Aims: The purpose of this study was to investigate differences in the acute response after resistance exercise between young and older adults. Methods: Seventeen young and 18 older adults performed a single session of resistance exercise, consisting of 3 sets of 10 isometric knee extensions. Maximal voluntary contraction (MVC), motor unit (MU) activity of the vastus lateralis, and electrically elicited torque of the knee extensor were measured before and after the resistance exercise. Results: Although both groups showed the same degree of decline in MVC (young: -15.2 +/- 14.3 %, older: -16.4 +/- 7.9 %, p = 0.839), electrically elicited torque markedly decreased in the young group (young: -21.5 +/- 7.7 %, older: -14.3 +/- 9.5 %, p < 0.001), and the decrease in the MU firing rate was greater in the older group (young: -26.1 +/- 24.1 %, older: -44.7 +/- 24.5 %, p < 0.001). Changes in the MU firing rate following the exercise were correlated with the MU recruitment threshold in the older group (p < 0.001, r(s) = 0.457), but not young group (p = 0.960). Discussion: These results showed that young adults exhibited a greater acute response in the peripheral component, whereas older adults showed a greater acute response in the central component of the neuromuscular system, and the acute response in MUs with a high recruitment threshold following resistance exercise was smaller than in those with a low recruitment threshold in older adults. These findings may partly explain why there are different chronic adaptations to resistance training between young and older adults.
