During the push-off phase of gait, both the ankle plantar flexors and the hip flexors are known to propel the body into swing. The plantar flexor muscles are particularly important as they are a major contributor to the energy for propulsion of the body, determining gait speed. Muscle torque shows a decline with age, evident in both men and women. A decrease in ankle plantar flexor torque would explain the decrease in gait speed common in older adults. However, in previous studies a trade-off was found between hip flexor muscles and ankle plantar flexor muscles, suggesting that the hip muscles compensate to make up for the decreased torque production in plantar flexor muscles during aging.

The purpose of this study was to determine whether a relationship exists between the hip flexor and ankle plantar flexor moments for (1) older and younger subjects and (2) self-selected and fast walking speeds, as well as the underlying mechanisms responsible for the relationship.

Motion capture data was analyzed for 12 subjects, 6 young (20 + 2 years) and 6 older (62 + 8 years). The subjects were equipped with reflective markers, and walked on an instrumented treadmill for two walking conditions at self-selected and fast speeds. A 6 degree-of-freedom model was created in Visual3D, reporting the kinematics during the gait cycle. Simulations were created in OpenSim to determine joint moments, individual muscle moment, muscle forces, and muscle moment arms in the ankle plantar flexors and the hip flexors from mid-stance to toe off.

By looking at the peak hip flexor and ankle plantar flexor moments our results suggested that older subjects may rely more on their hip muscles than the younger subjects during the push off phase of gait. Similarly, subjects walking at faster speeds rely significantly more on their hip muscles than those walking at self-selected speeds. Interestingly, the causes of these adaptations are two different underlying mechanisms. The older subjects showed an altered hip range of motion causing slightly greater peak hip muscle moment arms than the younger adults. This caused the older adults to have a slightly higher hip flexor to ankle plantar flexor moment ratio, enabling the older adults to rely more on their hip flexors than the younger adults. However, when comparing the fast and self-selected speeds of healthy adults, the underlying mechanism responsible for the increased reliance on the hip flexors was an increase in hip muscle forces.

The results of this study explain the underlying mechanisms behind the adaptation between ankle and hip muscles for aging and altered walking speeds. Healthy adults should expect to exhibit an adaptation in which they use their hip muscles more when they walk fast due to an increase in muscle force production. Older adults may show the same adaptation compared with younger adults but the underlying mechanism is due to a change in kinematics. These adaptations are important to note in healthy adults to set a healthy baseline when analyzing and designing rehabilitation for adults with pathologies.