led to 14 results, among them, only 3 studies explicitly separate gait initiation from the standard gait cycle, highlighting a clear gap in the literature and in the conceptual teaching of locomotion transitions. (see Table 2 ). Table 2 Studies explicitly distinguishing gait initiation from the standard gait cycle. Authors Title Journal / Source 1 Simonet A., Fourcade P., Loete F., Delafontaine A., Yiou E. Evaluation of the Margin of Stability during Gait Initiation in Young Healthy Adults, Elderly Healthy Adults and Patients with Parkinson's Disease Sensors (Basel), 2024, 24(11):3322 2 Beyraghi Z., Khanmohammadi R., Hadian M. R. Effects of Combining Transcranial Direct Current Stimulation With Balance Training on Anticipatory Postural Adjustments in Persons With Chronic Ankle Instability “Epub ahead of print" nel 2024. Sports Health, 2025, 17(2):383–393 3 Perales-López L., Sanz-Esteban I., Jiménez-Antona C. Automatic gait evoking in healthy adults through Vojta's peripheric somatosensory stimulation J Neuroeng Rehabil, 2024, 21(1):174 Conceptual Results and Implications The framework presented here, which forms the foundation of the Sincrony movement education methodology, originates from the theoretical construct known as the “Grocco model”, introduced by De Bernardi (2008) [ 5 ]. This original concept has been reformulated and extended into a structured framework referred to as Unconstrained Segmental Biomechanics. In its simplest form, a Grocco is conceptualised as a pair of jointed segments of equal length, connected by a pivot that functions as a simple hinge. Around this fulcrum, the two segments are free to rotate. Due to muscular contraction, the segment pair is subjected to two vector forces, F1 and F2, applied symmetrically near the free ends of each segment. The main components of these forces, tangential and orthogonal to each segment, generate both: a compressive stress along the segments, with a resultant F3 acting on the fulcrum, and a resultant torque which, in the absence of other applied forces (isolated system), would induce opposing rotational movement of the segments, causing either opening or closing, depending on the direction of the applied force. (Fig. 1 ) In practical terms, a Grocco is never truly an isolated system; it is always physically connected to at least one other system at one of its “free” ends. During movement, it thus exchanges forces with these systems. Similarly, in the case of the leg, the internal muscular forces acting within the Grocco-leg are transmitted both to the environment (i.e., the ground, via the ankle joint) and to the torso (via the pelvis). This segmentally unconstrained mechanics allows for a more accurate and dynamic representation of transitional movements, such as those occurring between postural states or during movement initiation. Through the Unconstrained Segmental Biomechanics framework, human movement, and locomotion in particular, can be interpreted as a sequence of Grocco units that are activated and interact over time. Although the behaviour of a single Grocco unit can always be traced back to the description provided above, what is observed in the system will depend on the timing of activation of each Grocco unit, as well as on the masses involved and the interconnection constraints (with their respective degrees of freedom). Therefore, the functioning of each unit is influenced by the behaviour of the adjacent units over time. From this theoretical perspective, every action can thus be described according to at least two fundamental viewpoints: kinematic and dynamic. Through this still little-used paradigm, it becomes possible, for example, to carry out a dynamic-kinematic analysis of gait initiation: in this case, the dynamic action starts with the contraction of a primary motor muscle (gluteus), which exerts a pair of forces on the systems to which it is connected, in particular the Grocco unit (leg) and the torso. The Grocco/leg, through the activation of the corresponding muscles (quadriceps and calf), will compensate for this force, transmitting it to the external environment (the floor) through force F1 (Fig. 2 ), to which the environment will react, according to the principle of action and reaction (Newton’s third law), with an equal and opposite force. The torso, in turn, must exert internal forces (through contraction of the abdominal muscles, for example) both as a reaction to the resultant forces at the point of connection with the Grocco-leg (the joint between femur and pelvis), and to counter the inertia that would tend to keep it at rest. Force F3, in the forward direction, is the resultant of all the applied forces and is the active force responsible for the transition from rest to motion of the body as a whole [ 32 ]. Discussion Relevance of the Grocco Model in Locomotion The fundamental aspect of the Grocco model is its ability to analyse human movement by integrating all the forces involved. Specifically, the model describes, unlike traditional approaches used in motor sciences, how, through the Grocco unit, multiple movement effects can be explained. The model is simplified, since during the gait initiation phase multiple Grocco units act along time, but despite the simplification, it may still enhance students’ understanding of the variables under study (dynamics-kinematics). In fact, through this model, we can analyse some important characteristics of locomotion as visible outcomes of a complex system that is not fully observable. Gait Initiation and CoP The forward movement of the pelvis (F₃), (Fig. 3 ), which represents the visible component of the Grocco unit in the gait initiation phase, is responsible for the change of state from static to dynamic through the displacement of the Centre of Mass (CoM) [ 33 ]. The push exerted through the foot (F₁), which represents the non-visible component whose direction acts downward and backward, is further responsible for the advancement of the pelvis and thus the transition from static to dynamic through the displacement of the CoM. It is therefore this displacement (F₃), described as that of the pelvis, which generates the advancement of the centre of mass (CoM) [ 34 ]. Pathcay demonstrated that, at the beginning of locomotion, the Centre of Pressure (CoP), the resulting point of the distribution of ground reaction forces (GRF), shifts postero-laterally, creating a moment that allows the CoM to advance with controlled acceleration [ 35 ]. Recent studies have interpreted this finding as a biomechanical preparation intended to optimise the transition from the static phase to dynamic movement, but without attributing descriptive connections to phenomena that intersect with the actual dynamics of movement, generally describing it as anticipatory [36; 37] (Jamshidi et al., 2012; Vieira et al., 2017). Through the Grocco model, however, we can integrate this observation by defining this distribution of CoP force as a described part of the expression of F₁ on the ground, and generative in the process of the gait initiation phase, as well as anticipatory only in relation to the visible movement. More precisely, the trajectory of the CoP seems to reflect the resultant vector of the forces exerted against the ground. This interpretation finds support in the literature, where the CoP has been associated with coordination between the musculoskeletal system and postural control, though without a clear explanation of the underlying reasons [ 38 ]. Passenger vs Agent A further important analytical element that arises from these reflections, and from the described