Sequence effects in concurrent strength and endurance training: performance outcomes, biological mechanisms, and practical applications

Sequence effects in concurrent strength and endurance training: performance outcomes, biological mechanisms, and practical applications | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 23 July 2025 V1 Latest version Share on Sequence effects in concurrent strength and endurance training: performance outcomes, biological mechanisms, and practical applications Authors : ZhangFeng 0000-0001-5904-6562 [email protected] , Su Yuhui , and WangJun Authors Info & Affiliations https://doi.org/10.22541/au.175324690.02278745/v1 620 views 178 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract This study examined how sequencing strength and endurance training affects performance outcomes and explored the underlying molecular mechanisms. Methods: Relevant domestic and international literature was reviewed and summarized using the literature review method. Starting from the effects of concurrent training with different sequences on improving strength and endurance qualities, an attempt was made to explore the in-depth mechanisms from the perspectives of neuromuscular and molecular biological adaptations, and the factors influencing the effects of concurrent training with different sequences were proposed. Results: While the training sequence appears unrelated to endurance capacity, muscle hypertrophy, or maximal strength gains, prioritizing strength before endurance training enhances neuromuscular adaptations, particularly benefiting relative strength and explosive power development. Recommendations: The training sequence should be reasonably arranged according to the training purpose and individual differences of the subjects; if the endurance-strength training mode is chosen, it is recommended that the interval between the two types of training be more than 3 hours to prevent acute molecular interference. Sequence effects in concurrent strength and endurance training: performance outcomes, biological mechanisms, and practical applications ZhangFeng 1 Su Yuhui †2 WangJun 3 1 Shanghai University of sport; 2 College of Sports Human Science, Jilin Sport University; 3 BeiJing Sport University; † This authors contributed equally to this work This study examined how sequencing strength and endurance training affects performance outcomes and explored the underlying molecular mechanisms. Methods: Relevant domestic and international literature was reviewed and summarized using the literature review method. Starting from the effects of concurrent training with different sequences on improving strength and endurance qualities, an attempt was made to explore the in-depth mechanisms from the perspectives of neuromuscular and molecular biological adaptations, and the factors influencing the effects of concurrent training with different sequences were proposed. Results: While the training sequence appears unrelated to endurance capacity, muscle hypertrophy, or maximal strength gains, prioritizing strength before endurance training enhances neuromuscular adaptations, particularly benefiting relative strength and explosive power development. Recommendations: The training sequence should be reasonably arranged according to the training purpose and individual differences of the subjects; if the endurance-strength training mode is chosen, it is recommended that the interval between the two types of training be more than 3 hours to prevent acute molecular interference. KEYWORDS concurrent training, training sequence, neuromuscular, molecular biology, practical applications 1 INTRODUCTION ”Concurrent Training” refers to how strength and endurance training tasks are arranged in the same period. Its research began with the research report by American exercise physiologist Hickson in 1980¹. Concurrent strength and endurance training is a popular training method, which can effectively improve athletes’ endurance and strength qualities in different events² , ³. However, in concurrent training, residual fatigue and energy substrate consumption after one training session may affect the quality and performance of subsequent sports training, and form an adverse neuromuscular and molecular internal environment, thus affecting the body’s ability to adapt to training⁴ , ⁵. Some scholars call this phenomenon the ”interference effect”⁶. Many factors lead to the ”interference effect”, such as the interval between strength and endurance training, the training level of the subjects, differences in training intensity and volume, etc. In addition, the sequence of strength and endurance training may be an important factor affecting the effect of concurrent training⁷. ”Sequence effect” refers to the phenomenon that when strength and endurance qualities are arranged in the same training session, or when the time between the two training methods is relatively close, different sequences of strength and endurance may produce different training effects⁸ , ⁹. Understanding the effect of the sequence of concurrent training on improving strength and endurance qualities and its possible biological mechanisms can better guide sports training and mass sports, and provide scientific training methods and means for improving athletes’ competitive ability and public health level. 