Older adults have difficulty maintaining balance when faced with postural
disturbances, a task that is influenced by the stiffness of the triceps
surae and Achilles tendon. Age-related changes in Achilles tendon
stiffness have been reported at matched levels of effort, but measures
typically have not been made at matched loads, which is important due to
age-dependent changes in strength. Moreover, there has been limited
investigation into age-dependent changes in muscle stiffness. Here, we
investigate how age alters muscle and tendon stiffness and their influence
on ankle stiffness. We hypothesized that age-related changes in muscle and
tendon contribute to reduced ankle stiffness in older adults and evaluated
this hypothesis when either load or effort were matched. We used B-mode
ultrasound with joint-level perturbations to quantify ankle, muscle, and
tendon stiffness across a range of loads and efforts in seventeen healthy
younger and older adults. At matched loads relevant to standing and the
stance phase of walking, there was no significant difference in ankle,
muscle, or tendon stiffness between groups (all p > 0.13). However,
at matched effort, older adults exhibited asignificant decrease in ankle
(27%; p = 0.008), muscle (37%; p = 0.02), and tendon stiffness (22%; p =
0.03) at 30% of maximum effort. This is consistent with our finding that
older adults were 36% weaker than younger adults in plantarflexion (p =
0.004). Together, these results indicate that, at the loads tested in this
study, there are no age-dependent changes in the mechanical properties of
muscle or tendon, only differences in strength that result in altered
ankle, muscle, and tendon stiffness at matched levels of effort.