displacement, concerns the biomechanical concept of the “passenger” in locomotion. Traditionally, the term “passenger” identifies body structures, such as the trunk and head, which do not actively contribute to locomotion but are passively propelled by the lower limbs. This distinction was introduced by Perry (1992), who divided the body into a passenger unit and a locomotor unit, conceptualising the differentiated role of the various body components in gait biomechanics [ 39 ]. Even according to conventional biomechanical analysis, during locomotion the trunk and head can be considered as passive structures being transported [ 40 ]. According to the Grocco model, this simplification is too reductive, as it does not allow the observation of two important dynamic components of locomotion: F₂ and inertia. A student in a learning phase might therefore risk interpreting human movement as if it were a drawing, where real physical forces are not acting. The principle of inertia (Newton’s First Law) dictates that, in the initial phase of locomotion, the trunk (due to its mass) naturally tends to maintain its state of inertia, remaining still with respect to the locomotor unit [ 39 ]. In order for the “passenger” element to also change its state of rest, an exchange of forces is therefore necessary between the lower Grocco unit, which constitutes the locomotor element, and the upper one. The resultant of these forces, transmitted through the action of the musculoskeletal system, is represented in the figure as F₂. Thus, as also confirmed by the inverted pendulum model, the trunk, although not directly involved in generating the propulsive push, must still actively participate in maintaining balance and regulating the distribution of forces. The CoM oscillates in a controlled manner thanks to the active interaction between the trunk and the movement of the lower limbs, optimising the system’s dynamic stability [ 41 ]. However, this model scrupulously describes the visible or measurable effects of locomotion, without considering the role and the way internal forces are generated and transmitted in movement. From this perspective, the Grocco model offers an additional interesting integration, describing the “passenger” as an “agent,” since the trunk and the head would remain kinematically behind the advancing lower limbs. To prevent this from occurring, further Grocco units and forces intervene within the system. These units must actively compensate for the resistance of the mass through flexion movements, generated by muscular contractions, that are often not visible, as their main purpose is to keep the upper part of the body aligned with the lower limbs. This flexion is instead clearly visible, for example, in running, due to greater acceleration and increased dynamic resistance, where the trunk tends to be visibly inclined in the kinematics of the action. Toward a Dynamic Framework for Teaching Locomotion The present work reformulates the Grocco model into Unconstrained Segmental Biomechanics (USB), a methodological framework for analysing locomotion beyond rigid-joint assumptions. By describing limbs as articulated segments connected by non-fixed pivots, USB provides a dynamic-kinematic interpretation that is particularly relevant for gait initiation, usually reduced to a simple change of state in cyclical models [ 17 ]. Through the simultaneous involvement of forces F₁, F₂, and F₃, USB allows for a more realistic representation of this transitional phase, releasing segments from rigidity, integrating the first step into gait analysis, and describing subsequent movements as emergent kinematic effects of underlying dynamics. On an applied level, USB may enrich locomotion teaching by highlighting non-visible but active force interactions, thereby fostering a more comprehensive understanding of human movement [ 42 , 43 ]. Its conceptual structure could also support rehabilitation strategies, wearable robotics, and sport performance analysis by clarifying the role of initial propulsive forces [33; 36; 37]. Nevertheless, empirical validation remains necessary to consolidate its pedagogical and scientific value. Conclusions This work presented Unconstrained Segmental Biomechanics (USB) as a conceptual framework for locomotion analysis, developed from the Grocco model. Using gait initiation as an example, USB was shown to provide a more realistic dynamic-kinematic description than traditional constrained models, highlighting the role of non-directly observable forces. The review of literature and teaching materials confirmed that the models currently adopted remain predominantly kinematic, neglecting such dynamic integration. USB is therefore proposed as a theoretical tool to reconsider the foundations of teaching and research in sport sciences. Experimental validation is now required to assess its descriptive, predictive, and applicative value, and to evaluate its potential impact in fields such as rehabilitation, wearable robotics, and sport performance analysis. Declarations Data Availability Statement This theoretical study did not generate or analyse datasets. All referenced materials are available in the public domain. Funding This research received no external funding. Ethics Approval and Consent to Participate Not applicable. Competing Interests The authors declare no competing interests. Consent for Publication All authors and contributors gave their consent for publication. Author Contributions A. Fogliata: conceived the study, developed the theoretical framework, conducted the literature review, and authored the manuscript. L. Cantoni: co-developed the conceptual model, guided all phases of the work, and contributed significantly to the structure and revision of the manuscript. A. Gambetta: contributed the applied physics components, refined biomechanical terminology, and supported theoretical validation through mechanical arm simulations in a laboratory setting. A. Ambretti: contributed to verifying selected literature sources and provided language editing. S. Tardini: participated in the design of the study, supported manuscript development, maintained collaborative coordination, and contributed to the theoretical framing. 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Hum Mov Sci. 2011;30(2):329–43. Jian Y, Winter D, Ishac M, Gilchrist L. Trajectory of the body COG and COP during initiation and termination of gait. Gait Posture. 1993;1(1):9–22. https://doi.org/10.1016/0966-6362(93)90038-3 . Hansen C, LaRue J, Do M-C, Latash ML. Postural Preparation to Stepping: Coupled Center of Pressure Shifts in the Anterior-Posterior and Medio-Lateral Directions. J Hum Kinetics. 2016;54(1):5–14. https://doi.org/10.1515/hukin-2016-0030 . Jamshidi Y, Nolte IM, Dalageorgou C, Zheng D, Johnson T, Bastiaenen R, Ruddy S, Talbott D, Norris KJ, Snieder H, George AL, Marshall V, Shakir S, Kannankeril PJ, Munroe PB, Camm AJ, Jeffery S, Roden DM, Behr ER. Common Variation in the NOS1AP Gene Is Associated With Drug-Induced QT Prolongation and Ventricular Arrhythmia. J Am Coll Cardiol. 2012;60(9):841–50. https://doi.org/10.1016/j.jacc.2012.03.031 . Vieira MF, De Brito AA, Lehnen GC, Rodrigues FB. Center of pressure and center of mass behavior during gait initiation on inclined surfaces: A statistical parametric mapping analysis. J Biomech. 2017;56:10–8. https://doi.org/10.1016/j.jbiomech.2017.02.018 . Speedtsberg MB, Christensen SB, Andersen KK, Bencke J, Jensen BR, Curtis DJ. (2017). Impaired postural control in children with developmental coordination disorder is related to less efficient central as well as peripheral control. Gait & Posture, 51, 1–6. https://doi.org/10.1016/j.gaitpost .2016.09.019Stergiou, N. (2020). Biomechanics and gait analysis. Academic Press. Perry J. Gait analysis: Normal and pathological function. Section. 