Recovery from forward loss of balance in young and older adults using the stepping strategy.

The purposes of this study were to quantify stability during recovery from forward loss of balance in young and older adults, older single steppers (OSS) and older multiple steppers (OMS), and to identify the biomechanical factors associated with stability during balance recovery. Forward loss of balance was achieved by releasing participants from a static forward lean angle. Participants regained balance by taking one or more rapid steps. Stability was quantified using the margin of stability (MoS), which was computed as the anterio-posterior distance between the forward boundary of the base-of-support and the vertical projection of the velocity adjusted centre of mass. MoS at foot contact and at maximal knee joint flexion angle following foot contact (KJ(MAX)) were smaller in older compared to young adults, and in OMS compared to OSS. Compared to young adults, older adults exhibited a shorter recovery step length, greater trunk flexion angles and exhibited smaller peak knee flexion angles. Trunk flexion angle at foot contact (r=-0.55) and step length (r=0.54) were significantly correlated with MoS at foot contact and together accounted for 51% of the variance in MoS at foot contact. MoS at foot contact was significantly correlated with MoS at KJ(MAX) (r=0.88) and together with peak knee flexion angle during the landing phase (r=0.60) and peak knee extension moment during the landing phase (r=0.47) accounted for 84% of the variance in MoS at KJ(MAX). Overall findings suggest that stability in the first step is lower for older compared to young adults and for multiple compared to single steppers, and that spatial-temporal, kinematic and kinetic factors are associated with stability during recovery from forward loss of balance.