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Clark WH and Franz JR, Triceps surae muscle-subtendon interaction differs between young and older adults. Connective Tissue Research. *Invited, Special Issue on Aging.

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Abstract. Mechanical power generated via triceps surae muscle-tendon interaction during walking is largely responsible for the total power needed to walk. This interaction is made complex by the biological architecture of the Achilles tendon, which consists of distinct bundles of tendon fascicles, known as “subtendons”, arising from the lateral and medial gastrocnemius (GAS) and soleus (SOL) muscles. Comparative data and our own in vivo evidence allude to a reduced capacity for sliding between adjacent subtendons compromising the Achilles tendon in old age. This is functionally important, as subtendon sliding could facilitate independent actuation between individual triceps surae muscles, perhaps augmenting contributions to trunk support and forward propulsion. Recently, we revealed that length change differences between the GAS and SOL of young adults positively correlated with non-uniform subtendon tissue displacement patterns. Here, we investigated aging effects on triceps surae muscle-subtendon interaction using dual-probe ultrasound imaging during a series of ramped isometric contractions. We hypothesized that, compared to young adults, older adults would have more uniform subtendon tissue displacements that are accompanied by anatomically consistent differences in GAS versus SOL muscle length change behavior. Our findings fully supported our hypotheses. Older adults had more uniform subtendon tissue displacements that extended to anatomically consistent and potentially unfavorable changes in muscle contractile behavior – evidenced by smaller differences between GAS and SOL peak shortening during isometric force generation. These findings provide an important biomechanical basis for previously reported correlations between more uniform Achilles subtendon behavior and reduced ankle moment generation during waking in older adults.

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