Muscles limiting the sit-to-stand movement: an experimental simulation of muscle weakness.

Adapted strategies in rising from a chair occur with muscle weakness. To assess whether muscle weakness caused the strategy change, an experimental simulation was performed that allowed to investigate separately effects of reduced muscle capacity and of strategy change on movement dynamics. It was hypothesized that a sit-to-stand (STS) strategy change spares muscles that become overloaded when muscle weakness develops. Ten healthy females participated; seven of them completed all tests. Muscle weakness causes an increased load-over-capacity ratio. In the present study, this ratio was increased by providing participants with a waist-coat containing 45% of their body mass. Participants performed sit-to-stand manoeuvres with and without added mass; moreover they were instructed to perform two different strategies, the moment-transfer-strategy and the stabilization-strategy. During these STS-tasks sagital 2D-video analysis were made and ground reaction forces (GRF) were measured. Joint moments and powers for ankle, knee and hip joint were calculated. The preferred strategy under the normal condition was the moment-transfer strategy. Increasing the load without adapting the strategy resulted in significantly higher (13%) knee-joint extension moments. Allowing a strategy shift in the loaded condition spared the knee-joint extensors (-6%) and transferred effort to hip-joint extensors (57%) and plantar flexor (67%) muscles. These results suggest that the capacity of knee-joint extensors limits the STS-performance when muscle weakness occurs.