Capillarisation and fibre types in hypertrophied m. plantaris in rats of various ages.

Influences of age, overload obtained through denervation of synergists, and training on the capillarisation of the m. plantaris were compared in 5-, 13- and 25-month-old rats in relation to different fibre types. Overload resulted in about 30% hypertrophy in each age group. Age effects were significant only in the deep (more oxidative) region of the muscle. From 5 to 13 months, the percentage of FOG fibres increased at the expense of FG fibres, while the fibre cross-sectional areas (FCSA) of each fibre type increased. From 13 to 25 months, the FCSA of FG fibres decreased, as did the local capillary-to-fibre ratio (LCFR) of each fibre type, indicating capillary loss and a declined capillary density for each fibre type (CFD). Overload effects were identical for both the superficial (more glycolytic) and the deep region for each age group. With overload, FCSA and LCFR of each fibre type increased, while CFD decreased, indicating that capillary proliferation occurred with overload, even at old age, although lagging behind increases in FCSAs. Training showed minor effects.