Fibre type-specific satellite cell response to aerobic training in sedentary adults

In the present study, we sought to determine the effect of a traditional, 12week aerobic training protocol on skeletal muscle fibre type distribution and satellite cell content in sedentary subjects. Muscle biopsies were obtained from the vastus lateralis [n=23 subjects (six male and 17 female); body mass index 30.7 +/- 1.2kgm-2] before and after 12weeks of aerobic training performed on a cycle ergometer. Immunohistochemical analyses were used to quantify myosin heavy chain (MyHC) isoform expression, cross-sectional area and satellite cell and myonuclear content. Following training, a decrease in MyHC hybrid typeIIa/IIx fibre frequency occurred, with a concomitant increase in pure MyHC typeIIa fibres. Pretraining fibre type correlated with body mass index, and the change in fibre type following training was associated with improvements in maximal oxygen consumption. Twelve weeks of aerobic training also induced increases in mean cross-sectional area in both MyHC typeI and typeIIa fibres. Satellite cell content was also increased following training, specifically in MyHC typeI fibres, with no change in the number of satellite cells associated with MyHC typeII fibres. With the increased satellite cell content following training, an increase in myonuclear number per fibre also occurred in MyHC typeI fibres. Hypertrophy of MyHC typeII fibres occurred without detectable myonuclear addition, suggesting that the mechanisms underlying growth in fast and slow fibres differ. These data provide intriguing evidence for a fibre type-specific role of satellite cells in muscle adaptation following aerobic training.