Fatigue effects on muscle excitability

Repeated isometric or shortening contractions of skeletal muscle cause muscle fatigue but several prior studies have reported an apparent absence of muscle fatigue when humans performed up to 70 lengthening contractions. We pursued the hypothesis that perhaps muscle excitability is a factor that aids force preservation with repeated eccentric actions, Soleus compound muscle action potential (M-wave) latency, peak-to-peak amplitude (PPA), duration, and area under the curve were examined in 12 subjects (mean age 24.3 y) over 4 testing days that included: no exercise, isometric exercise (neutral ankle angle), isokinetic (0.5 rad . s(-1)) concentric and eccentric exercise of the plantar flexors in the seated position on a Biodex dynamometer. Supramaximal shocks were delivered to the tibial nerve in the popliteal fossa at baseline (3 shocks, 1 min apart), during exercise(1 shock-after each of 5 bouts/10 contractions), and during 10-min recovery. From initial to final contractions, concentric, isometric, and eccentric fatigue was -32, -41 and +2% (Condition by Trial interaction, F-2,F-22 = 25.1, p = 0.000). No changes occurred in latency or duration (p > 0.05), but PPA (Condition by Time interaction, F-51,F-561 = 3.7, p = 0.000) increased during isometric and eccentric exercise and remained elevated during recovery. Area increased (F-51,F-561 = 3.1, p = 0.000) significantly during all three exercise conditions and approximated baseline by minute 8 of recovery, It was concluded that although the potentiation of the action potential of individual muscle fibers seems to be the common mechanism underlying the increase in muscle excitability during plantar flexion exercise, it is possible that different factors could cause such a non-specific response.