1992;1:1–47. Winter D. Human balance and posture control during standing and walking. Gait Posture. 1995;3(4):193–214. https://doi.org/10.1016/0966-6362(96)82849-9 . Hof AL, Gazendam MGJ, Sinke WE. The condition for dynamic stability. J Biomech. 2005;38(1):1–8. https://doi.org/10.1016/j.jbiomech.2004.03.025 . Scanlon D, Beckey A, Wintle J, Hordvik M. Weak’ physical education teacher education practice: co-constructing features of meaningful physical education with pre-service teachers. Sport Educ Soc. 2024;30(5):527–42. https://doi.org/10.1080/13573322.2024.2344012rrect . O’Brien C, Behan S, Taylor J. Game ready: pedagogical opportunities for proactive performance analysis in team sports. Sport Educ Soc. 2024;1–15. https://doi.org/10.1080/13573322.2024.2437023 . Additional Declarations No competing interests reported. Supplementary Files DeclarationofFunding.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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06:26:24","extension":"html","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":128072,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7749692/v1/7c5cddfb1ded376c4f975a8a.html"},{"id":92566058,"identity":"b7aa091c-09d2-4f88-882b-f8fc9c2ebfac","added_by":"auto","created_at":"2025-10-01 06:26:24","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":51287,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eForces acting on a Grocco unit (a) vs. forces acting in a traditional biomechanical model (b)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIdealised mechanical model illustrating a Grocco unit composed of two jointed segments connected by a central pivot. The forces F₁ and F₂ are applied tangentially at the distal ends of each segment and represent external muscular forces acting on the system. These opposing forces generate a compressive resultant force F₃ at the fulcrum and induce rotational moments in opposite directions (a.) Traditional biomechanical model where forces act perpendicularly to the lever, producing a torque around the fulcrum, without highlighting compressive forces between segments (b.).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7749692/v1/8e3dcd5f3488dc899f31efd0.png"},{"id":92566061,"identity":"c80f2afe-07a9-4feb-b7d4-c07e146bcc77","added_by":"auto","created_at":"2025-10-01 06:26:24","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":66472,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eInitial phase of locomotion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eInitial sequence of human walking: red arrows indicate the internal forces used to overcome inertia, while blue arrows represent the directional resultant of the movement. Red arrows indicate the internal forces used to overcome inertia, while blue arrows represent the directional resultant of the movement. The visible forces correspond to the step (S-F2); the non-visible forces, though fundamental, are the push (P-F1) and the friction (F).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7749692/v1/2a135ac4d4b783d61decb448.png"},{"id":92566178,"identity":"14abb7da-bcb6-45cd-b076-acc21a064b9c","added_by":"auto","created_at":"2025-10-01 06:34:24","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":80277,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSimplified representation of forces in the Grocco Model\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBody segments are color-coded to reflect their affiliation with specific Grocco units; some segments may belong to multiple units simultaneously, depending on the step phase and biomechanical function. Blue arrows (P-F1) indicate internal muscular push. Brown arrows (F) represent frictional force applied to the ground. Green arrows (S-F2) show the visible resultant of the step, the observable body motion. This model allows us to interpret pelvic displacement (F₃) as an emergent result of an integrated system of forces and segments. Though simplified, it supports didactic analysis of dynamic and kinematic variables in locomotion.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7749692/v1/d5a23e4b5e0a43dbda1acfd5.png"},{"id":93513590,"identity":"f592f277-dac6-46e9-922f-18c2792c39dd","added_by":"auto","created_at":"2025-10-14 15:54:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":807878,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7749692/v1/f62fb261-4725-47f2-becc-ef9a93948d64.pdf"},{"id":92566059,"identity":"e4284e31-71b9-4d1d-8168-f5ff2af50256","added_by":"auto","created_at":"2025-10-01 06:26:24","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":20358,"visible":true,"origin":"","legend":"","description":"","filename":"DeclarationofFunding.docx","url":"https://assets-eu.researchsquare.com/files/rs-7749692/v1/262563b8727061611542552f.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Unconstrained Segmental Biomechanics Applied to Gait Initiation: A Conceptual Approach","fulltext":[{"header":"Introduction","content":"\u003cp\u003eHuman movement is a highly complex phenomenon, resulting from the interaction between the musculoskeletal system, neurophysiological functions, and biomechanical principles governing its kinematics and dynamics [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Due to this complexity, mechanical or biomechanical models used to analyse movement have often employed theoretical simplifications to describe the real interactions between muscular and joint structures. These simplifications serve to reduce the system\u0026rsquo;s variability, thereby facilitating its study and modelling [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Among the most common simplifications in segmental mechanics is the representation of the human body as a system of rigid segments connected by joints, which are modelled as simplified kinematic constraints. These joints are frequently treated as fixed points or as having reduced degrees of freedom to simplify the equations of motion, while still maintaining a functional description of locomotion [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. For example, in upper limb modelling, the shoulder is often considered a proximal fixed point, the elbow an intermediate hinge, and the wrist a mobile terminal point, neglecting the actual force distribution and variability of joint connections. While this idealisation simplifies biomechanical analysis and might enhance its usability, it leads to a loss of precision in describing actual joint interactions [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. To reduce this simplification and enhance analytical capacity, the \u0026ldquo;Grocco model\u0026rdquo; was first introduced in 2008 by De Bernardi, an Italian physiotherapist, in a technical manual written in Italian; a concept that, to date, remains unacknowledged in international literature. Building upon this theoretical insight, the present work proposes an conceptual framework of that model, here defined as unconstrained segmental biomechanics, which enables a combined analysis of dynamic and kinematic interactions between articulated segmental units without fixed constraints [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] term refers to the basic unit of movement, conceived as a system composed of two jointed segments connected by a pivot and activated by an active muscle. Grocco units and their interactions are analysed