Humans can effortlessly vary the mechanical properties of their ankles to complete various tasks, from maintaining balance on a moving bus to adapting to changes in terrain. Ankle mechanics in the sagittal plane are dictated predominantly by the mechanics of the triceps surae muscles and the Achilles tendon, yet their relative contributions are largely unknown. Identifying the muscle and tendon contributions to ankle mechanics is essential for advancing our understanding of how these anatomical structures contribute to the mechanics of posture and locomotion, and for developing targeted interventions when the ability to regulate ankle mechanics is impaired by injury or aging. My thesis addresses this gap through four related studies. First, I developed a novel technique to quantify joint, muscle, and tendon stiffness simultaneously;​ stiffness was chosen as the primary measure of ankle mechanics due to its relevance to postural control and locomotion. After validating this approach, I used it to study the regulation of ankle mechanics in three common situations:​ 1) postural control, 2) during movement, and 3) when musculotendon properties are altered due to healthy aging. During postural control, I found that the Achilles tendon is the primary determinant of ankle stiffness beyond the lowest levels of muscle activation (~2% MVC). During movement, there is a shift in the respective roles of the muscle and tendon compared to posture, with the triceps surae and Achilles tendon contributing more equally to ankle stiffness. Lastly, I found that lower Achilles tendon stiffness in older adults altered the involuntary feedback control of ankle stiffness mediated through stretch reflexes. Having lower tendon stiffness may impair older adults' ability to adapt to the ever-changing demands of their physical environment. Together, this work provides fundamental insight into how ankle mechanics are governed, as well as how age-related musculotendon changes alter those mechanics.