2 EFFECTS OF CONCURRENT TRAINING WITH DIFFERENT SEQUENCES ON SPORTS PERFORMANCE 2.1 Effects on Strength Quality Sports training is the most effective way to improve skeletal muscle mass and strength. Different forms of exercise will lead to different adaptations of skeletal muscle. Therefore, arranging strength and endurance training in one session may interfere with both qualities, especially more significantly with strength quality⁵. The order of strength and endurance training may be one of the important factors affecting skeletal muscle hypertrophy and strength improvement. 2.1.1 Effects on Skeletal Muscle Hypertrophy and Maximum Strength Skeletal muscle hypertrophy results from skeletal muscle protein synthesis being greater than degradation. Exercise training can increase the content of satellite cells, and at the same time lead to a proportional increase in the content of muscle nuclei, thereby promoting muscle fiber hypertrophy¹⁰. Skeletal muscle hypertrophy can effectively improve maximum strength to a certain extent. The sequence of strength and endurance in concurrent training is not the leading cause of interference effects on skeletal muscle hypertrophy and maximum strength¹¹ , ¹² , ¹³. A systematic review and meta-analysis by Schumann et al.¹⁴ showed that regardless of the type of aerobic training (cycling vs. running), the frequency of concurrent training (>5vs.<5 times a week), training status (with vs. without training experience), average age (40 years old), and training mode (same training session vs. same day training vs. different days training), concurrent training with different sequences will not interfere with skeletal muscle hypertrophy and maximum strength. Previously, Schumann et al.⁸ found in a study on the acute effects of concurrent training with different sequences on human body functions that concurrent training of strength and endurance in both sequences had similar effects on the levels of serum-related hormones (testosterone, cortisol, growth hormone) in male subjects with moderate training levels. This study explained the deep mechanism of promoting skeletal muscle hypertrophy, and there is no correlation between training sequence and skeletal muscle protein synthesis. At the same time, Lundberg et al.¹⁵ re-verified the above conclusion through another review study. In addition, Cadore et al.¹⁶ , ¹⁷ conducted a study on the effect of concurrent training with different sequences on skeletal muscle hypertrophy in older adults. The results demonstrated that both sequences of concurrent training can effectively increase the thickness of skeletal muscle in the upper and lower limbs of older adults. However, there was no significant difference between the two experimental groups ( p >0.05). Wilhelm et al.¹⁸ also studied older adults, implementing a 12-week concurrent training program (twice weekly) with different sequences and assessing strength-related outcomes, including knee extension 1RM and muscle ultrasound measurements. Findings indicated that compared with before training, all indicators were significantly improved ( p ≤0.05), but there was no significant difference between different sequence groups ( p ≥0.05). The authors believe that the improvement of maximum strength in older adults is not related to training sequence. Chtara et al.¹⁹ found in a study with physical education majors as subjects that although pure strength training contributes more to improving strength quality than concurrent training, the sequence of concurrent training is not the main reason for improving the maximum strength of the subjects. Makhlouf et al.²⁰ studied and believed that compared with alternating aerobic and strength training on alternate days, concurrent training on the same day has similar or better effects on muscle strength, and that the sequence of concurrent training is irrelevant to the training effect. It is recommended that coaches arrange strength and endurance training in daily training according to needs, without considering the sequence. However, Shirai et al.²¹ took rats as the research object to study the effect of concurrent training with different sequences on skeletal muscle hypertrophy, and found that the strength (stimulated percutaneously with electrodes) - endurance (running on a rodent treadmill) training sequence had a relatively noticeable impact on the skeletal muscle hypertrophy signal (mTOR), and the strength - endurance training sequence was more conducive to skeletal muscle hypertrophy²². 2.1.2 Effects on Explosive Power The strength-endurance sequence appears optimal for explosive power development. This view holds that in the process of arranging concurrent training, the fatigue generated by the first part of training will affect the quality of the latter part, leading to a decline in the quality of the latter part of concurrent training, which some scholars call the fatigue accumulation effect⁵. According to this view, the arrangement of the concurrent training sequence should be related to the training purpose, and the quality that needs to be improved first should be arranged in the first half of the concurrent training to avoid the fatigue accumulation effect. Craig et al.