Bodyweight-supported gait training enables functional and task-specific training of walking shortly after a neurological injury. After a neurological injury, individuals have to relearn their active control of lateral balance to avoid falling. However, bodyweight-support forces seem to influence the dynamics of lateral balance. Existing literature does not sufficiently explain the interplay between bodyweight support and lateral balance, and conflicting results have been reported. One possible explanation of the inconsistent results is the concurrent application of vertical unloading and lateral support forces. This manner of force application is inherent to the mechanics of most bodyweight-support systems. This experiment aims to study the independent effect of vertical and lateral bodyweight-support forces on lateral balance. A RYSEN (Motek Medical B.V., Amsterdam, The Netherlands) bodyweight-support system was used, which allows the independent control of vertical unloading and lateral support forces. Fifteen participants walked overground in six different unloading conditions of the RYSEN. In the first part of the experiment, the vertical unloading force was increased, while no active lateral support forces were applied. In the second part of the experiment, the vertical unloading force was kept at a constant value, while the lateral support forces were changed. Gait characteristics related to lateral balance were extracted from motion capture and force plate data.
Increasing the vertical unloading had a significant effect on the step width, center of mass displacement and velocity, margin of stability, foot progression angle, and the relative time spent in the double support phase. An analysis that used the equations of motion for human walking revealed that the contribution of the foot-placement and ankle strategy to the mediolateral ground reaction force decreased due to unloading. This decrease may have been caused by an increased use of the counter-rotation strategy to control lateral balance. However, further analysis showed how the lateral alignment of the body weight support force with the center of mass biased this analysis. Lateral support showed much less effects than expected from literature. This seemed to be caused by the lateral support conditions producing too similar support forces.