Triphasic behavioral response of motor units to submaximal fatiguing exercise

Area of Science:

• Exercise Physiology
• Motor Control
• Neuroscience

Background:

• Muscle fatigue is characterized by a decline in maximal voluntary contraction (MVC).
• Surface electromyography (EMG) increases during fatiguing contractions, but the underlying motor unit behavior is complex.
• Understanding motor unit responses is crucial for elucidating muscle adaptation mechanisms.

Purpose of the Study:

• To investigate the changes in motor unit action potential (MUAP) firing rate and amplitude during and after prolonged submaximal isometric exercise.
• To characterize the temporal relationship between MUAP changes and the onset of muscle fatigue.
• To identify distinct motor unit subpopulations and their responses to exercise.

Main Methods:

• Recorded 4551 MUAPs from biceps brachii in 10 healthy adults using concentric needle electrodes.
• Intermittent isometric exercise at 20% MVC for 45 minutes.
• Automatic decomposition of electromyographic activity (ADEMG) analyzed MUAP firing rate and amplitude before, during, and after exercise.

Main Results:

• MUAPs showed a progressive increase in mean firing rate (P ≤ .01) and amplitude (P ≤ .05) during and after exercise at 30% MVC.
• Increased firing rate preceded increased amplitude and occurred before MVC reduction (fatigue).
• A short-term reduction in MUAP firing rate was observed at the onset of contractions.

Conclusions:

• Motor units exhibit a triphasic response: initial stabilization, followed by increased firing rate and variability, and then recruitment of additional units.
• These adaptations likely compensate for force-generating capacity loss during exercise.
• The findings explain the increase in surface EMG associated with muscle fatigue.