²³ found in a study on the effect of concurrent training sequence on strength development that performing strength training immediately after endurance training will reduce the quality, load, and intensity of strength training due to residual fatigue. It was also pointed out that the fatigue generated by previous endurance training may weaken the ability of trained muscles to generate sufficient muscle tension during strength training. A review study by Schumann et al.¹⁴ believed that although skeletal muscle fatigue caused by long-term endurance training does not hurt maximum strength, it significantly reduces the improvement of explosive power. The sequence of strength first and then endurance is more conducive to improving subjects’ countermovement jump, explosive power, and muscle work ability¹². Alves et al.²⁴ took prepubescent male subjects as the research object and found that the strength-endurance training sequence is more conducive to improving subjects’ explosive power (medicine ball throw, countermovement jump, standing long jump). In addition, two studies by Cadore et al.¹⁶ , ¹⁷ both believed that the sequence of strength first and then endurance is more conducive to adapting the nervous system in older adults, thereby promoting the improvement of explosive power. Human experiments have proved that concurrent training with different sequences does not interfere with skeletal muscle hypertrophy and maximum strength. However, human and animal experiments have drawn different conclusions, and the reasons are various. First, animal training interventions cannot be entirely similar to human sports. Most studies use electrical stimulation to simulate strength training for animal exercise interventions. Whether this can produce the same effect as human strength training modes, such as squats, needs further verification; the methods of testing relevant sports indicators in humans and animals are also different, which may be the reason for the inconsistent research conclusions. The impact of concurrent training with different sequences on explosive power has reached a consistent conclusion that arranging strength training before endurance training can better reduce the interference effect of concurrent training on explosive power. At the same time, the impact of concurrent training on skeletal muscle protein metabolism and strength quality is affected by many factors such as the subject’s training level, functional status, training methods, exercise intervention time, and nutritional supplements. Therefore, in training practice, different training methods should be adopted according to specific training purposes and athletes’ training levels to achieve the best training effect. 2.2 Effects on Endurance Quality Current evidence suggests that concurrent training does not interfere with aerobic endurance. Hickson¹ showed in the first research conclusion on concurrent training that the maximum oxygen uptake of subjects in the endurance training group and the concurrent training group increased by 17% and 20% respectively, indicating that concurrent training has no negative impact on aerobic endurance, which has also been recognized by other scholars²⁵ , ²⁶. The academic community has reached a relatively consistent conclusion on whether the sequence of concurrent training will lead to differences in the improvement of aerobic endurance quality. The improvement of aerobic endurance level is unrelated to the sequence of strength and endurance training⁵ , ¹³ , ²⁷. Eddens and his colleagues found that the endurance-strength and strength-endurance groups significantly improved indicators such as maximum oxygen uptake and body fat percentage after exercise intervention. However, no significant difference was found between the two experimental groups²⁸. At the same time, Vilaça et al.²⁹ showed that after subjects received concurrent training intervention with different sequences, the maximum oxygen uptake and other indicators were tested, and it was found that both indicators were significantly improved after training intervention, and there was no difference between sequences. Wilhelm et al.¹⁸ believed that both sequences of concurrent training can improve maximum aerobic capacity (maximum oxygen uptake and second ventilatory threshold VT2). However, there was no significant difference between different sequence groups ( p ≥0.05). The reason may be that the improvement of endurance quality is closely related to the training volume. No matter how the sequence changes, the total amount of training intervention is unchanged, so the stimulation degree of the body’s aerobic metabolism ability is similar. In addition, compared with pure endurance training, concurrent training will not hurt maximum oxygen uptake¹¹, but will increase the proportion of type IIa muscle fibers³⁰. At the same time, strength training can increase skeletal muscle stiffness, improve neuromuscular function, and improve the coordination and control ability of neuromuscles. Improving the above functions can improve athletes’ running economy³¹, thus promoting the effective improvement of subjects’ aerobic capacity. 