Effects of acute eccentric exercise stimulus on muscle injury and adaptation

The purpose of this study was to investigate the stimulus of acute eccentric aerobic exercise that would elicit minimal muscle injury but adequate to induce muscle tissue adaptation. Twenty healthy subjects were randomized into two groups: (a) the low stimulus eccentric exercise group (LSEEG); and (b) the high stimulus eccentric exercise group (HSEEG). Both groups performed acute exercise (bout 1) and a repeated exercise (bout 2). In the acute bout, the LSEEG exercised at 50% of heart rate reserve (HRR) for 30 min while the HSEEG exercised at 70% of HRR for 40 min on a treadmill declined at a gradient of 15°. Two weeks after the acute exercise for both groups, the subjects performed a repeated exercise bout at 80% of HRR for 40 min. Creatine kinase (CK) and lactate dehydrogenase (LDH), total white blood cells (TWBC), and perceived muscle soreness (SOR) before and 1, 24, and 48 hrs post-exercise were assessed as markers of muscle injury and adaption. The results showed that muscle injury was significantly higher in the HSEEG than in the LSEEG in the acute exercises. However, both exercise groups developed similar muscle adaptations with no significant differences in attenuations in CK, LDH, and SOR in the repeated bout. Exercise at 50% of HRR for 30 min may be a threshold stimulus for acute eccentric aerobic exercise.