Fatigue effects on motor unit activity during submaximal contractions.
OBJECTIVE
To examine motor unit changes during the development of fatigue in healthy subjects.


METHODS
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).


METHODS
Academic outpatient neuromuscular clinic.


METHODS
Healthy laboratory personnel.


METHODS
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.


CONCLUSIONS
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.
Conwit
R A
RA
Department of Neurology, Johns Hopkins Bayview Medical Center, Baltimore, MD 21224, USA.
Stashuk
D
D
Suzuki
H
H
Lynch
N
N
Schrager
M
M
Metter
E J
EJ
D000200:Action Potentials; D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D005221:Fatigue / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*