Older adults exhibit hallmark declines in gait speed, step length, and ankle kinetics for reasons poorly understood. These age-related changes are thought to be precipitated by decreased mechanical output from the ankle plantar flexors. In order to compensate for these ankle deficits, it has been observed that older adults increase mechanical output at the hip, primarily through the hip extensors. This redistribution of lower limb joint work and power is termed the ‘Distal-to-Proximal Shift.’ However, not all older adults display this redistribution, and others have observed different compensatory actions such as increased hip flexor power. For those who have explored this phenomenon previously have done so through a limited lens by examining changes in gait mechanics and muscle morphology separately. Thus, the first aim of this thesis is to identify age-related differences in muscle volume and fat fraction of the plantar flexors in young and older adults. The second aim of this work is to determine differences in lower limb gait mechanics in both young and older adults. Therefore, this thesis strives to implement a more holistic approach by combining overground gait, dynamometry, and MRI to attempt to elucidate the factors contributing to the ‘Distal-to-Proximal Shift’ in older adult gait.

For age-related differences in muscle morphology, muscle volume differences were found in the in the medial gastrocnemius, where older adults had a smaller muscle volume. Furthermore, older adults displayed smaller maximum cross-sectional areas in the gastrocnemii, and older males had a smaller soleus muscle volume and crosssectional area, but not older females. There were no differences in plantar flexor MVIC, specific torque, peak plantar flexion moment, peak ankle power, and total positive ankle work between young and older adults, suggesting that the older adults included here had youthful retention of their ankle function. However, older adults still displayed greater total positive hip work and redistribution ratio compared to young adults, implicating that they had a greater reliance on the hip musculature for propulsion. This increased reliance on the hip musculature may have negative implications on the metabolic cost of walking in the aging population.