Final answer:
Higher capillary density in muscle is associated with higher aerobic capacity.
Step-by-step explanation:
Higher capillary density in muscle is associated with higher aerobic capacity. Capillaries are tiny blood vessels that deliver oxygen and nutrients to muscle tissue. Increased capillary density allows for more efficient oxygen and nutrient delivery, which can support higher levels of sustained aerobic activity. This is why endurance athletes, who rely on aerobic activity, tend to have a higher proportion of slow-twitch muscle fibers, which are dense with capillaries.