Can increases in capillarization explain the early adaptations in metabolic regulation in human muscle to short-term training?

To investigate the hypothesis that increases in fibre capillary density would precede increases in oxidative potential following training onset, tissue was extracted from the vastus lateralis prior to (0 days) and following 3 and 6 consecutive days of submaximal cycle exercise (2 h·day^–1). Participants were untrained males (age = 21.4 ± 0.58 years; peak oxygen consumption = 46.2 ± 1.6 mL·kg^–1·min^–1; mean ± standard error (SE)). Tissue was assessed for succinic dehydrogenase activity (SDH) by microphotometry and indices of capillarization based on histochemically assessed area and capillary counts (CC) in specific fibre types. Three days of training (n = 13) resulted in a generalized decrease (p < 0.05) in fibre area (–14.2% ± 3.0%; mean ± SE) and increase (p < 0.05) in CC/Area (20.4% ± 2.7%) and no change in either CC or SDH activity. Following 6 days of treatment (n = 6), increases (p < 0.05) in CC (18.2% ± 4.2%), CC/Area (28.9% ± 3.2%), and SDH activity (22.9% ± 6.0%) occurred that was not specific to major fibre type. No changes in either fibre area or fibre-type distribution were observed with additional training. We conclude that increases in angiogenic-based capillary density and oxidative potential occur coincidentally following training onset, while increases in capillary density, mediated by reductions in fibre area, represent an initial isolated response, the significance of which may be linked to the metabolic alterations that also result.