Effects of triceps surae muscle strength and tendon stiffness on the reactive dynamic stability and adaptability of older female adults during perturbed walking.

This study aimed to examine whether the triceps surae (TS) muscle-tendon unit (MTU) mechanical properties affects gait stability and its reactive adaptation potential to repeated perturbation exposure in older adults. Thirty-four older adults each experienced eight separate unexpected perturbations during treadmill walking, while a motion capture system was used to determine the margin of stability (MoS) and base of support (BoS). Ankle plantarflexor muscle strength and Achilles tendon (AT) stiffness were analysed using ultrasonography and dynamometry. A median split and separation boundaries classified the subjects into two groups with GroupStrong (n=10) showing higher ankle plantarflexor muscle strength (2.26{plus minus}0.17 Nm·kg-1 vs. 1.47{plus minus}0.20 Nm·kg-1; P<0.001) and AT stiffness (544{plus minus}75 N·mm-1 vs. 429{plus minus}86 N·mm-1; P=0.004) than GroupWeak (n=12). The first perturbation caused a negative ΔMoS (MoS in relation to unperturbed baseline walking) at touchdown of perturbed step (PertR), indicating an unstable position. GroupStrong required four recovery steps to return to ΔMoS zero level, while GroupWeak was unable return to baseline within the analysed steps. However, after repeated perturbations, both groups increased ΔMoS at touchdown of PertR with a similar magnitude. Significant correlations between ΔBoS and ΔMoS at touchdown of the first recovery step and TS MTU capacities (0.41<r<0.57; 0.006<P<0.048) were found. We concluded that older adults with TS muscle weakness have a diminished ability to control gait stability during unexpected perturbations, increasing their fall risk, but that degeneration in muscle strength and tendon stiffness may not inhibit the ability of the locomotor system to adapt the reactive motor response to repeated perturbations.