without assuming fixed points, thus allowing a more realistic depiction of joint connections and the distribution of acting forces. Although this model is still a simplification, it is less restrictive and allows for the analysis of more forces involved in the movements, thus offering a more accurate description of joint dynamics. Recent studies have shown that increasing the number of analysed variables improves the precision in describing various biomechanical phenomena, including the displacement of the body\u0026rsquo;s centre of mass [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] performance evaluation [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] and energy expenditure assessment [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In particular, the analysis of multiple variables could be especially useful in studying human gait, which, although widely documented, is often analysed using a limited number of parameters [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Gait is an essential motor function and is well-documented in the literature with considerable accuracy; however, the analysis is mainly based on kinematic parameters [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The integration of a broader set of biomechanical parameters could therefore enable a more detailed evaluation [11; 12]. Traditionally, locomotion has been analysed through various theoretical models describing the so-called \u0026ldquo;gait cycle\u0026rdquo;, as a sequence of defined phases, each characterised by specific biomechanical and kinematic conditions [13; 14]. These models conceptualise locomotion as a cyclical phenomenon, where the sequence of motor events regularly repeats and returns the system to its initial state after a defined period, governed by the principles of energy conservation and classical mechanics [15; 16]. One of the internationally recognised models, used to teach gait kinematics in academic motor science programmes, is that proposed by Perry and Burnfield (2010). This model divides the gait cycle into eight main phases, each with distinct kinematic and dynamic conditions: initial contact (0%), loading response (10%), mid-stance (30%), terminal stance (50%), pre-swing (60%), initial swing (73%), mid-swing (87%), and terminal swing (100%). Here, the first phase of the gait cycle is identified as initial contact, the moment when the heel of the leading limb touches the ground [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. This simplification aims to ensure applicability in academic teaching (Stergiou, 2020); however, while widely used and effective, this simplification introduces a degree of abstraction that may limit the ability to interpret and explain the analysed phenomenon [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The use of less simplified models, such as the one proposed here, may provide a more accurate representation, especially in locomotion, which, despite some cyclical features, is intrinsically non-stationary, characterised by dynamic variations that can support a deeper understanding of kinematics [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Specifically, beginning the analysis of locomotion from the discrete temporal phase of initial contact (0%) omits the gait initiation phase, and consequently allows for its omission while teaching it. Studying this phase separately, as a transition between two distinct states (static position and dynamic movement), is often simplified to the point where initial contact appears kinematically identical whether it follows terminal swing (100%) or initiates the gait itself [13; 20]. This simplification, which considers only the foot-ground contact (phase 0%) in terms of spatial position, may be misleading if detached from the dynamic components of locomotion [21; 22]. From the perspective of gait dynamics, these two conditions are not equivalent at all: at gait initiation, the momentum has not yet been generated, whereas in initial contact after terminal swing, the system already possesses pre-existing momentum [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. In gait initiation, the body must overcome standing inertia, requiring the generation of acceleration, which is no longer needed to the same extent in subsequent steps, in accordance with Newton\u0026rsquo;s first law of motion, which states that a body at rest remains at rest unless acted upon by an external force [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Therefore, if the first step is not taught as an integral yet distinct part of locomotion analysis, one risks to presenting a system as periodic merely because the initiation process that triggered it has been ignored, potentially creating comprehension difficulties for students [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Given the relevance of gait initiation as a transitional phase, we aimed to verify how gait is currently taught in leading sport science curricula, in order to assess whether the models used in education include dynamic representations or remain limited to purely kinematic schemes. Graduates in motor sciences will eventually need to handle movement in real-world contexts, such as rehabilitation, motor education, or sport, where the highest degree of difficulty often lies in transitions between different motor states [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Thus, employing a model that examines both the unobservable causes of movement (dynamics) and the observable effects (kinematics) through the study of basic units and their interactions, could prove especially beneficial. Furthermore, the inclusion of its unconstrained segmental mechanics highlights a more accurate representation of motor transitions. This methodological expansion could be particularly valuable in the teaching of biomechanics in motor science universities, as it is widely acknowledged that in-depth analysis of forces is fundamental to understand mechanical responses in human movement [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003eThis section presents a comparative analysis between the traditional models adopted in academic and scientific contexts and the proposed approach (USB), in order to highlight the limitations and gaps that justify the need for a new conceptual framework. To assess the academic representation of locomotion and its relevance within the context of human movement biomechanics, a systematic international analysis was conducted.