Augmentation of the contraction force of human thenar muscles by and during brief discharge trains

We investigated the influence of the history of activity on the contractile properties of abductor pollicis brevis (APB) to define how the forces produced by individual stimuli change within a stimulus train, with a view to clarifying the optimal discharge frequency for force production in brief trains. Supramaximal electrical stimuli were delivered to the median nerve at the wrist singly or in trains of 2-5 at various interstimulus intervals (ISIs). The force and electromyographic (EMG) responses to trains of n stimuli were defined by online subtraction of the responses to n - 1 stimuli. The force attributable to the nth stimulus was normalized to that produced by a single stimulus. The contraction force produced by 2 stimuli exceeded the force expected with linear summation of 2 single twitches by 30-40% at ISIs of 2-100 ms. Increasing the number of stimuli resulted in less augmentation of the force produced by the last stimulus in the train for ISIs up to 20 ms, but greater augmentation for ISIs of 50-100 ms. At ISIs of less than 10 ms, the time to peak force produced by the last stimulus in a 5-pulse train was delayed by approximately 100 ms, the peak force produced by that stimulus was less than that produced by a single stimulus, and it occurred on the falling phase of the overall contraction. These properties are best explained by the catchlike property of muscle. This implies that the augmentation of contraction force due to this property can increase throughout a stimulus train, and is not restricted to the doublet discharges that have conventionally been studied. We conclude that, with brief discharge trains, maximal forces occur at ISIs of 56-75 ms, intervals that are longer than those conventionally associated with the catchlike property. Discharge rates of 15-20 HZ appear to be optimal for force generation by APB during steady contractions.