Types of Muscle Contractions

Isotonic muscle contractions are the common muscle contractions that enable people (and other animals) to move about generally.

There are two types of isotonic muscle contraction:

• Concentric muscle contraction, and
• Eccentric muscle contraction.

Concentric Muscle Contraction:

Muscle shortens as tension in the muscle increases, as when lifting a weight.

Muscles shorten as muscle fibres contract.

For example when lifting an object by holding it in the right hand then contracting the biceps brachii muscles of the right-arm concentrically the elbow joint flexes, moving the lower-arm and so the hand and object held in it upwards.

Eccentric Muscle Contraction:

Muscle lengthens as tension in the muscle increases, as when slowly lowering a weight.

Although the actin and myosin filaments within the muscle fibres contract (to produce the force needed) the fibres themselves also slide alongside each other resulting in the overall lengthening of the muscle.

Continuing the above example of an object that has been moved upwards by contraction of the right biceps brachii to flex the right elbow, the object may then be lowered in a steady controlled way by contracting the biceps brachii muscles of the right-arm eccentrically to extend the elbow joint, lowering the lower-arm together with the hand and object held in the hand.

Isometric Muscle Contraction:
meaning "same distance", i.e. static

In isometric muscle contraction the muscle maintains the same length as tension in the muscle increases, as when holding a weight in a static position for an extended period of time.

That is, there is no change in the length of the contracting muscle during isometric muscle contraction. The amount of force a muscle can produce during an isometric contraction depends on the length of the muscle at the point of contraction. Each muscle has an optimum length at which the maximum isometric force can be produced.

The force of the contraction occurs in the muscle fibre but the muscle fibres themselves do not move relative to each other, so the overall length of the muscle doesn't change.

For example when holding or gripping an object such that there is a downward force on the object (due to gravity) which the muscles oppose in order to hold the object in a static position (holding) and/or to maintain steady contact with the object e.g. with fingers wrapped around a handle (gripping). The hand / arm are not moving but the muscles are contracted in order not to release/drop the object.

Isokinetic Muscle Contraction:
meaning "same speed"

As in isotonic contractions (see above), in isokinetic muscle contraction the muscle changes length during the contraction. In isokinetic muscle contraction the muscle contracts maximally throughout its full range of movement.
The defining characteristic of isokinetic muscle contractions is that they result in movements of a constant speed. A piece of equipment called an Isokinetic Dynamometer is used to measure the (constant) speed of isokinetic muscle contraction. Such equipment is not common in all schools, colleges, leisure centres and gyms but tends to be used in rehabilitation centres and specialist sports training facilities.

The changes in overall length of the whole muscle depend on the combined effects of contraction (shortening) of muscle fibres and movement of individual muscle fibres alongside each other (potentially increasing overall muscle length).