One of the limiting factors in human physical performance is the amount of oxygen that is absorbed by the muscles from the bloodstream. Accordingly, entrepreneurs have begun selling at gymnasiums and health clubs bottles of drinking water, labeled SuperOXY,  that has extra oxygen dissolved in the water. Such water would be useless in improving physical performance, however, since <b> the amount of oxygen in the blood of someone who is exercising is already more than the muscle cells can absorb </b>.
The statement "frequent physical exercise increases the body's ability to take in and use oxygen" would serve the same function in the argument as the statement in boldface.