Late depression of muscle excitability in humans after fatiguing stimulation.
1. Changes in muscle excitation and in isometric twitch force have been studied for up to 8 h after fatiguing stimulation of the human biceps brachii. 2. Within 10 s of a cessation of the 20 Hz fatiguing tetanus, the amplitudes of the M waves (muscle compound action potentials) had returned to control values, whereas the twitch forces were reduced in all subjects. The M waves then decreased in amplitude over the next 3 h, reaching a mean value that was 42.4 +/- 18.6% of control levels (means +/- S.E.M.; P < 0.001). 3. By 8 h, the mean M wave amplitude had recovered to 93.8 +/- 33.3% of control levels, while the corresponding mean twitch force was 104.1 +/- 36.9%. 4. The cellular mechanism responsible for the depression of the M wave is presently unknown, but it is likely to be postsynaptic and may involve Na+ channels.
McFadden
L K
LK
Department of Biomedical Sciences, McMaster University, Hamilton, Ontario, Canada.
McComas
A J
AJ
D000200:Action Potentials; D000328:Adult; D004558:Electric Stimulation; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D018763:Muscle Fatigue; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D015222:Sodium Channels / Q000502:physiology; D013997:Time Factors
D015222:Sodium Channels