Although endurance exercise training promotes angiogenesis and improves metabolic health, the effect of resistance training on this process is less well defined. We hypothesized that capillarization would increase proportionally, and concurrently, with muscle fiber hypertrophy in response to resistance training in young men.
Methods: In this double-blind, randomized control trial, 36 men (22 ± 1 yr) were randomized to placebo or protein supplementation, and participated in 12 wk of resistance training. Skeletal muscle biopsies were collected before and after 2, 4, 8, and 12 wk of training. Immunohistochemistry assessed fiber type-specific size and capillarization. Western blot and reverse transcription polymerase chain reaction assessed proteins involved in the molecular regulation of angiogenesis.
Results: Resistance training effectively increased Type I (15% ± 4%; P < 0.01) and Type II fiber cross-sectional area (28% ± 5%; P < 0.0001), an effect that tended to be further enhanced with protein supplementation in Type II fibers (P = 0.078). Capillary-to-fiber ratio significantly increased in Type I (P = 0.001) and II (P = 0.015) fibers after 12 wk of resistance exercise training and was evident after only 2 wk. Capillary-to-fiber perimeter exchange index increased in the Type I fibers only (P = 0.054) after 12 wk of training. Training resulted in a reduction in vascular endothelial growth factor mRNA. A (P = 0.008), while vascular endothelial growth factor receptor 2 (P = 0.016), hypoxia-inducible factor 1α (P = 0.016), and endothelial nitric oxide synthase (P = 0.01) increased in both groups. Hypoxia-inducible factor 1α protein content was higher in the protein group (main group effect, P = 0.02), and endothelial nitric oxide synthase content demonstrated a divergent relationship (time-group interaction, P = 0.049).
Conclusions: This study presents novel evidence that microvascular adaptations and the molecular pathways involved are elevated after 2 wk of a 12-wk resistance training program. Increases in muscle fiber cross-sectional area are effectively matched by the changes in the microvasculature, providing further support for resistance training programs to optimize metabolic health.