Hindlimb kinematics, kinetics, and muscle dynamics during sit-to-stand and sit-to-walk transitions in emus (Dromaius novaehollandiae)

Animals not only need to walk and run but also lie prone to rest and then stand up. Sit-to-stand (STS) and sit-to-walk (STW) transitions are vital behaviours little studied in species other than humans so far, but likely impose biomechanical constraints on limb design because they involve near-maximal excursions of limb joints that should require large length changes and force production from muscles. By integrating data from experiments into musculoskeletal simulations, we analysed joint motions, ground reaction forces, and muscle dynamics during the STS and STW transitions in a large terrestrial, bipedal, and cursorial bird: the emu (Dromaius novaehollandiae, approximately 30 kg). Simulation results suggest that in both STS and STW, emus operate near the functional limits (> 50 % of shortening/lengthening) of some of their hindlimb muscles, particularly in distal muscles with limited capacity for length change and leverage. Both movements involved high muscle activations (> 50 %) and force generation of major joint extensor muscles early in the transition. STW also required larger net joint moments and non-sagittal motions, entailing greater demands for muscle capacity. Whilst our study involves multiple assumptions, our findings lay the groundwork for future studies to understand, for example, how tendon contributions may reduce excessive muscle demands, especially in the distal hindlimb. As the first investigation into how an avian species stands up, this study provides a foundational framework for future comparative studies investigating organismal morphofunctional specialisations and evolution, offering potential robotics and animal welfare applications.