\u003c/p\u003e\u003cp\u003eAcademic Representation of Locomotion\u003c/p\u003e\u003cp\u003eThe goal was to evaluate the dissemination and acceptance of segmental biomechanics and kinematic-based analytical models in academic sport and exercise sciences. For this purpose, the top 20 institutions listed in the Shanghai Ranking\u0026rsquo;s Global Ranking of Sport Science Schools and Departments 2024 (ShanghaiRanking Consultancy. Academic Ranking of World Universities 2024. Retrieved from \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.shanghairanking.com\u003c/span\u003e\u003cspan address=\"https://www.shanghairanking.com\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) were selected, owing to the methodological robustness of their academic evaluations [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The choice of sample size (between 10 and 30 institutions) aligns with prior research indicating this range as statistically optimal for comparative studies involving academic rankings [29;30]. Each selected university department specialising in movement science was contacted with a request to provide bibliographic references, books or articles, used in teaching human locomotion to students. Twelve institutions (60%) responded to this request. Of these, three universities (25%) stated that they did not include locomotion in their academic curricula; three universities (25%) reported that their teaching was based on the model proposed by Perry \u0026amp; Burnfield (2010); and the remaining six universities (50%) indicated the use of various references, amounting to seven different textbooks [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. In order to further analyse the theoretical foundations of locomotion education, all textbooks and teaching materials recommended by the academic programs of the universities examined were incorporated into the core of bibliographic references. A total of 34 volumes were identified and studied. For each of them, the presence or absence of references to the \"Grocco model\" or comparable models based on intersegmental causality was systematically assessed. (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Those texts were reviewed and analysed to identify possible grocco-like models used in locomotion instruction [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. To contextualize the proposed framework within current biomechanical research, a systematic literature review was conducted.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eList of the 34 manuals analysed: absence of the Grocco model or equivalent segmental causal models.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBook\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTitle\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAPA Citation\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eClinical disorders of balance, posture and gait\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eBronstein, A. M., Brandt, T., \u0026amp; Woollacott, M. H. (Eds.). (2004). Clinical disorders of balance, posture and gait (2nd ed.). Arnold Publishers.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBiomechanical basis of human movement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHamill, J., \u0026amp; Knutzen, K. M. (2020). Biomechanical basis of human movement (4th ed.). Wolters Kluwer.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSensors for gait, posture, and health monitoring\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLockhart, T. (Ed.). (2020). Sensors for gait, posture, and health monitoring. Volume 1. 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Lippincott Williams \u0026amp; Wilkins.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHuman walking\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eInman, V. T. (1989). Human walking. Edwin Mellen Press.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHuman identification based on gait\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNixon, M. S., Tan, T., \u0026amp; Chellappa, R. (2006). Human identification based on gait (1st ed.). Springer. \u0026amp; 2010.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eLearning human gait.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eKasebzadeh, P. (2019). Learning human gait. Link\u0026ouml;ping Studies in Science and Technology. 2012\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBiomechanics of movement: The science of sports, robotics, and rehabilitation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eDelp, S. L., \u0026amp; Uchida, T. K. (2020). Biomechanics of movement: The science of sports, robotics, and rehabilitation. The MIT Press.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNeuromechanics of human movement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEnoka, R. M. (2024). Neuromechanics of human movement (6th ed.). Human Kinetics\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e23\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eThe science of walking: Investigations into locomotion in the long nineteenth century.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMayer, A. (2020). The science of walking: Investigations into locomotion in the long nineteenth century. University of Chicago Press.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAnatomy and human movement: Structure and function\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePalastanga, N., \u0026amp; Soames, R. W. (2019). Anatomy and human movement: Structure and function (7th ed.). Elsevier.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBiomechanics of human motion: Basics and beyond for the health professions\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLeVeau, B. F. (2010). Biomechanics of human motion: Basics and beyond for the health professions (1st ed.). Routledge.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBasic biomechanics\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHall, S. J. (2023). Basic biomechanics (10th ed.). McGraw-Hill Education.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eKinesiology: Scientific basis of human motion\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eWells, K. F. (2001). Kinesiology: Scientific basis of human motion (10th ed.). McGraw-Hill Education - Europe.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3D analysis of human movement, sport, and health promotion.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePetrigna, L. (Ed.). (2023). 3D analysis of human movement, sport, and health promotion. MDPI.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBiomechanics and motor control of human movement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eWinter, D. A. (2009). Biomechanics and motor control of human movement (4th ed.). Wiley\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHuman movement \u0026amp; biomechanics\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eKerr, A., \u0026amp; Rowe, P. (Eds.). (2019). Human movement \u0026amp; biomechanics (7th ed.). Elsevier.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eGait analysis laboratory manual\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCicirelli, G., \u0026amp; Perri, A. G. (2018). Gait analysis laboratory manual. Springer.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e32\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eKinetics of human motion\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eZatsiorsky, V. M. (2002). Kinetics of human motion. Human Kinetics.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eGait analysis: Normal and pathological function\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePerry, J. (1992). Gait analysis: Normal and pathological function. Section 1 1\u0026ndash;47.