3 BIOLOGICAL MECHANISMS OF CONCURRENT TRAINING WITH DIFFERENT SEQUENCES IN IMPROVING STRENGTH AND ENDURANCE QUALITIES 3.1 Neuromuscular Adaptation to Concurrent Training with Different Sequences Good adaptation of the neuromuscular system to sports training can improve the body’s corresponding physical qualities, and different training methods lead to different adaptations of the neuromuscular system. The adaptation of the central nervous system caused by strength training is mainly manifested in: the increase in the output frequency of brain central nervous impulses; the increase in the connection of interneurons in the spinal cord, which increases the cross-training effect; the enhancement of the ability of motor neurons to switch between excitation and inhibition³²; greater corticospinal drive, stronger spinal motor neuron excitability and/or reduced effect of inhibitory descending drive³³. The adaptive changes of the central nervous system can enhance its ability to coordinate and control motor units, thereby improving the efficiency of strength output. After a period of endurance training, the H reflex of the subjects increases, and the anti-fatigue ability is enhanced. It can also increase the activity of α motor neurons, mobilize more slow muscle fibers of skeletal muscle to participate in exercise, and delay the occurrence of fatigue³⁴ , ³⁵. The sequence of concurrent training may affect the improvement effect of strength quality, but has no impact on the increase of muscle mass²⁸, which indicates that regardless of the training sequence, endurance training will not affect the skeletal muscle hypertrophy caused by strength training. From this, we infer that the impact of concurrent training with different sequences on strength quality may be related to the nervous system’s adaptation. Studies have shown that sports performance seems to have little to do with the sequence of strength and endurance, but there are indeed differences in the observation of neural adaptation between the two groups³⁶. When taking older adults as the research object, it was found that in the concurrent training three times a week, when strength training was carried out before endurance training, the neuromuscular system could produce better adaptation³⁷, which was also verified in the experiment of middle-aged subjects³⁶. These studies all believe that arranging strength training before endurance training will produce good adaptation to the nervous system. However, the opposite sequence may interfere with the nervous system. If aerobic training is always arranged before strength training, it may not affect the improvement of maximum strength. However, at least to a certain extent, it will have a specific inhibitory effect on the nervous system³⁸. Relative muscle strength can well reflect the nervous system’s role in the process of strength development, and the increase of relative muscle strength indicates the good adaptation of the nervous system to training³⁹ , ⁴⁰ , ⁴¹. Cadore et al.¹⁷ studied and believed that compared with the opposite training sequence, the program with strength training first had a greater increase in relative muscle strength (27% and 15% respectively). To sum up, the interference effect can be partially explained by the fact that neural adaptation is affected after strength training. From a practical point of view, although concurrent training may interfere with neuromuscular adaptation, arranging strength training at the beginning of a training session reduces this adverse impact; that is, it can maximize the neuromuscular adaptation of concurrent training. 3.2 Molecular Biological Adaptation to Concurrent Training with Different Sequences Different training forms lead to different molecular biological reactions in the body. The typical external performance caused by resistance training is skeletal muscle hypertrophy, and its mechanism is related to the molecular pathway centered on mTOR; endurance training can effectively improve the body’s aerobic endurance level, mainly through molecules such as AMPK, CaMK, and p38MAP to activate PGC-1α protein, and lead to changes in the quantity, function, and structure of mitochondria, thereby improving the body’s aerobic metabolism ability. In addition, endurance training leads to obvious reactions to energy metabolism. A large amount of ATP is decomposed to produce AMP, resulting in changes in the ratio of the two, thus stimulating the phosphorylation of AMPK. This molecule acts as an energy sensor. AMPK affects skeletal muscle protein metabolism through three pathways. The first one can inhibit the phosphorylation level of mTOR through TSC2, leading to a decrease in the rate of skeletal muscle protein synthesis and a reduction in muscle circumference, as shown in Figure 1 (a); the other two can increase the rate of skeletal muscle protein degradation through the ubiquitin-proteasome system and the autophagy-lysosome system, as shown in Figure 1 (b). From this, we know that strength and endurance training may conflict at the molecular level, thus interfering with skeletal muscle hypertrophy or muscle strength²⁶ , ⁴² , ⁴³. Different sequences of concurrent training may