Slowed muscle contractile properties are not associated with a decreased EMG/force relationship in older humans

We tested the hypothesis that, as a result of slower muscle contractile properties, the electromyogram (EMG)/force relationship is decreased during voluntary contractions in older compared to young humans. We studied 22 young (32 +/- 1 yr, mean +/- SE) and 20 older (72 +/- 1) men and women. To quantitate ankle dorsiflexor muscle properties, we measured isometric twitch time to peak force and maximal relaxation rate, the rates of tetanic (50 Hz, 1 s) force development and relaxation, and the stimulated force-frequency relationship. The voluntary EMG/force relationship was determined during isometric dorsflexion from 10% to 100% MVC (maximal voluntary isometric contraction force) in 10% MVC increments. Twitch time to peak force and the rates of tetanic force development and relaxation were slower in the older subjects. Greater relative force was produced in older compared to young adults at 10 Hz. During voluntary contractions, EMG was greater in older compared to young subjects at lower intensities (10% and 20% MVC). Thus, although the older adults exhibited a slowing of contractile properties and summation of force when stimulated at 10 Hz, the voluntary EMG/force relationship was increased rather than decreased at low contraction intensities, compared to young adults. We conclude that the slowing of contractile properties does not result in increased neural "efficiency" of voluntary force production in older adults. This novel observation may have important functional relevance to the performance of activities of daily living, particularly in a more frail older population.