Normally aging older adults demonstrate the bilateral deficit during ramp and hold contractions.

The bilateral deficit results from the maximum voluntary force of a bilaterally performed task being smaller than the sum of the maximum voluntary force of the unilaterally performed tasks. It is underlain by the limitation of the neural drive during maximum bilateral contractions and has been hypothesized to reflect the inability to fully activate high threshold motor units. Because high threshold motor units atrophy in older adults, a smaller bilateral deficit in older adults compared to young adults would further support the hypothesis. Indeed, K. Häkkinen et al. in 1995 and 1996 reported no bilateral deficit in older adults performing rapid maximum contractions. The present study extends this investigation to slowly developed maximum contractions. The results demonstrated a bilateral deficit (p < .05). This result, combined with the age related decrease in the number of high threshold motor units, tends to refute the contention that selective restriction of high threshold motor units causes the bilateral deficit during maximum voluntary isometric ramp and hold contractions.