Isometric training improves fatigue resistance in dystrophin-deficient muscle.

Eccentric contractions, in which the muscle is stretched during contraction, cause substantially greater damage than isometric (ISO) contractions, in which the length of the muscle does not change during contraction. Here, we tested the hypothesis that ISO training improves fatigue resistance in skeletal muscle from dystrophin-deficient mdx52 mice (15-22 wk old). ISO training (100 Hz stimulation frequency, 0.25-s contractions every 0.5 s, 6 sets of 60 contractions) was performed on the left plantar flexor muscles in vivo with supramaximal electrical stimulation every other day for 4 wk. Compared with the normal control muscle, resistance to fatigue was reduced in the nontrained muscle from mdx52 mice, which was accompanied by a reduction in citrate synthase activity and the LC3BII/I ratio and an increase in the phosphorylation levels of Akt Ser473 and the expression levels of p62. ISO training restored these alterations and markedly increased in vivo fatigue resistance and PGC-1α expression in mdx52 muscles. Moreover, an increased number of Evans Blue dye-positive fibers was significantly reduced by ISO training in mdx52 muscles. In contrast, ISO training did not restore a reduction in the amount of SH3 and cysteine-rich domain 3 in mdx muscles. Thus, our data suggest that mitochondrial function is impaired in dystrophin-deficient muscles, which is likely to be induced by the defective autophagy due to persistent activation of Akt. ISO training inhibits the aberrant activation of Akt presumably by up-regulating the PGC-1α expression, which results in improved mitochondrial function and thus fatigue resistance in dystrophin-deficient muscles.