Muscle-tendon mechanics explain unexpected effects of exoskeleton assistance on metabolic rate during walking

The goal of this study was to gain insight into how ankle exoskeletons affect the behavior of the plantarflexor muscles during walking. Using data from previous experiments, we performed electromyography-driven simulations of musculoskeletal dynamics to explore how changes in exoskeleton assistance affected plantarflexor muscle–tendon mechanics, particularly for the soleus. We used a model of muscle energy consumption to estimate individual muscle metabolic rate. As average exoskeleton torque was increased, while no net exoskeleton work was provided, a reduction in tendon recoil led to an increase in positive mechanical work performed by the soleus muscle fibers. As net exoskeleton work was increased, both soleus muscle fiber force and positive mechanical work decreased. Trends in the sum of the metabolic rates of the simulated muscles correlated well with trends in experimentally observed whole-body metabolic rate (R2=0.9), providing confidence in our model estimates. Our simulation results suggest that different exoskeleton behaviors can alter the functioning of the muscles and tendons acting at the assisted joint. Furthermore, our results support the idea that the series tendon helps reduce positive work done by the muscle fibers by storing and returning energy elastically. We expect the results from this study to promote the use of electromyography-driven simulations to gain insight into the operation of muscle–tendon units and to guide the design and control of assistive devices.

本研究旨在探究行走过程中踝关节外骨骼对跖屈肌（plantarflexor muscles）行为的影响。基于既往实验数据，我们开展了肌电图（electromyography）驱动的肌肉骨骼动力学仿真，以探究外骨骼辅助力度变化如何影响跖屈肌-肌腱单元的力学特性，尤其针对比目鱼肌（soleus）。我们采用肌肉能量消耗模型估算单块肌肉的代谢率。当平均外骨骼扭矩升高且外骨骼未提供净功时，肌腱回弹量的减少会导致比目鱼肌纤维产生的正机械功增加。随着外骨骼净功的提升，比目鱼肌纤维的肌力与正机械功均出现下降。仿真肌肉代谢率总和的变化趋势与实验观测到的全身代谢率变化趋势吻合度极高（决定系数R²=0.9），验证了本研究模型估算结果的可靠性。本研究的仿真结果表明，不同的外骨骼工作模式可改变受辅助关节处肌肉与肌腱的运作状态。此外，本研究结果佐证了串联肌腱可通过弹性储能与释能，减少肌纤维所需完成的正机械功这一观点。我们期望本研究成果能够推动肌电图驱动仿真技术的应用，以深入解析肌肉-肌腱单元的运作机制，并为辅助装置的设计与控制提供指导。