Muscle fatigue in frog semitendinosus: Alterations in contractile function

The purpose of this study was to characterize the contractile properties of the frog semitendinosus (ST) muscle before and during recovery from fatigue, to relate the observed functional changes to alterations in specific steps in the cross-bridge model of muscle contraction, and to determine how fatigue affects the force-frequency relationship. The frog ST (22°C) was fatigued by direct electrical stimulation with 100-ms 150-Hz trains at 1/s for 5 min. The fatigue protocol reduced peak twitch (P(t)) and tetanic (P(o)) force to 32 and 8.5% of initial force, respectively. The decline in P(t) was less than P(o), in part due to a prolongation in the isometric contraction time (CT), which increased to 300% of the initial value. The isometric twitch duration was greatly prolonged as reflected by the lengthened CT and the 800% increase in the one-half relaxation time ( 1/2 RT). Both P(t) and P(o) showed a biphasic recovery, a rapid initial phase (2 min) followed by a slower (40 min) return to the prefatigue force. CT and 1/2 RT also recovered in two phases, returning to 160 and 265% of control in the first 5 min. CT returned to the prefatigue value between 35 and 40 min, whereas even at 60 min 1/2 RT was 133% of control. The maximal velocity of shortening, determined by the slack test, was significantly reduced [from 6.7 ± 0.5 to 2.5 ± 0.4 optimal muscle length/s] at fatigue. The force-frequency relationship was shifted to the left, so that optimal frequency for generating P(o) was reduced. In conclusion, the biphasic recovery of P(t) and P(o) suggests that fatigue was caused by multiple factors with varying recovery times. The fast phase probably mirrors a rapid change in one or more of the steps of excitation- contraction coupling, whereas the slow phase may be mediated by cellular recovery from contraction-induced increases in H⁺ and inorganic phosphate.