Effect of resistance training on muscle fatigue and recovery in intact rats

Purpose: To examine the effect of resistance training on muscle fatigue fro m intermittent contractions and subsequent recovery in intact rats. Methods : By using electrical stimulation, plantar flexor muscles were trained with eccentric and concentric contractions (5 x 10 repetitions, 5 d(.)wk(-1) fo r 6 wk) during ankle rotations. By using nerve stimulation, concentric cont ractions (40) imposed on isometric contractions (stimulation time, 1.9 s; r est period, 13.6 s; intermittent contractions) induced fatigue. During reco very, equivalent contractions were used every 5 min for 30 min. Results: Tr aining increased isometric forces (19% and 23% at ankle positions of 1.57 a nd 0.70 rad), but muscle weights were not changed. After training, smaller declines in isometric (control, 68.9 +/- 1.4%; trained, 58.8 +/- 2.9%) and average concentric force (control, 71.6 +/- 0.7%: trained, 65.5 +/- 2.8%) o ccurred from fatigue. Recovery for 5 min returned isometric and average con centric force to 61.7 +/- 2.2% and 65.1 +/- 2.5% of initial values for cont rols and 76.9 +/- 2.2% and 77.1 +/- 2.2% after training. After recovery for 30 min, these forces were 87.6 +/- 0.7% and 89.2 +/- 1.1% of initial value s for controls and recovered almost completely (94.2 +/- 1.3% and 94.6 +/- 1.6%) in trained muscles. During fatigue, the decline in force during succe ssive concentric contractions was larger after training (from 19.7 +/- 1.1% to 50.1 +/- 2.0%; controls, from 19.9 +/- 2.0% to 41.7 +/- 1.4%). Recovery of this decline in force was training-independent and complete within 5 mi n. Conclusions: Rat plantar flexor muscles adapt to 6 wk of 5 d(.)wk(-1) re sistance training with: 1) increased isometric force, 2) smaller losses in isometric and average concentric force during fatigue, 3) larger force decl ine during concentric contractions during fatigue, and 4) improved recovery following fatigue. Different mechanisms might account for the recovery of the average concentric force and the. decline in force during concentric co ntractions.