Training for skeletal muscle capillarization: a Janus-faced role of exercise intensity?

Excerpt

Muscle capillarization is central for the delivery of oxygen and nutrients to the exercising muscle, and thus, capillarization is vital for exercise capacity. A high muscle capillary density means a large muscle-to-blood exchange surface area, short oxygen diffusion distance, and high red blood cell mean transit time. After all, this is what you need for translating a high cardiac output to a great exercise performance. Exercise is the most potent stimulator of angiogenesis in skeletal muscles, and few weeks of exercise training lead to measureable increases in muscle capillarization. Untrained subjects may enjoy 10–30 % increase in capillarization after 6–8 weeks of exercise training, whereas elite aerobic athletes with years of progressive exercise training express a capillary-to-muscle fiber ratio of more than 200 % of that of untrained individuals (Ingjer 1979). During exercise, vascular endothelial growth factor (VEGF) is secreted from the muscle fibres to the muscle interstitium. VEGF is the single most important growth factor for expansion of the capillary bed and together with a number of other angioregulatory factors; VEGF levels determine the degree of capillary growth (Egginton 2009). …