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Fatigue effects on motor unit activity during submaximal contractions
R A Conwit1, D Stashuk, H Suzuki
1Department of Neurology, Johns Hopkins Bayview Medical Center, Baltimore, MD 21224, USA.
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.