If we walk in the light as He is in the light, we have fellowship with one another. The theory is that a good workout puts our body in a condition similar to what happens at the onset of a fever. That's not all bad. A fever is the body's way of fighting back when micro-intruders get into our system. Increased body temperature aids our white blood-cell defense system while slowing down the action of bacteria and viruses.
Exercise does the same thing.
It releases chemicals into the blood that stimulate the brain to make our temperature rise. The first two chapters of 1 John indicate that a regular practice of good spiritual exercise is beneficial to the health of our soul. Disobedience, however, cools our spiritual temperature. Fellowship with God and other believers is neglected. Sin is neither confessed nor forsaken.