Linking muscle-tendon dynamics and energetics to inform exoskeleton design for improved locomotor economy in aging

摘要 在老年人中，步行速度是健康，幸福和长寿的一个很好的预测因素。不幸 随着成年人的成熟，他们喜欢的步行速度迅速下降，因为他们相应的代谢成本增加; 这些变化削弱了老年人驾驭社区的能力。根据最新的科学发现， 与年龄相关的肌腱硬度下降导致代谢成本增加和行走减少 老年人与年轻人的速度。这是因为与年轻（僵硬）的肌腱相比，老年（顺从） 肌腱将串联的肌肉收缩动力学转移到可能不太经济的状态，我的理论是 与老年人相对于年轻人较慢且较不经济的运动有关。因此，这 项目的第一个目标是揭示肌腱刚度，串联肌肉动力学（即， 操作长度和缩短速度），以及周期性收缩的相应代谢成本。到 为了实现这一目标，我将系统地改变老年人比目鱼肌和跟腱动力学周期， 非侵入性的孤立肌肉收缩。我将使用一个 一系列生理测量，通过使用1）超声成像，使我能够跟踪 皮肤下的肌肉和肌腱运动，以及2）开路呼吸测定法，使我能够量化 参与者的代谢率。我假设有效地降低跟腱硬度 经济的串联肌肉动力学。这一目标的结果将揭示基本的关系， 老年肌肉骨骼系统和执行运动任务的相应代谢成本。为 第二个目标，我将确定比目鱼肌收缩动力学如何独立地影响老年人的代谢率 在走路的时候。要做到这一点，我将采用被动（定制鞋）和主动（脚踝外骨骼模拟器） 辅助设备，每个系统地改变老年人比目鱼肌动力学在步行过程中，而我评估 相关的生物力学和生理学指标：肌肉/肌腱动力学、代谢成本、肌肉 激活、步幅动力学和运动学。我假设，辅助设备，优化的相互作用， 比目鱼肌操作长度和缩短速度将最大限度地减少步行的代谢成本， 成年人更喜欢步行速度。目标2的结果将揭示最佳比目鱼肌动力学，以提高 老年人步行经济和速度。研究结果可用于辅助设备设计， 旨在增强老年人腿部功能和行走的外科手术和康复/锻炼计划 性能总之，本研究旨在全面测试周期性肌肉收缩的空间， 揭示肌腱僵硬，肌肉动力学和能量学之间的基本联系，特别是关于 老年人流动性。

Abstract Among older adults, walking speed is an excellent predictor of health, happiness, and longevity. Unfortunately as adults mature, their preferred walking speeds rapidly decline as their corresponding metabolic costs increase; changes that impair older adult’s ability to navigate their communities. Based on recent scientific findings, I posit that the age-related decline in tendon stiffness contributes to the increased metabolic cost and reduced walking speeds of older versus young adults. That is because compared to youthful (stiffer) tendons, aged (compliant) tendons shift the in-series muscle contractile dynamics to presumably less economical states, which I theorize relates to slower and less economical locomotor movement in older versus young adults. Accordingly, this project’s 1st Aim seeks to uncover the relationships between tendon stiffness, in-series muscle dynamics (i.e. operating lengths and shortening velocities), and the corresponding metabolic cost of cyclic contractions. To fulfill this aim, I will systematically alter older adult soleus muscle and Achilles tendon dynamics during cyclic, non-invasive isolated muscle contractions. I will characterize the corresponding physiological responses using a battery of physiological measurements, highlighted by the use of 1) ultrasound imaging that enables me to track muscle and tendon movement ‘underneath the skin’ and 2) open-circuit respirometry that enables me to quantify participant metabolic rates. I hypothesize that effectively reducing Achilles tendon stiffness elicits less economical in-series muscle dynamics. The results of this aim will uncover fundamental relationships between aged-musculoskeletal systems and the corresponding metabolic costs of performing locomotor tasks. For the 2nd Aim, I will determine how soleus muscle contractile dynamics independently affect older adult metabolic rates during walking. To do this, I will employ passive (custom footwear) and active (ankle exoskeleton emulator) assistive devices that each systematically alter older adult soleus dynamics during walking, while I assess relevant biomechanical and physiological measures: muscle/tendon dynamics, metabolic cost, muscle activation, stride kinetics and kinematics. I hypothesize that the assistive device that optimizes the interplay of soleus operating length and shortening velocity will minimize the metabolic cost of walking and improve older adult preferred walking speed. The results of Aim 2 will reveal the optimal soleus muscle dynamics to improve older adult walking economy and speed. Study implications may be used to inform assistive device design, surgical procedures, and rehabilitation/exercise programs aimed to enhance older adult leg function and walking performance. Altogether, this study is set to comprehensively test the space of cyclic muscle contractions to reveal the fundamental links between tendon stiffness, muscle dynamics, and energetics, with special regard to older adult mobility.