[coophitter]

To all concerned: I've been doing a little research on the way muscles work a' la extensor action especially since I was wrong about YodasLuke's example of keeping a leaning man a constant distance from you with your bent right arm. In this example, the triceps muscle fibers would maintain the same length as tension develops. The force of contraction herein takes place within the fibers, but the fibers themselves do not move in relation to each other, so the overall muscle remains the same length. This type of force development by muscles is termed isometric.

If you were to let the man come closer to you but controlled his speed so he didn't crash into you, like my example of bringing a benchpressed barbell slowly back to your chest, then the triceps muscle fibers would be lengthening as they developed tension. This type of force development by a muscle is termed eccentric.

If you pushed the man away from you, your triceps muscle fibers would shorten or contract. This is the most familiar type of force development by a muscle, and it is termed concentric.

Muscles cannot develop a force concentrically, isometrically, or eccentrically at the same time. Muscles can only develop a force one way at a time, so if and when the triceps fibers shorten to move the left arm, extensor action (which supposedly does not move the left arm) can no longer be in effect. The triceps or any other muscle just isn't talented enough to develop dual or treble forces wherein the same fibers would have to independently and simultaneously contract, lengthen, and/or remain the same length.

Coophitter