The increased capacity of the muscle to extract oxygen following training is primarily due to a(n) _____.

• A. Decrease In Capillary Density
• B. Increase In Capillary Density
• C. Increase In Myofibrillar Density
• D. Increase In Muscle Fiber Size

Answer: (B)

More capillaries around the muscle fibers from training increases the muscle’s ability to extract oxygen. When capillary density rises, there is more surface area for oxygen to diffuse into the muscle and the distance O2 has to travel from blood to mitochondria is shortened. This makes it easier for the muscle to pull oxygen from the blood during exercise, increasing the arteriovenous oxygen difference. This adaptation is a hallmark of endurance training via angiogenesis. Other changes, like more contractile proteins or larger muscle fibers, don’t directly improve diffusion of oxygen and can even lengthen the diffusion path if capillaries don’t keep pace.