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e34\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eExercise and sport science\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eGarrett, W. E., \u0026amp; Kirkendall, D. T. (Eds.). (2000). Exercise and sport science (1st ed.). Williams \u0026amp; Wilkins\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eSystematic Literature Review on Gait Models\u003c/p\u003e\u003cp\u003eAdditionally, all relevant articles indexed on PubMed (2024) were consulted using the keywords: \u0026ldquo;gait\u0026rdquo;, \u0026ldquo;gait cycle\u0026rdquo;, \u0026ldquo;gait cycle analysis and \u0026ldquo;gait cycle analysis EMG\u0026rdquo;, in accordance with the PRISMA model. These terms were selected to cover a broad spectrum of gait-related research: \u0026ldquo;Gait\u0026rdquo; provided general insights into locomotor function across diverse populations and methodologies. \u0026ldquo;Gait cycle\u0026rdquo; focused the search on the biomechanical and temporal segmentation of walking, particularly the phases of stance and swing. \u0026ldquo;Gait cycle analysis\u0026rdquo; was used to retrieve studies that provided integrated descriptions of the gait cycle, often including both spatiotemporal and neuromechanical parameters. \u0026ldquo;Gait cycle analysis EMG\u0026rdquo; allowed for the inclusion of neuromuscular activation studies, essential for assessing the integration between motor control and biomechanical output. This combination of keywords ensured that both kinematic and electromyographic perspectives were included in the dataset, with the aim of identifying whether recent literature incorporates any dynamic segmental models similar to the Grocco. The first analytical phase involved the qualitative examination of 301 articles selected through a systematic search on PubMed. Starting from the keyword \"gait\", in 2024 it produced a total of 6,478 results, a series of progressive filters were then applied to ensure relevance and focus. The dataset was then limited to: studies with human participants only and free availability of the full-text, obtaining 2,195 results. Subsequently, to narrow the methodological field of application, the types of articles were selected: comparative study, evaluation study, observational study, randomized controlled clinical trial and validation study. The second analytical phase, focused specifically on the term \u0026ldquo;pitch cycle,\u0026rdquo; followed the same filtering procedure. The initial search yielded 442 results, reduced to 157 after applying the human and free full-text filters. However, applying the same five filters by article type, only one study matched all criteria. Consequently, this phase was integrated into the broader gait-based screening and later expanded with \u0026ldquo;gait cycle analysis and gait cycle EMG analysis,\u0026rdquo; in a desire to integrate studies on neuromuscular control and kinematic segmentation in healthy subjects. The research focused on \u0026ldquo;gait cycle analysis\u0026rdquo; in 2024 produced 227 results and, following the previous selection, 13 studies, all of which were analysed qualitatively. The fourth phase, focused on \u0026ldquo;gait cycle analysis EMG\u0026rdquo; in 2024, returned 13 articles. After applying the same filters as the previous selections, a final sample of 2 studies was obtained and analysed. Comparative analysis of these 317 articles revealed a clear predominance of approaches based on segmental biomechanics and kinematics, with a marked absence of models that explicitly integrate dynamic analysis of locomotion for kinematic interpretation; moreover, the treatment of the gait initiation phase and ground contact are treated as separate and independent phenomena. In light of these observations, we wanted to analyse \u0026ldquo;gait initiation,\u0026rdquo; to identify studies that specifically deal with the onset of ambulation and to check whether its description was as a biomechanical and neuromotor event distinct from the gait cycle. The filtered search like the previous ones led to 14 results, among them, only 3 studies explicitly separate gait initiation from the standard gait cycle, highlighting a clear gap in the literature and in the conceptual teaching of locomotion transitions. (see Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eStudies explicitly distinguishing gait initiation from the standard gait cycle.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAuthors\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eTitle\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eJournal / Source\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSimonet A., Fourcade P., Loete F., Delafontaine A., Yiou E.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEvaluation of the Margin of Stability during Gait Initiation in Young Healthy Adults, Elderly Healthy Adults and Patients with Parkinson's Disease\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSensors (Basel), 2024, 24(11):3322\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBeyraghi Z., Khanmohammadi R., Hadian M. R.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEffects of Combining Transcranial Direct Current Stimulation With Balance Training on Anticipatory Postural Adjustments in Persons With Chronic Ankle Instability\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026ldquo;Epub ahead of print\" nel 2024. Sports Health, 2025, 17(2):383\u0026ndash;393\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePerales-L\u0026oacute;pez L., Sanz-Esteban I., Jim\u0026eacute;nez-Antona C.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAutomatic gait evoking in healthy adults through Vojta's peripheric somatosensory stimulation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eJ Neuroeng Rehabil, 2024, 21(1):174\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eConceptual Results and Implications\u003c/p\u003e\u003cp\u003eThe framework presented here, which forms the foundation of the Sincrony movement education methodology, originates from the theoretical construct known as the \u0026ldquo;Grocco model\u0026rdquo;, introduced by De Bernardi (2008) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. This original concept has been reformulated and extended into a structured framework referred to as Unconstrained Segmental Biomechanics. In its simplest form, a Grocco is conceptualised as a pair of jointed segments of equal length, connected by a pivot that functions as a simple hinge. Around this fulcrum, the two segments are free to rotate. Due to muscular contraction, the segment pair is subjected to two vector forces, F1 and F2, applied symmetrically near the free ends of each segment. The main components of these forces, tangential and orthogonal to each segment, generate both:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003ea compressive stress along the segments, with a resultant F3 acting on the fulcrum, and\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003ea resultant torque which, in the absence of other applied forces (isolated system), would induce opposing rotational movement of the segments, causing either opening or closing, depending on the direction of the applied force. (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e)\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn practical terms, a Grocco is never truly an isolated system; it is always physically connected to at least one other system at one of its \u0026ldquo;free\u0026rdquo; ends. During movement, it thus exchanges forces with these systems. Similarly, in the case of the leg, the internal muscular forces acting within the Grocco-leg are transmitted both to the environment (i.e., the ground, via the ankle joint) and to the torso (via the pelvis). This segmentally unconstrained mechanics allows for a more accurate and dynamic representation of transitional movements, such as those occurring between postural states or during movement initiation. Through the Unconstrained Segmental Biomechanics framework, human movement, and locomotion in particular, can be interpreted as a sequence of Grocco units that are activated and interact over time. Although the behaviour of a single Grocco unit can always be traced back to the description provided above, what is observed in the system will depend on the timing of activation of each Grocco unit, as well as on the masses involved and the interconnection constraints (with their