AGE‐RELATED EFFECTS OF FATIGUE AND RECOVERY FROM FATIGUE IN RAT MEDIAL GASTROCNEMIUS MUSCLE

Force‐velocity, power‐velocity and unloaded shortening data were obtained from in situ medial gastrocnemius muscle‐tendon complexes (stimulated at 60 Hz) with intact circulation of mature male rats (∼ 125 days old). Measurements were carried out at the end of a long (15 s) contraction (fatigued muscles) or with a short (1 s) contraction either in the fresh state (fresh muscles) or in muscles which had recovered for 15 min after a long contraction. Compared to the fresh state fatigue reduced isometric force by 57%, maximal shortening velocity by ∼ 40% and maximal power output by 81%. These reductions were similar to data previously obtained with younger rats (40 days old). However, the velocity data of the muscles which had recovered for 15 min after a long contraction showed a greater reduction in the mature rats. This difference between the two age groups together with a difference in the changes in the initial parts of the isometric force‐time curves suggest an age‐dependent response of the fast‐fatigable fibre population of these mixed muscles. In a separate series of experiments the underlying mechanism of the recovery from fatigue was studied in a group of young rats. Fatigue was induced with five long (15 s) contractions (each at 5 min intervals). The recovery of isometric force and power output was monitored with short contractions which indicated a plateau of recovery but the absolute values were still reduced after 60 min (85 and 71% of pre‐fatigue values, respectively). Phosphocreatine concentration recovered rapidly, whereas the ATP concentration was still markedly reduced after 1 h of recovery. The time courses of recovery of inosine‐5'‐monophosphate (IMP) and lactate concentrations resembled those of force and power output. Thus it is possible that age‐dependent differences in IMP and/or lactate production may play a role in fatigue and recovery from fatigue.