Although the patterns of GRF and NJMs were consistent between younger and older adults who successfully arrested the forward fall within one step, age differences in magnitude of lower-limb NJMs were observed .
At the same relative step length used to arrest a forward fall, older females generated significantly lower ankle plantar flexor and hip extensor NJMs than younger females .
Both younger adults and older adult nonfallers are reported to use ankle plantar flexor, knee flexor, and hip extensor NJMs to redirect the GRF anterior to the COM in response to a trip.
During the contact phase of the swing leg, ankle plantar flexor, knee extensor, and hip flexor NJMs (Figure 5) were observed in both single- and multiple-step strategies .
As observed during forward fall recovery [5,7,18,27], regulating angular impulse would involve coordinating the ankle, knee, and hip NJMs and activating the corresponding lower-limb muscles [5,7,18,27].
At the lower-limb level, the subject used a set of knee extensor NJMs, relatively small hip extensor or flexor NJMs, and ankle plantar flexor NJMs to redirect the GRF posterior to the COM (Figure 6(b)) [40-42].
Execution of a successful stepping strategy with the swing leg, as observed in younger adults, requires (1) a relatively short reaction time to initiate the recovery movement following the perturbation, (2) fast flexion and extension velocities of lower-limb joints during the swing phase to position the foot anterior to the COM, (3) a long step length to create a large base of support, and (4) coordinated and sufficient lower-limb NJMs to regulate angular impulse.
For the support leg, the inability to sufficiently redirect the GRF relative to the COM observed in older adult fallers is attributed to their inability to redistribute the lower-limb NJMs to regulate the angular impulse.