Age-related decline in leg-extensor power development in single- versus multi-joint movements

Rapid muscle characteristics, such as the rate of power development (RPD), are shown to decline more than maximal muscle characteristics during ageing in single-joint actions. However, functional disability is mainly the result of multi-joint lower limb failure. The complex activation patterns inherent to multi-joint actions and the deteriorating effect of age on that neural drive suggest a larger effect of age on RPD multi-joint. Yet, this is the first study that compared multi- with single-joint leg extension tests in terms of RPD across the adult life span and assessed its transferability to functional performance. 96 healthy adults (male 49, female 47, age = 20-69 years) performed dynamic single-joint knee-extension tests on a Biodex System 3 dynamometer and multi-joint leg-extension tests on a custom-made device at low, moderate and high speed. Peak power (Ppeak) was calculated as the highest value of the power-time curve and RPD as the linear slope fill isokinetic speed was reached. Functional performance was tested using squat jump height. RPD showed greater age-related declines in multi-joint (-1.92%/year) versus single-joint (-1.42%/year) actions, which is in contrast with the finding of Ppeak (- 0.77% vs. -1.04%/year). Squat jump height was more strongly associated with RPD multi-joint than single-joint (r = 0.77-0.82 vs. 0.44-0.61). These results show greater age-related declines of RPD multi-joint versus single-joint and demonstrate its functional relevance. We believe that this finding may be of high importance for the detection and prevention of functional disability during ageing.