As one ages, the body reverses our thinking. When we are young the needs of the body dominate. As a baby, when you're hungry you cry, when you have to go to the bathroom, you just do it, hence, diapers. Later, into midlife and more, we are taught control over the body, with delayed gratification, and discipline. The mind takes over, and we think we can will ourselves around, over and above various situations. In the final stage of life, once again, to our surprise, things are reversed and it's matter over mind.
Injuries, illnesses, energy, time, and our mental and emotional needs dictate more than we would like to admit. You can't bend those knees on the tennis court if asthma kept you up the night before, or for that matter walk those three miles in the morning with even just a hairline fracture in your toe. Our body and needs once again humble us.