All are the same source of strength. The body houses and empowers the mind and spirit. The mind gives form and directs the body and spirit. The spirit gives purpose to the body and mind. Neglecting any single aspect robs you of any strength you might seek. If I crack a stone wall with my fist, it takes not simply the strength of my body, but the focus of my mind and the strength of my soul to do so. All things are one in the Force, all connected. You can't separate one from the other.
Are you terribly religious?