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

Area of Science:

• Human physiology
• Muscle physiology
• Neuromuscular function

Background:

• Sustained maximal isometric contractions induce muscle fatigue.
• Understanding soleus muscle fatigue is crucial for human performance.
• Previous studies show fatigue in intrinsic hand and foot muscles.

Purpose of the Study:

• To evaluate changes in electrical activation and force generation of the soleus muscle during sustained maximal isometric contractions.
• To assess neuromuscular junction integrity during prolonged soleus muscle effort.
• To compare soleus muscle fatigue characteristics with other human muscles.

Main Methods:

• Surface and intramuscular electromyography (EMG) were used to record soleus electrical activity.
• Subjects performed maximal isometric plantarflexion contractions lasting 1-3 minutes.
• Supramaximal electrical pulses stimulated the tibial nerve to elicit M waves and assess neuromuscular junction function.

Main Results:

• Maximal force decreased to 70% of the initial maximum by 180 seconds.
• M waves remained stable, showing no evidence of neuromuscular junction failure up to 3 minutes.
• Intramuscular recordings showed a 50% reduction in firing rate by 30 seconds, with minimal further decline.

Conclusions:

• The human soleus muscle exhibits significant force decline but minimal neuromuscular junction failure during sustained maximal contractions.
• Electrical activation changes, particularly firing rate reduction, contribute to fatigue.
• The soleus muscle's fatigue characteristics are optimized for resisting fatigue, differing from intrinsic hand and foot muscles.