Does having more muscle capillaries automatically improve endurance?

Understanding the question

A well-developed capillary network can improve the conditions for oxygen and nutrient exchange, but endurance is not determined by capillary number alone. Blood flow, red-cell transit, oxygen content, diffusion distance and the muscle’s ability to use oxygen all influence the result. Exercise also changes which parts of the microcirculation carry blood and how flow is distributed. Capillary density can change because fibers change size, even if the number of capillaries does not. Research should therefore distinguish structural vascular adaptation from acute perfusion and from actual performance in a defined endurance task.

What a useful investigation needs to consider

Capillaries per area and capillaries relative to fibers are different measurements. Hypertrophy can lower a density value by increasing the denominator, so a reduced density does not automatically establish that vessels were lost.

Whole-body maximal oxygen uptake includes cardiac output and oxygen extraction. An improvement in that test does not identify the muscle microcirculation as the sole cause, and a local vascular change may not produce an equally large whole-body change.

Read the detailed explanation

The companion article explores muscle capillaries and the delivery of oxygen to working fibers in more depth, with topic-specific explanations and source material.