MUSCLE ACTION
Muscles generally develop forces sufficient to maintain a rotational equilibrium around a joint.
Obviously, joints do not remain in equilibrium indefinitely; if they did, joints would never move, or at least never change direction. Still, to make approximate but meaningful calculations, we assume that the forces around joints are close to rotational equilibrium. We assume, in other words, that the moments produced by gravity and the muscles are equal or nearly equal and opposite.
These assumptions produce three possible situations:
- The opposing moments are exactly equal.
Mm = Mg
The joint does not move so the muscle's length remains constant; this is isometric muscle action.
- The muscles produce a moment that exceeds the moment which gravity produces:
Mm > Mg
In this case, the joint moves as the muscle shortens in a concentric action.
- The muscle produces a moment that opposes the effect of gravity, but is nevertheless smaller than the moment that gravity produces:
Mm < Mg
In this case, the joint moves in the direction dictated by gravity's moment. While the muscle's activity exerts a force on its attachments, and controls gravity's effect on the joint, the muscle still elongates. This is eccentric muscle action.
Note that the same muscle is active regardless of direction of joint motion; muscle develops a force in a direction which approximates its attachments, even if those attachments are moving apart.
We prefer to speak of muscle action, not contraction, since the term contraction might imply to some that a muscle can only develop force when it is shortening.
ISOTONIC AND ISOKINETIC EXERCISE
We reserve these terms for exercises or exercise equipment which produce the following:
- Isotonic: constant tension or force
- Isokinetic: constant joint angular velocity