Fatigue effects on motor unit activity during submaximal contractions

Objective: To examine motor unit changes during the development of fatigue in healthy subjects. Design: Automated decomposition-enhanced spike-triggered averaging was used to characterize motor unit size and firing rate in the dominant vastus medialis during maintained contractions at 10% and 30% of maxima voluntary contraction (MVC). Setting: Academic outpatient neuromuscular clinic. Participants: Healthy laboratory personnel. Main Outcome Measures: Surface electromyogram, surface-detected motor unit action potential amplitude (S-MUAP), mean firing rate, force (MVC), motor unit index. Results: Surface electromyogram values and S-MUAP amplitudes increased during both 10% and 30% MVC fatiguing contractions, while mean firing rates decreased. A motor unit index, indicating the degree of motor unit pool activation, increased similarly to S-MUAP size, implying that new and larger units were recruited to maintain the contraction. Repeated contractions led to earlier motor unit changes and fatigue. Conclusion: During submaximal fatiguing contractions, additional motor units are activated to maintain strength. These changes begin early, within the first minute, particularly after a previous fatiguing effort.