Mechanical model of the recovery reaction from stumbling: effect of step length on trunk control

Falling after a gait perturbation is a major problem for elderly people. The goal of this paper is to model some mechanical limitations of the recovery strategies performed after a trip or stumble, such as elevating or lowering strategies. A biomechanical model of the recovery was used to interpret stumbling data measured on healthy young and elder participants. The experiments consisted of simulating a stumble by blocking the swing leg, while the subject was walking on a treadmill. The motion and the vertical ground reaction forces were recorded to calculate the inverse dynamics. It is hypothesized that the stumble recovery depends on the ability to control the trunk. In the elevating strategy, the swing foot lands ahead of the body center of mass (COM) and the trunk flexion torque can be compensated. In the lowering strategy, with a shorter step, the trunk flexion cannot be arrested without the application of antero-posterior horizontal ground reaction forces. This action accelerates forwardly the COM, thus requiring quick successive recovery steps to place the foot ahead of the COM. If the recovery step is too slow, it is impossible to arrest trunk flexion and a fall would occur. The model and experimental data suggest that step length and speed after stumbling limit the recovery. Further research on screening protocols of maximal step speed is advised to evaluate the risk of falling in the elderly.