lead to different molecular reactions. Figure 1 Molecular pathway of skeletal muscle protein metabolism induced by strength and endurance training 3.2.1 Effects on Skeletal Muscle Protein Synthesis Pathway Akt/mTOR/p70S6k/S6 is the main molecular pathway for exercise to promote skeletal muscle protein synthesis. Some studies have demonstrated that when aerobic training is carried out first, and then strength training, the skeletal muscle protein synthesis rate is the most obvious. On the contrary, the protein synthesis efficiency is low⁴⁴. Other research conclusions also support this point. When they analyzed the mTOR signaling pathway molecules related to protein synthesis, they found that the phosphorylation levels of mTOR, p70S6K, S6, and GSK-3β in the endurance-strength group were significantly higher than those in the control group and the strength-endurance group. The mTORC1 signal response caused by strength training will be down-regulated by the molecular effect generated by endurance training. The authors believe the last training form determines the molecular response²² , ⁴⁵. The phosphorylation level of ribosomal protein S6 was significantly increased after the end of the first module of concurrent training. However, the phosphorylation level of strength training was higher than that of endurance training. There was a significant difference between the groups, which can verify that strength training is more conducive to skeletal muscle protein synthesis than endurance training. At the same time, when detecting its phosphorylation level three hours after exercise intervention, it was found that the endurance-strength group was significantly higher than the strength-endurance group⁴⁶, which further indicated that the last training form determines the molecular response level. Studies on Akt molecules have shown that concurrent training with different sequences can increase their phosphorylation level, which has little to do with the training sequence²². The activation levels of Akt and its downstream molecules are not synchronized, which may be due to the existence of other non-Akt-dependent protein synthesis pathways⁴⁷ , ⁴⁸. The above research conclusions indicate that the training sequence of endurance first and then strength may be conducive to synthesizing skeletal muscle protein. Another protein synthesis pathway is mTORC1/4E-BP1/eIF 4E/S6 (Figure 1a). All eukaryotic mRNA has a ”cap” structure at the 5’ end, which can be recognized by eIF 4E and combined with it to promote protein synthesis⁴⁹. 4E-BP1 is a translation inhibitory protein, which can occupy the binding site between eIF 4E and the ”cap” structure by binding to the ”cap” structure at the 5’ end, thereby inhibiting the initiation stage of protein translation. When 4E-BP1 is highly phosphorylated, it will release the binding site of the ”cap” structure, so that eIF 4E can bind to the ”cap” structure of mRNA, promoting protein synthesis⁵⁰. Therefore, the phosphorylation degree of 4E-BP1 is closely related to the initiation stage of translation. However, mTORC1 can inhibit the phosphorylation level of 4E-BP1, thus hindering the combination of eIF 4E and the ”cap” structure of mRNA, which is not conducive to protein synthesis. Shirai et al.²¹ , ²² used different endurance training methods to conduct a concurrent training intervention on rats. After three weeks of exercise intervention, it was found that the phosphorylation level of 4E-BP1 in the RE-MICT sequence group was higher than that in the control group and the opposite sequence group. However, when HIIT training was used instead of MICT, there was no difference in the phosphorylation level of 4E-BP1 between groups. It shows that the training method is also one of the reasons for the interference effect. At the same time, no significant changes were found between groups when detecting Akt, mTOR, and TSC2. This indicates that 4E-BP1 may be regulated by other molecular pathways and the molecular pathway centered on mTOR. 3.2.2 Effects on Skeletal Muscle Protein Degradation Pathway The first molecular pathway that reduces the skeletal muscle protein synthesis rate is AMPK/TSC2/mTORC1 (the green line pathway in Figure 1). Aerobic exercise can activate the AMPK signal transduction pathway in skeletal muscle cells⁵¹. As a key energy sensor in cells, AMPK can inhibit the activity of mTORC1 through TSC2, or AMPK can directly phosphorylate mTORC1 after being activated, thereby blocking the downstream signal transmission of mTOR and further hindering the rate of protein synthesis⁵². AMPK is very sensitive to exercise intensity. The activity of AMPK increases at the beginning of exercise and can return to the basic level in a short time after exercise⁵³ , ⁵⁴. Even if AMPK has a negative regulatory effect on mTORC1, it only works briefly after training. This inhibitory effect will decrease or disappear if the interval is too long. Therefore, the interval time of concurrent training is of great significance for controlling skeletal muscle protein synthesis. The concurrent training sequence of strength training followed by endurance training enhances TSC2 phosphorylation levels more than the reverse sequence, aligning with the