Older adults overcome reduced triceps surae structural stiffness to preserve ankle joint quasi-stiffness during walking.
Krupenevich RL, Clark WH, Sawicki GS, Franz JR.
Abstract. Ankle joint quasi-stiffness is an aggregate measure of the interaction between triceps surae muscle stiffness and Achilles tendon stiffness. This interaction may be altered due to age-related changes in the structural properties and functional behavior of the Achilles tendon and triceps surae muscles. We hypothesized that, due to a more compliant Achilles’ tendon, older adults would exhibit lower ankle joint quasi-stiffness than young adults, during walking and during isolated contractions at matched triceps surae muscle activations. We also hypothesized that, independent of age, triceps surae muscle stiffness and ankle joint quasi-stiffness would increase with triceps surae muscle activation. We used conventional gait analysis in one experiment and, in another, electromyographic biofeedback and in vivo ultrasound imaging applied during isolated contractions. We found no difference in ankle joint quasi-stiffness between young and older adults during walking. Conversely, we found that: (i) young and older adults modulated ankle joint quasi-stiffness via activation-dependent changes in triceps surae muscle length-tension behavior, and (ii) at matched activation, older adults exhibited lower ankle joint quasi-stiffness than young adults. Despite age-related reductions during isolated contractions, ankle joint quasi-stiffness was maintained in older adults during walking – which may be governed via activation-mediated increases in muscle stiffness.