What physiological change is associated with endurance training?

Endurance training leads to several adaptations within the cardiovascular system, one of the most significant being an increase in the number of capillaries in the muscles. This adaptation enhances the muscle's ability to utilize oxygen during prolonged physical activities. Increased capillary density allows for improved blood flow and efficient exchange of gases, nutrients, and waste products between blood and muscle tissues.

With more capillaries, athletes can enhance their stamina and performance as their muscles receive a greater supply of oxygen and nutrients required for sustained aerobic activity. This adaptation is crucial for endurance athletes, as it directly contributes to their ability to maintain prolonged periods of exercise at higher intensities without fatigue.

Other options do not accurately reflect the physiological changes commonly seen with endurance training. For instance, while heart rate may reduce at rest due to improved cardiovascular efficiency, it is not exclusively considered a primary training adaptation like capillary growth. Decreasing muscle mass is adverse and generally not a goal of endurance training, while increasing blood viscosity could lead to negative effects on circulation and is not a typical training response.