The relationship of force output characteristics during a sit-to-stand movement with lower limb muscle mass and knee joint extension in the elderly

This study examined the reliability of ground reaction force (GRF) parameters during sit-to-stand (STS) movements and the relationships between the GRF parameters and lower limb muscle mass and knee extension muscle strength. Fifty elderly females performed an STS movement twice from a chair adjusted to their knee height and the GRF, lower limb muscle mass and isometric knee extension muscle strength were measured. Reliabilities of GRF parameters were high (intra-class correlation coefficient = ICC = 0.70–0.95). Parameters on force output during trunk flexion phase (ground reaction force at hip-lift off, sum of force output between beginning of STS movement and hip-lift off) differed significantly between trials, but their effect sizes were small (0.15–0.23). GRF parameters during hip-lift off and knee–hip joint extension phases significantly correlated with knee extension strength (|r| = 0.29–0.64) but not lower limb muscle mass. In conclusion, the reliability of GRF during STS movement is good in hip-lift off and knee–hip joint extension phases and these phases relate significantly with lower limb muscle function. These two phases are useful for evaluation of leg muscle function of the elderly.