INVOLVEMENT OF MEMBRANE EXCITATION FAILURE IN FATIGUE-INDUCED BY INTERMITTENT SUBMAXIMAL VOLUNTARY CONTRACTION OF THE 1ST DORSAL INTEROSSEOUS MUSCLE

Factors involving muscle fatigue were studied at intermittent isometri c contraction of the first dorsal interosseous muscle (FDI). Subjects made repeated contraction of 6 s, 40% of maximal voluntary contraction (40% MVC) (= target force) followed by 4 s rest until the target forc e could no longer be maintained. Measurement of MVC, recording of M wa ve evoked from the relaxed muscle, force measurement during tetanic el ectrical stimulation and measurement of serum K+ concentration were pe rformed every 2 min during the fatiguing exercise and after 10 min of recovery. The declining rate of MVC force was always smaller than that of the force generated by 50 Hz stimulation, suggesting that no centr al nervous system (CNS) failure was involved. The amplitude and the ar ea of the M wave declined while the duration of the wave increased, ac companied by the rise in serum K+ concentration. After 10 min of recov ery, all parameters except the MVC force were restored. Thus we conclu ded that the failure in muscle membrane excitation, as well as the fai lure in excitation/contraction coupling, is involved in the deteriorat ing force generating capacity of FDI during intermittent isometric con traction.