Isokinetic, Eccentric, and Concentric Muscle Contractions There are three basic types of muscle contraction: isometric, isotonic, and isokinetic. An isometric contraction occurs when a muscle contracts, producing force without changing the length of muscle. To demonstrate this action, in the sitting position place your right hand under your thigh and place your left hand on your right biceps muscle. Now, pull up with your right hand or in other words, attempt to flex your right elbow. Note that there was no real motion at the elbow joint, but you did feel the muscle contract. This is an isometric contraction of your right biceps muscle. The muscle contracted, but no joint motion occurred. Next, hold a weight in your hand while flexing your elbow to bring the weight up toward your shoulder.
You will feel the biceps muscle contract, but this time there is joint motion. This is an isotonic contraction. An isotonic contraction occurs when a muscle contracts, the muscle length changes, and the joint angle changes. Occasionally you will read a text that describes an isometric contraction as a static or tonic contraction and an isotonic contraction as phasic. While these terms mean essentially the same thing, they have fallen into disuse, and specific differences among these terms seem no longer relevant. An isotonic contraction can be subdivided into concentric and eccentric contractions.
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A concentric contraction occurs when there is joint movement, the muscles shorten, and the muscle attachments (Origin and Insertion) move toward each other. Picking up the weight, as described earlier, is an example of a concentric contraction of the biceps muscle. If you continue to palpate the biceps muscle while setting the weight back down on the table, you will feel that the biceps muscle (not the triceps muscle) continues to contract, even though the joint motion is elbow extension. What is occurring is an eccentric contraction of the biceps muscle. An eccentric contraction occurs when there is joint motion but the muscle appears to lengthen, that is, the muscle attachments separate. Realize that after bringing the weight up to shoulder level, if you relaxed your biceps muscle, your hand and weight would drop to the table.
If you used your triceps muscle, (which would extend the elbow concentrically), your hand and weight would fall onto the tabletop with some force and speed. However, what you did by slowly returning the weight to the tabletop was to slow down the pull of gravity. Eccentric contractions are sometimes referred to as lengthening contractions. This is somewhat misleading because actually what the muscle is doing is returning to its normal resting position from a shortened position. Quadriceps setting exercises are isometric contractions of the quadriceps muscle. Flexing and extending the knee are isotonic contractions. Sitting on a table and straightening the knee is a concentric contraction of the quadriceps muscle, while bending the knee would be an eccentric contraction of the quadriceps muscle.
If you were lying on the floor in a prone position and flexed your knee, it would be a concentric contraction of the hamstring muscles. Straightening your knee would be an eccentric contraction of the same muscles. What is happening? Straightening the knee while sitting and bending the knee while prone involve moving the part against gravity. Bending the knee while sitting and straightening the knee while prone involve moving the part with gravity and actually slowing down gravity. Changing this example slightly will illustrate another feature of concentric contractions. In the sitting position, have someone give resistance while you flex your leg.
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What type of contraction is it, and what muscle group is contracting? The answer is a concentric contraction of the hamstring muscles. In this case, gravity is not being slowed down, but a force greater then the pull of gravity is being overcome. Therefore, it can be summarized that isotonic contractions have the following features: CONCENTRIC CONTRACTIONS 1. Muscle attachments move closer together. 2. Movement is occurring against gravity (a raising motion).
3. If movement is occurring with gravity, the muscle is overcoming a force greater than the pull of gravity.
ECCENTRIC CONTRACIONS 1. Muscle attachments move farther apart. 2. Movement occurs with gravity (a lowering motion).
Another, less common type of muscle contraction is an isokinetic contraction. It is a fairly new concept because it can be done only with special equipment.
The Cybex and Orthotron were the first machines to induce such contractions. With an isokinetic contraction, the resistance to the part varies, but the velocity, or speed, stays the same. This differs from an isotonic contraction in which the resistance remains constant but the velocity varies. Consider the example of the person with the 5-lb weight attached to the leg. While that person straightens and flexes the knee (isotonic contraction), the amount of resistance stays the same. That 5-lb weight remained 5 lbs throughout the range.
Because of other factors, such as angle of pull, it is easier to move the leg in the middle and at the end of the range than at the beginning. In other words, the speed at which the person is able to move the leg varies throughout the range. In an isokinetic contraction, the speed is preset and will stay the same no matter how hard a person pushes. However, the resistance will vary. If the person pushes harder, the machine will give more resistance, and if the person does not push as hard, there will be less resistance. Table 1 summarizes the differences among these three types of muscle contractions. Why are isokinetic muscle contractions significant? There are two significant advantages.
Isokinetic exercises can alter or adjust the amount of resistance given through the range of motion, while an isotonic contraction cannot. This is important because a muscle is not as strong at the beginning or end of its range as it is in the middle. Because the muscle is strongest in the midrange, more resistance should be given there, and less resistance at the beginning and end. An isotonic contraction cannot do this. Therefore, what happens is that there may be too much resistance in the weaker parts of the range and not enough resistance in the stronger parts. Accommodating resistance is important because of the pain factor. If pain suddenly develops during the exercise, the person’s response is to stop exercising, or not work as hard.
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However, with an isotonic contraction, this response cannot happen quickly or even safely. With an isokinetic exercise, if the person stops working, the machine also stops. If the person does not contract as hard, the machine does not give as much resistance. Hopefully, this will give you some idea of the value of isokinetic exercise. There are, however, some drawbacks. For example, isokinetic exercise takes special equipment, and that equipment is expensive.
As you will learn in the therapeutic exercise section, there is a time and place for all of these types of muscle contractions. It is important that you recognize the differences among them. TABLE 1. TYPES OF MUSCLE CONTRACTION Speed Resistance Joint Motion Isometric Fixed Fixed (0 degrees / second) No Isotonic Variable Fixed Yes Isokinetic Fixed Variable (accommodating) Yes BIBLIOGRAPHY Duchateau J, Enoka RM. Neural control of shortening and lengthening contractions: influence of task constraints. J Physiol.
2008 Oct 27. Taylor JL, Gandevia SC. A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions. J Appl Physiol. 2008 Feb;104(2):542-50. Enoka RM . Neuromechanical basis of human kinesiology.
Champaign, IL: Human Kinetics Books;1988:31-63. Lieber RL . Skeletal muscle structure and function. Baltimore: Williams and wilkins;1992. Delorme TL, Watkins al. Progressive Resistence Exercise. New York: Appleton century;1951.
Hall CM, Brodie LT . Therapeutic exercise moving toward function. Williams and Wilkins 1998. Cynthia C. Norkin; Pamela K. Levangie. Joint Structure & Function: a Comprehensive Analysis. Davis Publications,1992..
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