Time-course of force production by fast isometric contraction of the knee extensor in young and elderly subjects

Rapid force production by isometric contraction of the knee extensor was examined in a wide range of force output for 12 healthy elderly (65-86 years) and 12 young (20-35 years) subjects. Time-course of tension development and duration of first burst EMG activities (AG-1) of the vastus medialis muscle were compared between both groups. Significant increase in the elderly as compared with the young was found in duration from the onset of the EMG to the rise of tension (TLT), but not in time from the rise to peak of force (FTmax), although FTmax tended to be longer in the elderly than the young at three different levels of force output. AG-1 duration was also prolonged in the elderly but the difference was not significant. The prolongation of TLT in the elderly suggests that spatio-temporal recruitments of the motor units and/or the percentage of fast twitch fibers decreases with aging.