respective degrees of freedom). Therefore, the functioning of each unit is influenced by the behaviour of the adjacent units over time. From this theoretical perspective, every action can thus be described according to at least two fundamental viewpoints: kinematic and dynamic. Through this still little-used paradigm, it becomes possible, for example, to carry out a dynamic-kinematic analysis of gait initiation: in this case, the dynamic action starts with the contraction of a primary motor muscle (gluteus), which exerts a pair of forces on the systems to which it is connected, in particular the Grocco unit (leg) and the torso. The Grocco/leg, through the activation of the corresponding muscles (quadriceps and calf), will compensate for this force, transmitting it to the external environment (the floor) through force F1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), to which the environment will react, according to the principle of action and reaction (Newton\u0026rsquo;s third law), with an equal and opposite force. The torso, in turn, must exert internal forces (through contraction of the abdominal muscles, for example) both as a reaction to the resultant forces at the point of connection with the Grocco-leg (the joint between femur and pelvis), and to counter the inertia that would tend to keep it at rest. Force F3, in the forward direction, is the resultant of all the applied forces and is the active force responsible for the transition from rest to motion of the body as a whole [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eRelevance of the Grocco Model in Locomotion\u003c/p\u003e\u003cp\u003eThe fundamental aspect of the Grocco model is its ability to analyse human movement by integrating all the forces involved. Specifically, the model describes, unlike traditional approaches used in motor sciences, how, through the Grocco unit, multiple movement effects can be explained. The model is simplified, since during the gait initiation phase multiple Grocco units act along time, but despite the simplification, it may still enhance students\u0026rsquo; understanding of the variables under study (dynamics-kinematics). In fact, through this model, we can analyse some important characteristics of locomotion as visible outcomes of a complex system that is not fully observable.\u003c/p\u003e\u003cp\u003eGait Initiation and CoP\u003c/p\u003e\u003cp\u003eThe forward movement of the pelvis (F₃), (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), which represents the visible component of the Grocco unit in the gait initiation phase, is responsible for the change of state from static to dynamic through the displacement of the Centre of Mass (CoM) [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The push exerted through the foot (F₁), which represents the non-visible component whose direction acts downward and backward, is further responsible for the advancement of the pelvis and thus the transition from static to dynamic through the displacement of the CoM. It is therefore this displacement (F₃), described as that of the pelvis, which generates the advancement of the centre of mass (CoM) [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Pathcay demonstrated that, at the beginning of locomotion, the Centre of Pressure (CoP), the resulting point of the distribution of ground reaction forces (GRF), shifts postero-laterally, creating a moment that allows the CoM to advance with controlled acceleration [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Recent studies have interpreted this finding as a biomechanical preparation intended to optimise the transition from the static phase to dynamic movement, but without attributing descriptive connections to phenomena that intersect with the actual dynamics of movement, generally describing it as anticipatory [36; 37] (Jamshidi et al., 2012; Vieira et al., 2017). Through the Grocco model, however, we can integrate this observation by defining this distribution of CoP force as a described part of the expression of F₁ on the ground, and generative in the process of the gait initiation phase, as well as anticipatory only in relation to the visible movement. More precisely, the trajectory of the CoP seems to reflect the resultant vector of the forces exerted against the ground. This interpretation finds support in the literature, where the CoP has been associated with coordination between the musculoskeletal system and postural control, though without a clear explanation of the underlying reasons [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003ePassenger vs Agent\u003c/p\u003e\u003cp\u003eA further important analytical element that arises from these reflections, and from the described displacement, concerns the biomechanical concept of the \u0026ldquo;passenger\u0026rdquo; in locomotion. Traditionally, the term \u0026ldquo;passenger\u0026rdquo; identifies body structures, such as the trunk and head, which do not actively contribute to locomotion but are passively propelled by the lower limbs. This distinction was introduced by Perry (1992), who divided the body into a passenger unit and a locomotor unit, conceptualising the differentiated role of the various body components in gait biomechanics [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Even according to conventional biomechanical analysis, during locomotion the trunk and head can be considered as passive structures being transported [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. According to the Grocco model, this simplification is too reductive, as it does not allow the observation of two important dynamic components of locomotion: F₂ and inertia. A student in a learning phase might therefore risk interpreting human movement as if it were a drawing, where real physical forces are not acting. The principle of inertia (Newton\u0026rsquo;s First Law) dictates that, in the initial phase of locomotion, the trunk (due to its mass) naturally tends to maintain its state of inertia, remaining still with respect to the locomotor unit [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. In order for the \u0026ldquo;passenger\u0026rdquo; element to also change its state of rest, an exchange of forces is therefore necessary between the lower Grocco unit, which constitutes the locomotor element, and the upper one. The resultant of these forces, transmitted through the action of the musculoskeletal system, is represented in the figure as F₂. Thus, as also confirmed by the inverted pendulum model, the trunk, although not directly involved in generating the propulsive push, must still actively participate in maintaining balance and regulating the distribution of forces. The CoM oscillates in a controlled manner thanks to the active interaction between the trunk and the movement of the lower limbs, optimising the system\u0026rsquo;s dynamic stability [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. However, this model scrupulously describes the visible or measurable effects of locomotion, without considering the role and the way internal forces are generated and transmitted in movement. From this perspective, the Grocco model offers an additional interesting integration, describing the \u0026ldquo;passenger\u0026rdquo; as an \u0026ldquo;agent,\u0026rdquo; since the trunk and the head would remain kinematically behind the advancing lower limbs. To prevent this from occurring, further Grocco units and forces intervene within the system. These units must actively compensate for the resistance of the mass through flexion movements, generated by muscular contractions, that are often not visible, as their main purpose is to keep the upper part of the body aligned with the lower limbs. This flexion is instead clearly visible, for example, in running, due to greater acceleration and increased dynamic resistance, where the trunk tends to be visibly inclined in the kinematics of the action.