Repeated isometric or shortening contractions of skeletal muscle cause
muscle fatigue but several prior studies have reported an apparent ab
sence of muscle fatigue when humans performed up to 70 lengthening con
tractions. We pursued the hypothesis that perhaps muscle excitability
is a factor that aids force preservation with repeated eccentric actio
ns, 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: n
o 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 we
re 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/1
0 contractions), and during 10-min recovery. From initial to final con
tractions, concentric, isometric, and eccentric fatigue was -32, -41 a
nd +2% (Condition by Trial interaction, F-2,F-22 = 25.1, p = 0.000). N
o changes occurred in latency or duration (p > 0.05), but PPA (Conditi
on 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 al
l three exercise conditions and approximated baseline by minute 8 of r
ecovery, It was concluded that although the potentiation of the action
potential of individual muscle fibers seems to be the common mechanis
m underlying the increase in muscle excitability during plantar flexio
n exercise, it is possible that different factors could cause such a n
on-specific response.