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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 will have more uniform subtendon tissue displacements
that are accompanied by anatomically consistent differences in GAS versus SOL muscle
length change behavior. Our findings fully support our hypotheses, older adults have
more uniform subtendon tissue displacements that extend 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.