For the elderly, although various factors such as sensation, speed, balance, and psychological status in addition to strength influence the time required to perform a given number of repetitions in the STS task, knee extensor strength is the most important variable for explaining the variance in it [3]. However, the sex difference in STS time cannot always reflect that in knee extensor strength [4, 5]. As an approach to this issue, Takai et al. [5] expressed 10-times-repeated STS performance as a power index (STS-PI) by using an equation involving leg length, body mass, height of chair, acceleration of gravity, and time taken for the task as variables and examined its associations with the cross-sectional area of the quadriceps femoris muscle and knee extension force in elderly individuals. In terms of the results, STS-PI was highly correlated to the two variables, but the corresponding associations for STS time were not significant. The power values calculated using an equation with body mass and STS performance as variables have been shown to be strongly correlated to the average and peak power values developed in the STS task, derived from force-platform and high-speed motion analyses [1]. These findings indicate that the STS-PI is a valid indicator for conveniently assessing the force and/or power generation capability of lower-body muscle. At the same time, we can say that, if one intends to examine age-related or sex-related difference in STS performance in relation to force generation capability, the STS performance should be expressed as a power value.
the force of gravity by kelly stevenson epub
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