observed trend in mTOR activity⁴⁶ and supporting the conclusion that mTOR regulation depends on TSC2 signaling. Another study reported acute interference effects: when strength training preceded endurance training, mTOR Ser2448 phosphorylation was significantly suppressed, whereas this suppression did not occur with the opposite sequence⁵⁵. This may be attributed to AMPK activation during endurance training, which inhibits mTOR Ser2448 phosphorylation. Another pathway to accelerate skeletal muscle protein degradation is the AMPK/FOXO/MURF-1 and MAFbx (the purple line pathway in Figure 1b). The role of the ubiquitin-proteasome system in regulating protein metabolism cannot be ignored. This system comprises ubiquitin, enzymes related to protein ubiquitination, and proteasomes (26s proteasomes) that degrade ubiquitinated proteins. They interact to complete intracellular protein degradation and participate in regulating the balance of the intracellular environment⁴⁴. MURF-1 and MAFbx are considered to be muscle atrophy genes, located on chromosomes 2 and 8, respectively⁵⁶. MURF-1 and MAFbx are two muscle-specific E3 ubiquitin ligases⁵⁷. The selective degradation of proteins by proteasomes depends on the specificity of E3 ubiquitin ligases. Coffey et al.⁵⁵ found that MAFbx did not change significantly after concurrent training intervention with different sequences. At the same time, the expression of the MuRF-1 gene increased 3 hours later, but it was unrelated to the training sequence. At the same time, they conducted another study on the effect of concurrent training on skeletal muscle protein metabolism. In the experiment, muscle biopsy analysis was performed on the subjects before exercise intervention, after the first part of concurrent training, after the end of the whole intervention, and 3 hours after exercise. It was found that the content of MAFbx mRNA increased after exercise, but there was no significant difference, and it was not related to the training sequence. However, compared with the endurance-strength training form, the MURF-1 in the strength-endurance training form group increased significantly 3 hours after exercise⁴⁶, indicating that the strength-endurance training mode may increase the rate of protein degradation, which is not conducive to skeletal muscle hypertrophy. However, the two studies have different conclusions, which may be related to the individual differences of the subjects and the different sampling times. From the perspective of molecular pathways of skeletal muscle protein synthesis and degradation in concurrent training with different sequences, although most research conclusions support that the endurance-strength training form is more conducive to the formation of skeletal muscle hypertrophy. However, most of the studies in this field take animals as the research objects, and take the expression of some molecules that promote protein synthesis or degradation as the research mechanism. However, the interaction between molecules is very complex. These studies explain the impact of concurrent training arrangements with different sequences on skeletal muscle hypertrophy from the perspective of single or partial molecular interference, which may lead to one-sided conclusions. At the same time, there are significant differences between humans and animals in training intervention and other aspects, which may be the reasons for the inconsistency between human sports performance and animal molecular expression levels. In addition, the role of autophagy in skeletal muscle protein degradation cannot be ignored, but there is little literature involving autophagy in the study of concurrent training. Researchers should pay more attention to and study this aspect. 3.3 Effects on Aerobic Metabolism-Related Indicators The aerobic metabolism indexes of skeletal muscle are related to aerobic exercise capacity. By detecting the relevant metabolism indexes of skeletal muscle, we can better understand the relationship between concurrent training with different sequences and aerobic exercise capacity. After 6 weeks of concurrent training with different sequences, the citrate synthase (CS) and cytochrome C oxidase in the subjects’ skeletal muscles were significantly increased, and the subjects’ aerobic capacity was improved. These changes were related to the arrangement sequence of strength and endurance training¹⁹. Glycogen reserve can reflect the level of the body’s aerobic metabolism capacity. Both sequences of concurrent training can increase the liver glycogen content of experimental rats, but there was no significant difference between groups²². The change of mitochondrial biogenesis markers can represent the aerobic metabolism capacity to a certain extent¹⁹. After concurrent training with different sequences, the expression of mitochondrial protein and the change of cytochrome C were unrelated to the training sequence, and the oxidative metabolism capacity and mitochondrial regeneration capacity were also unrelated to the training sequence²¹. After concurrent training with different sequences, the skeletal muscle glycogen content, PGC-1α, and proteins related to oxidative phosphorylation, such as SDHB, UQCRC, MTCO1, ATP4A, and HIF-1α, were detected. Findings indicated that compared with the control group, the above indexes were significantly improved, but there was no significant change between different sequence groups²². Thus, we further confirmed the conclusion that the improvement of endurance level may have little to do with the training sequence through in-depth analysis. 