The load/capacity ratio affects the sit-to-stand movement strategy

Background In this study the effect of a changed load/capacity ratio on sit-to-stand performance and on the underlying net joint moments was investigated. In subjects with muscle weakness the load/capacity ratio is increased due to reduced muscle capacity. In the current study this ratio was manipulated by changing the load. This approach allowed studying the isolated effect of an increased load/capacity ratio on sit-to-stand strategy. Methods Ten healthy women performed sit-to-stand movements under four load conditions. The load/capacity ratio was manipulated by adding 0%, 15%, 30% and 45% of the body mass to a weight vest. To determine changes in sit-to-stand strategy flexion of the trunk and temporal characteristics were assessed. Joint moments at ankle, knee and hip joints and activation patterns of major leg muscles were determined from the kinematics and kinetics. Findings Increasing the extra load from 30% to 45% changed the sit-to-stand performance. In the 45% condition maximal trunk flexion was increased and movement time significantly elongated. The strategy change was associated with a disproportionate increase of the net hip extension moment and a delayed peak of the net knee extension moment. Interpretation This study shows that experimentally observed changes in sit-to-stand strategy can be attributed to an increase in the load/capacity ratio. For treatment purposes this implies that increasing muscle strength, reducing body mass or a combination of these could be a suitable approach to improve sit-to-stand performance. The experimental model applied will be useful to study the isolated effect of the load/capacity ratio.