\u003c/p\u003e\u003cp\u003eToward a Dynamic Framework for Teaching Locomotion\u003c/p\u003e\u003cp\u003eThe present work reformulates the Grocco model into Unconstrained Segmental Biomechanics (USB), a methodological framework for analysing locomotion beyond rigid-joint assumptions. By describing limbs as articulated segments connected by non-fixed pivots, USB provides a dynamic-kinematic interpretation that is particularly relevant for gait initiation, usually reduced to a simple change of state in cyclical models [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Through the simultaneous involvement of forces F₁, F₂, and F₃, USB allows for a more realistic representation of this transitional phase, releasing segments from rigidity, integrating the first step into gait analysis, and describing subsequent movements as emergent kinematic effects of underlying dynamics. On an applied level, USB may enrich locomotion teaching by highlighting non-visible but active force interactions, thereby fostering a more comprehensive understanding of human movement [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Its conceptual structure could also support rehabilitation strategies, wearable robotics, and sport performance analysis by clarifying the role of initial propulsive forces [33; 36; 37]. Nevertheless, empirical validation remains necessary to consolidate its pedagogical and scientific value.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis work presented Unconstrained Segmental Biomechanics (USB) as a conceptual framework for locomotion analysis, developed from the Grocco model. Using gait initiation as an example, USB was shown to provide a more realistic dynamic-kinematic description than traditional constrained models, highlighting the role of non-directly observable forces. The review of literature and teaching materials confirmed that the models currently adopted remain predominantly kinematic, neglecting such dynamic integration. USB is therefore proposed as a theoretical tool to reconsider the foundations of teaching and research in sport sciences. Experimental validation is now required to assess its descriptive, predictive, and applicative value, and to evaluate its potential impact in fields such as rehabilitation, wearable robotics, and sport performance analysis.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis theoretical study did not generate or analyse datasets. All referenced materials are available in the public domain.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research received no external funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics Approval and Consent to Participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors and contributors gave their consent for publication.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA. Fogliata: conceived the study, developed the theoretical framework, conducted the literature review, and authored the manuscript. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eL. Cantoni: co-developed the conceptual model, guided all phases of the work, and contributed significantly to the structure and revision of the manuscript. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA. Gambetta: contributed the applied physics components, refined biomechanical terminology, and supported theoretical validation through mechanical arm simulations in a laboratory setting. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA. Ambretti: contributed to verifying selected literature sources and provided language editing. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eS. Tardini: \u0026nbsp;participated in the design of the study, supported manuscript development, maintained collaborative coordination, and contributed to the theoretical framing. Supervised the academic framing of the work and contributed to the theoretical positioning. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHamill J, Knutzen KM, Derrick T. (2022). Biomechanical basis of human movement (Fifth edition, International edition). Wolters Kluwer Health.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNordin M, Frankel VH. Basic biomechanics of the musculoskeletal system. 4th ed. Wolters Kluwer health - Lippincott Williams \u0026amp; Wilkins; 2012.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBartolo E, Giacomozzi C, Coppini DV, Gatt A. Analysis of the interplay between proximal lower limb and foot joint structures as a mechanical predictor of neuropathic ulceration. 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Game ready: pedagogical opportunities for proactive performance analysis in team sports. Sport Educ Soc. 2024;1\u0026ndash;15. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1080/13573322.2024.2437023\u003c/span\u003e\u003cspan address=\"10.1080/13573322.2024.2437023\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Biomechanics of locomotion, gait initiation, locomotor transitions, joint constraints models, dynamic and kinematic interactions, human movement.","lastPublishedDoi":"10.21203/rs.3.rs-7749692/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7749692/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTraditional biomechanical models of locomotion describe the human body as an articulated chain with constrained degrees of freedom, where movements are represented as rotations around fixed joint axes. They assume joint constraints as fixed points in space, thereby nullifying the forces acting at those points and reducing the ability of the model to represent the dynamic forces within the system, particularly during the non-cyclical phases of locomotion. This theoretical study introduces Unconstrained Segmental Biomechanics (USB), a conceptual approach that represents movement as the temporal activation of basic motor units, each composed of a pair of segments articulated by a non-fixed joint constraint. This representation enables the simultaneous analysis of dynamic and kinematic interactions and highlights the specificity of transitional phases, such as gait initiation, compared to cyclical locomotion. To examine how these models are currently transmitted in education, we investigated how gait is taught through a review of the literature and of teaching practices in leading sport science institutions. This analysis revealed that the adopted models remain exclusively kinematic, with no application of integrated dynamic representations such as USB. This work is proposed as a conceptual approach to reframe the foundations of locomotion biomechanics and to stimulate new perspectives.\u003c/p\u003e","manuscriptTitle":"Unconstrained Segmental Biomechanics Applied to Gait Initiation: A Conceptual Approach","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-01 06:26:19","doi":"10.21203/rs.3.rs-7749692/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"31ef999d-4ac7-4b1b-98b3-256c6d93463b","owner":[],"postedDate":"October 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-10-14T15:53:40+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-01 06:26:19","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7749692","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7749692","identity":"rs-7749692","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}