4 INFLUENCING FACTORS OF CONCURRENT TRAINING EFFECTS WITH DIFFERENT SEQUENCES Concurrent strength and endurance training is a complex training method, whose effectiveness is influenced by multiple factors, such as exercise mode, intensity, training interval, participants’ training status (e.g., elite athletes vs. sedentary individuals, young vs. older adults), and individual differences. In practical training, these factors should be fully considered to avoid confounding effects from non-target factors, and training protocols should be adjusted accordingly to maximize effectiveness. 4.1 Subject Differences Differences in subjects’ age, training level, etc., may be one of the factors leading to inconsistent research conclusions. Two meta-analyses⁵ , ²⁷ believe that concurrent training with different sequences has different intervention effects on adults and adolescents. In terms of improving skeletal muscle dynamic strength, adults will achieve better results by adopting the strength-endurance training sequence; however, for adolescents, the aerobic-strength training mode may be more beneficial. The reason may be that, compared with adults, children or adolescents usually show stronger anti-fatigue and rapid body recovery after high-intensity training⁵⁸. When adopting the aerobic-strength training mode, previous aerobic training has little impact on the quality of subsequent strength training in older adults, so they are less likely to be affected by the fatigue interference effect than adults. Findings in elderly subjects are generally consistent with those in adults. Specifically, Cadore et al.¹⁷ suggested that the strength-endurance training sequence may maximize neuromuscular adaptations in older adults. When designing concurrent training for older adults, performing strength training before endurance training results in better maximum dynamic strength and greater relative muscle strength compared to the reverse sequence¹⁶. Additionally, Cadore et al.¹⁷ recommended that older adults adopt the ‘strength-first, then endurance’ mode during concurrent training, as it is more conducive to improving muscle mass and other health-related functions In addition, training status is another key factor influencing the arrangement of training sequences. For example, it has been proposed that individuals without training experience can achieve greater training benefits regardless of the training mode (strength-endurance or endurance-strength), and their benefits are less correlated with training sequence⁵⁹. This may be because untrained individuals have lower motor function and greater potential for training adaptation. 4.2 Training Purposes Some studies have demonstrated that choosing the concurrent training sequence gives priority to the training goal to train the primary goal quality when the body is not fatigued. The priority of the training goal will determine the sequence and/or time arrangement of other training courses⁶⁰. The concurrent training sequence for maximizing the improvement of strength-related qualities seems to have reached a relatively unified research conclusion: if the training purpose is to increase the relative strength and explosive power of skeletal muscle, or to increase muscle content, it is recommended to arrange strength training before endurance training³⁶ , ⁶¹. A review analysis by Yu Hongjun⁶² pointed out that the impact of training sequence is related to sports events. It is recommended to adopt the sequence of endurance first and then strength in running events, but in rowing and canoeing events, the sequence of strength first and then endurance will achieve better results. 4.3 Exercise Modes Different strength and endurance training modes also affect the interference effect of concurrent training with different sequences²¹ , ²². Arranging strength training immediately after low-intensity, non-energy-exhausted endurance training can better improve the effect of endurance training than pure low-intensity endurance training. It can stimulate the generation of maximum strength⁶³. At the same time, this low-intensity endurance training will not interfere with the molecular effect caused by later strength training⁶⁴ , ⁶⁵. Plyometric training can make full use of the characteristics of the muscle stretch-shortening cycle, and has a good effect on improving muscle strength and explosive power³¹. Using adolescent football players as the research object, strength training was carried out as plyometric training. The results showed that regardless of the sequence of strength training and formal football special training, the effect of strength training can be effectively exerted, and there was no significant difference between groups⁶⁶. Compared with long-term continuous training, high-intensity interval training can more effectively improve aerobic capacity, glycolytic metabolism