Electrical and mechanical changes in human soleus muscle during sustained maximum isometric contractions.

Experiments were designed to evaluate changes in the electrical activation and force generating capabilities of human soleus muscle during sustained, maximum isometric contractions. Eighteen experiments were conducted on 7 healthy subjects. Surface EMG, and in select cases, intramuscular fine wire EMG recordings, were made to assess the electrical activation of soleus. Subjects performed maximum isometric plantarflexion contractions of 1-3 min during which time supramaximal electrical pulses were delivered to the tibial nerve at 5-s intervals to elicit maximum M waves. M wave areas were assessed for evidence of neuromuscular junction failure. The results revealed that, on average, maximum force declined to 80% of unfatigued maximum by 60 s of effort, 74% by 90 and 120 s, and 70% by 180 s. M waves were stable for efforts up to 3 min, thereby providing little evidence for neuromuscular junction failure. In 3 experiments, total spike counts from intramuscular recordings displayed a 50% reduction in firing by 30 s of effort, with little additional slowing for up to 3 min. Although all of the fatigue-induced electrical and mechanical alterations in muscle activation reported earlier for intrinsic hand and foot muscles were verified in these experiments on soleus, the magnitudes and time courses of these changes were quite different. All changes were consistent with a muscle designed to optimally resist fatigue.