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MUSCLE CONTRACTION CLASSIFICATION
Although a muscle contracts in one manner—it shortens and lengthens—there are three
classifications of muscle contraction. An individual can train using one or more of these
classifications in order to improve strength and muscular endurance fitness. These
classifications are isotonic, isokinetic and isometric conditioning.
Isotonic
Isotonic movement is a contraction that results in free, dynamic movement at a joint in
the body. Isotonic training uses the more traditional resistance training equipment, such
as barbells, dumbbells, pulleys, resistance machines, and heavy calisthenics such as situps, push-ups, squats, etc. Isotonic movement occurs in two ways:
Concentric Contraction
A concentric contraction occurs when the resistance or weight is less than the force
exerted by a muscle or muscle group. This causes the muscle or muscle group to shorten.
When an individual performs a biceps curl, pulling the weight toward the body, the biceps
shorten—a concentric contraction.
Eccentric Contraction
The muscle is also capable of exerting tension while lengthening, known as eccentric
contraction. This typically occurs when the muscle produces a braking force to slow down
the movement produced by gravity (such as when lowering the weight during a biceps
curl). It also occurs when the resistance is greater than the force exerted by the
muscle or muscle group. On a now-canceled American television series, a couple of the
show’s characters, “Brian” and “Joe” were in the weight room. Brian decided to use the
leg press machine. After Brian had just performed the concentric portion of his
movement—the quadriceps shortened to extend his legs—Joe loaded so much weight onto
the machine that Brian could no longer hold his legs in an extended position. Brian’s legs
started to bend—his quadriceps lengthened—an eccentric movement. Because the body
can handle more weight eccentrically, some individuals use eccentric movements to
improve their performances.
Isokinetic
Although isokinetic conditioning, like isotonic training, results in dynamic movement at a
joint, isokinetic machinery regulates the movement so that, no matter how much force is
used, the speed of the exercise remains the same. In joint motion, muscles involved in
the movement are stronger at certain angles and weaker at others. In isokinetic
exercise, the resistance of the machine adjusts as needed throughout the movement to
ensure that the speed remains constant. Because isokinetic machinery accommodates the
user, it is often used in rehabilitative settings. Isokinetic training generally requires
special equipment.
Isometric
In isometric training, the muscle contracts against an immovable resistance. An example
of this is pushing against a wall—although the muscle exerts a force, it does not cause
the limb to move. An advantage to isometric training is that large gains in muscular
strength can occur. The disadvantage, however, is that the strength gain is specific to
the angle at which the exercise is performed, so several angles should be used for each
joint when performing isometric training.