capacity, and muscle strength, and the effect is more evident in amateur subjects⁶⁷. Therefore, combining high-intensity interval and strength training, the concurrent training mode may obtain greater benefits. In addition, the training environment is also an important factor affecting the training effect. For example, when swimming is used as an intervention for aerobic endurance training, the water temperature will also affect the activation level of molecules such as mTOR and AMPK⁶⁸ , ⁶⁹. At the same time, compared with running, swimming can reduce the body load and have less impact on subsequent strength training⁷⁰, thus also having different impacts on the training effect. When studying the interference effect of concurrent training at the molecular level, more attention should be paid to controlling the impact of irrelevant variables on the research results. In addition, compared with cycling, aerobic training with running may have a more obvious interference effect in concurrent training, especially on the hypertrophy of type I muscle fibers¹⁵. Through meta-analysis, Wilson et al.⁷¹ explored the factors that endurance training interferes with muscle hypertrophy and muscle strength growth in concurrent training, and found that endurance training mainly based on running efficiently interferes with the results of strength training, while endurance training using power bicycles will not produce an interference effect. The authors believe that compared with cycling, which is mainly based on concentric contraction, the relevant muscle groups in running do eccentric contraction, which easily causes muscle damage and reduces muscle strength, thus having a greater interference with strength quality. 4.4 Interval Time Some studies have demonstrated that the training sequence has a greater impact on acute molecular reactions⁴⁶ , ⁵⁶, so the interval time between strength and endurance training may be an important factor causing molecular signal reactions in skeletal muscle⁶⁴ if high-intensity endurance training is arranged first, in order to ensure that APMK and SIRT1 return to the fundamental value, a 3-hour rest time after endurance training is necessary before arranging strength training. This is because APMK increases rapidly after high-intensity endurance training and takes at least 3 hours to return to the quiet level⁷². In addition, the activity of mTORC1 after strength training will last for 18 hours⁷³ , ⁷⁴. Therefore, a sufficient interval time is necessary to avoid the interference effect of AMPK molecules on mTORC1. This also tells us that arranging strength training 3 hours after endurance training can eliminate molecular interference. In addition to considering the interference effect between molecules, the recovery time of fatigue is also a key issue to be considered in arranging concurrent training. Some studies believe that the running economy was tested 8 hours, 24/48 hours after lower limb strength training, and it was found that there was a certain degree of reduction⁷⁵. Therefore, the length of the interval time needs to be arranged according to factors such as training intensity. In summary, the sequence effect of concurrent training is not a fixed phenomenon but is regulated by multiple factors. These factors should be considered when applying training sequence to maximize the effectiveness of strength and endurance improvement. 5 CONCLUSIONS AND RECOMMENDATIONS Current evidence has suggested that the improvement of endurance quality, skeletal muscle hypertrophy, and maximum strength is not related to the concurrent training sequence. However, the sequence of strength first and then endurance can improve the neuromuscular system’s adaptability, which is more helpful for improving relative strength and explosive power. When implementing concurrent training, the exercise sequence should be rationally designed based on training objectives and individual differences. If the primary goal is to enhance aerobic endurance, the training sequence may be less critical; however, to minimize AMPK-mediated inhibition of mTOR signaling, strength training should ideally be scheduled at least 3 hours after endurance exercise. Conversely, to optimize neuromuscular adaptations (e.g., relative strength and explosive power), a strength-endurance sequence is recommended. Furthermore, the effects of different concurrent training sequences on physical performance and their underlying biological mechanisms likely result from multifactorial interactions rather than isolated factors. Previous studies, however, have often examined this phenomenon through a narrow lens, potentially leading to oversimplified conclusions. Future research should adopt a more comprehensive approach by incorporating additional influencing variables and designing experiments from a broader perspective. REFERENCES 1. Hickson RC, Rosenkoetter MA, Brown MM. Strength training effects on aerobic power and short-term endurance. Med Sci Sports Exerc . 1980, 12(5):336-9. 2. Huiberts RO, Wüst RCI, van der Zwaard S. Concurrent Strength and Endurance Training: A Systematic Review and Meta-Analysis on the Impact of Sex and Training Status. 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