The effects of age and task demand on dynamic mean ankle moment arm during walking

Background: Older adults walk with a diminished foot and ankle push-off compared to younger adults and this difference may be a target for assistive devices. Dynamic mean ankle moment arm (DMAMA) is an aggregate measure of foot and ankle control that varies in younger adults across walking speeds and inclines.

Research question: Does age affect DMAMA across a range of walking tasks designed to challenge power output from the ankle and foot?

Methods: We enrolled 12 healthy younger adults and 10 healthy older adults. Participants walked on an instrumented treadmill for 2 minutes at a range of 4 speeds and again, for 2 trials, with their waist attached to a motor-driven system that delivered constant 5 % body weight horizontal forces aiding or impeding net propulsion. A 12-camera system collected motion capture data and a split-belt instrumented treadmill recorded ground reaction forces. We calculated DMAMA as the ratio of ankle moment impulse to sagittal ground reaction force impulse. DMAMA and its determinants were analyzed using a mixed factorial ANOVA for differences across conditions.

Results: Independent of age, DMAMA significantly increased with faster walking speed and as demand for net propulsion decreased. However, DMAMA did not differ significantly between age groups across conditions. Only ankle moment impulse was affected by age, and was smaller in older than younger adults across the majority of walking tasks.

Significance: As an aggregate measure previously proposed as a control parameter for assistive devices to augment foot and ankle push-off, DMAMA may apply equally well for older and younger adults.