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.