Skeletal muscle hypertrophy and structure and function of skeletal muscle fibres in male body builders

Needle biopsy samples were taken from vastus lateralis muscle (VL) of five male body builders (BB, age 27.4 ± 0.93 years; mean ± S.E.M.), who had being performing hypertrophic heavy resistance exercise (HHRE) for at least 2 years, and from five male active, but untrained control subjects (CTRL, age 29.9 ± 2.01 years). The following determinations were performed: anatomical cross-sectional area and volumeof the quadriceps andVLmuscles in vivo bymagnetic resonance imaging (MRI);myosin heavy chain isoform(MHC) distribution of the whole biopsy samples by SDS-PAGE; cross-sectional area (CSA), force (Po), specific force (Po/CSA) and maximum shortening velocity (Vo) of a large population (n = 524) of single skinned muscle fibres classified on the basis of MHC isoform composition by SDS-PAGE; actin sliding velocity (Vf) onpuremyosin isoforms by in vitro motility assays. In BB a preferential hypertrophy of fast and especially type 2X fibres was observed. The very large hypertrophy of VL in vivo could not be fully accounted for by single muscle fibre hypertrophy. CSA of VL in vivo was, in fact, 54% larger in BB than in CTRL, whereas mean fibre area was only 14% larger in BB than in CTRL. MHC isoform distribution was shifted towards 2X fibres in BB. Po/CSA was significantly lower in type 1 fibres from BB than in type 1 fibres from CTRL whereas both type 2A and type 2X fibres were significantly stronger in BB than in CTRL. Vo of type 1 fibres and Vf of myosin 1 were significantly lower in BB than in CTRL, whereas no difference was observed among fast fibres and myosin 2A. The findings indicate that skeletal muscle of BB was markedly adapted to HHREthrough extreme hypertrophy, a shift towards the stronger and more powerful fibre types and an increase in specific force of muscle fibres. Such adaptations could not be fully accounted for bywell known mechanisms ofmuscle plasticity, i.e. by the hypertrophy of singlemuscle fibre quantitative mechanism) and by a regulation of contractile properties of muscle fibres based on MHC isoform content (qualitative mechanism). Two BB subjects took anabolic steroids and three BB subjects did not. The former BB differed from the latter BB mostly for the size of their muscles and muscle fibres