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指标。不幸的是 随着成年人的成熟，他们喜欢的步行速度会随着相应的新陈代谢成本增加而迅速下降； 损害老年人在社区中导航的能力的变化。根据最近的科学发现，我认为 与年龄相关的肌腱僵硬的下降导致代谢成本的增加和步行的减少 老年人与年轻人的速度。这是因为与年轻(更僵硬)的肌腱相比，衰老(顺从) 肌腱将串联肌肉的收缩动力转换到可能不那么经济的状态，我的理论是这样的 与老年人和年轻人相比，运动速度较慢且不那么经济的运动有关。因此，这 项目的第一个目标是揭示肌腱僵硬、串联肌肉动力学(即 工作长度和缩短速度)，以及相应的循环收缩的代谢成本。至 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在循环中的动力学， 非侵入性孤立肌肉收缩。我将使用一种 一系列生理测量，突出的是使用了超声波成像，使我能够追踪 肌肉和肌腱在皮肤下的运动，2)开路呼吸测量，使我能够量化 参与者代谢率。我假设有效地减少跟腱僵硬所产生的影响较小 经济的串联肌肉动力学。这一目标的结果将揭示 老化的肌肉骨骼系统和执行运动任务的相应新陈代谢成本。对于 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。为此，我将使用被动(定制鞋)和主动(脚踝外骨骼模拟器)。 在我评估时，每个辅助装置都系统地改变了老年人比目鱼肌行走时的动力学 相关生物力学和生理指标：肌肉/肌腱动力学、代谢成本、肌肉 动作、步态动力学和运动学。我假设，优化相互作用的辅助设备 比目鱼肌的工作长度和缩短速度将使步行的新陈代谢成本降至最低，并改善衰老 成年人更喜欢步行速度。目标2的结果将揭示最优的比目鱼肌动力学来改善 老年人步行经济，速度快。研究暗示可用于辅助装置设计， 旨在增强老年人腿部功能和步行的外科手术和康复/锻炼计划 性能。总之，这项研究旨在全面测试周期性肌肉收缩的空间，以 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 老年人行动不便。

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.