Neuromuscular fatigue during repetitive stimulation in elderly and young adults

The purpose of this investigation was to examine the integrity of neuromuscular transmission and impulse propagation during fatigue by examining the muscle compound action potential (M wave) in elderly and young adults. The tibialis anterior muscle of nine elderly [{ie567-1}=67.7 (SE 1.7) years] and nine young [{ie567-2}=26.7 (SE 1.2) years] adults was maximally stimulated repetitively at frequencies of 20, 30 or 40 Hz for 60 s on separate occasions. There was a significantly smaller resting M wave amplitude [7.9 (SE 0.4) mV versus 9.9 (SE 0.6) mV] and M wave area [0.038 (SE 0.005) mV s versus 0.06 (SE 0.004) mV · s] in the elderly versus the young adults respectively. Measurement of the evoked muscle contractile properties revealed significantly (P<0.05) longer twitch durations and a significantly (P<0.05) greater peak twitch torque [4.6 (SE 0.4) Nm versus 3.2 (SE 0.5) Nm] in the elderly versus the young adults, respectively. The elderly adults had a significantly greater torque decline during the 20-Hz trial; however, the decline in torque during the 30-Hz and 40-Hz trials was similar in the elderly and the young adults (30 Hz: 40%; 40 Hz: 56%). Throughout each of the stimulation trials, the decline in torque was accompanied by a significant reduction in M wave amplitude (20 Hz: 14%; 30 Hz: 53%; 40 Hz: 67%); M wave area also declined significantly during the 30-Hz (31%) and 40-Hz (53%) trials. There was no significant difference between the elderly and the young adults in the reduction in the M wave amplitude or area during each trial. The similar electrical responses of the tibialis anterior between the elderly and the young adults suggest that any age-associated changes that occur in the neuromuscular apparatus do not appear to affect the integrity of neuromuscular propagation of the development of fatigue during repetitive stimulation at these frequencies.