The other day, I walked up to my wife at the gym. She looked at me and said, "I don't know what these things are for, but they hurt like hell, so I keep doing them."

After I stopped laughing, I realized how close she had come to capturing the double-edged sword that is exercise.

For some people, the pain or discomfort is a sign that something good must be happening. For them, the end result is worth enduring the pain. For other people, the pain runs totally counter to their first and foremost value in life: comfort. So they stop.

They would rather be comfortable and out of shape than briefly uncomfortable and in shape. Notice, I said uncomfortable. Pain is not the objective. If you feel pain, something is wrong.

You might have injured the muscle in a prior workout. You might be hurting it at the moment, perhaps by using too much weight or overexerting yourself. The pain you feel during an exercise is your body telling you that its muscles and tendons have not yet adapted to the exertion.

One of the body's amazing built-in safeguards is a link between the nervous system and the muscle tendons. It's called the Golgi tendon organ, and it protects the tendon against too much force from muscle contraction.

It's possible, of course, to ignore this pain. Under extreme stress, in dire emergencies, with the adrenalin pumping and someone's life on the line, people have been known to push their bodies to Herculean performance.

But later, they pay for it. Weight trainers who push past the pain threshold of the Golgi run a likely risk of tearing a tendon. Quadriceps and biceps tendons are particularly vulnerable and frequent sites of overstress injuries.