Mechanical properties of the muscle-tendon unit change with aging, but it is not known how these modifications influence the control of lower leg muscles during upright stance. In this study, young and elderly adults stood upright on a force platform with and without vision while muscle architecture and myotendinous junction movements (expressed relative to the change in the moment on the x-axis of the force platform) were recorded by ultrasonography and muscle activity by electromyography. The results show that the maximal amplitude of the sway in the antero-posterior direction was greater in elderly adults (age effect, P < 0.05) and was accompanied by an increase in lower leg muscle activity compared with young adults. Moreover, the data highlight that fascicles shorten during forward sway and lengthen during backward sways but more so for young (-4 ± 3 and -4 ± 3 mm/Nm, respectively) than elderly adults (-0.7 ± 3 and 0.8 ± 3 mm/Nm, respectively; age × sway, P < 0.001). Concurrently, the pennation angle increased and decreased during forward and backward sways, respectively, with greater changes in young than elderly adults (age × sway, P < 0.001). In contrast, no significant differences were observed between age groups for tendon lengthening and shortening during sways. The results indicate that, compared with young, elderly adults increase the stiffness of the muscular portion of the muscle-tendon unit during upright stance that may compensate for the age-related decrease in tendon stiffness. These observations suggest a shift in the control strategy used to maintain balance.