This study was conducted to quantify and compare the extent of fibre degenerative and regenerative processes in different muscles of the rat hindlimb following single or repeated daily bouts of treadmill exercise. Wistar rats were used as non-exercised controls, or subjected to one, five, or ten (n = 8 per group), 30-minute daily bouts (-16 degrees, 12-15m.min-1) of downhill exercise. Soleus (S), vastus lateralis (VL), medial gastrocnemius (MG), plantaris (P), and tibialis anterior (TA) muscles were analyzed from transverse cryosections stained with either H&E for morphological alterations indicative of fibre degeneration or regeneration, or mATPase activity for determination of fibre type. Results showed that in all groups, the percentage of morphologically altered fibres (%AF) was greater in S (4-8%) than in MG, VL, P, or TA (1-2%). The %AF across all muscles was greater following only one, versus multiple exercise bouts, or versus no exercise. The proportions of AF of different histochemical types followed the same distribution as the fibre type in the muscle area examined. These direct assessments indicate that the extent of fibre degenerative and regenerative processes varies among the different muscles involved, and is greater following a single bout, compared to repeated daily bouts of exercise.

Resting and submaximal isometric exercise 31P magnetic resonance spectroscopy (MRS) was carried out on 7 endurance-trained males (26.0 +/- 3 yrs) and 7 sedentary males (27.0 +/- 4 yrs). Spectral analysis provided peak areas of phosphocreatine (PCr), inorganic phosphate (Pi), adenosine triphosphate (ATP), and the chemical shift of Pi relative to PCr. The ratio of PCr/Pi was moderately lower during rest (preexercise p = .13, postexercise p = .18), and significantly higher during exercise (p < .05) in the trained subjects. Intracellular pH patterns were the same for both groups; a transient alkalosis was observed at the onset of exercise with a return to resting levels after 2 min. Differences suggest improved ATP resynthesis rate in the trained subjects during exercise. Intracellular pH changes can be attributed to the utilization of hydrogen ions that accompany PCr hydrolysis during work. The findings are congruent with previous reports indicating a superior oxidative capacity in trained skeletal muscle.