Orson Scott Card said this quote

It was not about physical strength, Wit reminded himself. It was 90 percent mental, 10 percent physical. That's what the SEAL instructors were looking for: men and women who could disregard the pleadings of the body. Pain was nothing, sleep was nothing. What was chaffed skin, wrecked muscles, bleeding sores? The body chooses to be sore. The body chooses to be exhausted. But the SEAL mind rejects it. The SEAL mind commands the body, not the other way around.