Motor unit discharge rates decline by about 50% over 60 s of a sustained maximum voluntary contraction (MVC). It has been suggested that this decline in discharge rate serves to maintain force by protecting against conduction failure and by optimizing the input to motor units as their contractile properties change. This hypothesis, known as muscle wisdom, is based in part on studies in which muscle force was shown to decline more rapidly when stimulation was maintained at a high rate than when stimulus rate was reduced over time. The stimulus rates used in those studies, however, were higher than those normally encountered during MVCs. The purpose of this study was to compare force loss under constant and declining stimulus rate conditions using rates similar to those that occur during voluntary effort. Isometric force and surface EMG signals were recorded from human adductor pollicis muscles in response to supramaximal stimuli delivered to the ulnar nerve at the elbow. Three fatigue protocols, each 60 s in duration, were carried out on separate days on each of 10 subjects: (1) continuous stimulation at 30 Hz, (2) stimulation at progressively decreasing rates from 30 to 15 Hz and (3) sustained MVC. The relative force-time integral (endurance index) was significantly smaller for the sustained MVC (0.75 ± 0.08) and decreasing stimulus rate conditions (0.76 ± 0.16) compared to the condition in which stimulus rate was maintained at 30 Hz (0.90 ± 0.13). These findings suggest that decreases in discharge rate may contribute to force decline during a sustained MVC.
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