Quantifying dynamic and postural balance difficulty during gait perturbations using stabilizing/destabilizing forces.

Intensity of balance exercises used to reduce fall risk is often poorly quantified. The study aimed to test whether balance difficulty can be rated during gait perturbations against balance difficulty during gait without perturbation, using the stabilizing/destabilizing forces. These forces represent the difficulty to maintain balance as the theoretical forces necessary to cancel body velocity and to set the body into an unstable posture, respectively. Ten healthy subjects walked on a split-belt treadmill, that also generated perturbations. Kinetic and kinematic data were collected during gait at comfortable and fast speeds without perturbation, and in five trials at comfortable speed with perturbations. Perturbations consisted of increasing or decreasing the speed of one belt to three different levels in each direction in a random order during the stance phase of 12 random steps per trial. The difficulty of maintaining balance was measured during the perturbation and the three following recovery steps. Compared to comfortable speed, higher stabilizing and lower destabilizing forces indicated higher balance difficulty during the perturbation step for faster-belt perturbations, and recovery steps for slower-belt perturbations. This was also associated with the center of mass shifted forward, and moving faster, and with the center of pressure closer to the forward limit of the base of support. Difficulty increased proportionally with the intensity of perturbation and was significantly higher for the more intense perturbations than at fast speed. Thus, the stabilizing/destabilizing forces seem adequate to evaluate balance difficulty during gait perturbations and could be used to determine the optimal difficulty for balance rehabilitation.