Myofiber Permeability and Force Production of Rat Muscles Following Eccentric Contractions: The Repeated Bout Effect Depends on the Interval

Past eccentric contraction (ECC)-induced muscle injury reduces the severity of symptoms of subsequent muscle injury;this phenomenon is known as a repeated bout effect (RBE). It has been reported that increases in the duration of the interval between the first and second bouts are linked to weakening of the RBE. However, the histology following the attenuation of the RBE remains unclear. We examined the sustained effects of the second bout with regard to myofiber permeability and muscle force. Sixty-four male rats were randomly assigned to eight groups that varied in the number of exercise sessions and the duration of the interval between the first and second bouts: the non-ECC (Control);the single-injury (Post 1st bout);groups that were allowed to recover for 1, 2, and 4 weeks after a single injury (Pre 2nd bout_1w, Pre 2nd bout_2w, and Pre 2nd bout_4w);and groups that were subjected to second injuries 1, 2, and 4 weeks after the first (Post 2nd bout_1w, Post 2nd bout_2w, and Post 2nd bout_4w). The tibialis anterior was electrically stimulated in each ECC group. Twenty-four hours before muscle sampling, Evans blue dye (EBD) (a marker of myofiber damage) was administered. The maximal isometric contraction tension was measured immediately before sampling. The number of EBD-positive (+) fibers was determined via histological analysis. An RBE was revealed by functional examination at the 1- and 2-week and histological examination at the 1-, 2-, and 4-week time points (P < 0.05). In terms of myofiber permeability, prolongation of the interval before the second bout weakened this effect (P < 0.05). Experiments with 1-, 2-, and 4-week intervals indicated that prolongation of the interval before the second bout weakened the RBE with regard to myofiber permeability.

Changes in the number of circulating CD34＋ cells after eccentric exercise of the elbow flexors in relation to muscle damage

Background： It has been reported that strenuous exercise increases the number of bone marrow-derived progenitor cells such as CD34＋ cells in the blood, but no previous studies have investigated the changes in circulating CD34＋ cells following resistance exercise. This study tested the hypothesis that the number of CD34＋ cells in the blood would increase after eccentric exercise of the elbow flexors, but decrease in recovery, and the magnitude of the changes would be dependent on the magnitude of muscle damage. Methods： Nine men （28.0 ± 6.6 years） performed exercises consisting of 10 sets of six maximal voluntary eccentric contractions of the elbow flexors with their non-dominant arm. Six of them performed the same exercise with the same ann 4 weeks later. Changes in indirect markers of muscle damage were measured before, within 10 min after, and at 24, 48, 72, and 96 h after eccentric exercise. Differential leukocyte counts （total leukocytes, neutrophils, lymphocytes, monocytes） and CD34＋ cells in the blood were measured before, immediately after, and at 2, 24, 48, 72, and 96 h following the exercises. Results： After eccentric exercise, significant （p 〈 0.05） decreases in maximal voluntary isometric contraction torque and increases in delayed onset muscle soreness and plasma creatine kinase activity were observed. However, no significant changes in leukocytes and CD34＋ cells were evident. The changes in muscle damage markers were significantly （p 〈 0.05） smaller following the second exercise session as compared with the first exercise session, but the changes in leukocytes and CD34＋ cells were not significantly different between sessions. Conclusion： These results did not support the hypothesis, and showed that eccentric exercise-induced muscle damage to the elbow flexors did not influence the number of circulating CD34＋ cells.

Effect of repeated eccentric exercise on muscle damage markers and motor unit control strategies in arm and hand muscle

To examine the contralateral repeated bout effect(CL-RBE)on muscle damage markers and motor unit(MU)control strategies,seventeen healthy adults performed two bouts of 60 eccentric contractions with elbow flexor(EF group;n=9)or index finger abductor(IA group;n=8)muscles,separated by 1 week.All participants randomly performed eccentric exercise on either the right or left arm or hand muscles,and muscle damage markers and submaximal trapezoid contraction tests were conducted pre,post,1-and 2-day post eccentric protocol.One week after the first bout,the same exercise protocol and measurements were performed on the contralateral muscles.Surface electromyographic(EMG)signals were collected from biceps brachii(BB)or first dorsal interosseous(FDI)during maximal and submaximal tests.The linear regression analyses were used to examine MU recruitment threshold versus mean firing rate and recruitment threshold versus derecruitment threshold relationships.EMG amplitude from BB(bout 1 vs.bout 2=65.71%±22.92%vs.43.05%±18.97%,p=0.015,d=1.077)and the y-intercept(group merged)from the MU recruitment threshold versus derecruitment threshold relationship(bout 1 vs.bout 2=7.10±14.20 vs.0.73±16.24,p=0.029,d=0.513)at 50%MVIC were significantly different between two bouts.However,other muscle damage markers did not show any CL-RBE in both muscle groups.Therefore,despite changes in muscle excitation and MU firing behavior,our results do not support the existence of CL-RBE on BB and FDI muscles.