The different types of muscular contraction

In order to produce the vast range of movements of which the body is capable, muscles can either shorten, lengthen or remain the same length when contracting. Muscle contractions are classified depending on the muscle action which predominates:

Isotonic contractions

These refer to those instances when the muscle is moving while contracting. This is further divided into concentric and eccentric actions:

• Concentric contractions involve the muscle shortening under tension, and it is the most common form of muscular contraction. For example, the biceps concentrically contract during the upward phase of a bicep curl, or in the triceps during the upward phase of a push-up. It corresponds to the ‘work’ or ‘effort’ phase of an exercise.
• Eccentric contractions are the opposite to concentric, and occur when the muscle lengthens under tension (usually when returning to its normal resting length). For example, the biceps during the downward phase of the bicep curl or in the triceps during the downward phase of the push-up.

The eccentric contraction of the bicep during the downward phase is used to counteract the force of gravity, and acts like a brake. This is because gravity acts on the mass of the weight and the forearm causing the elbow to extend. If the bicep does not contract to control the rate of motion caused by gravity, then the movement will be very quick and may result in injury.

Isometric contractions

These take place when a muscle increases in tension without actively shortening or lengthening. The muscle remains the same length while contracting and there is no movement taking place.

This occurs all the time, as the majority of a person’s muscles contract isometrically in order to maintain posture.

It also occurs when a muscle is acting as a fixator, or when it is working against a resistance that it cannot move.

An example of an isometric contraction would be when holding a weight in a stationary position; or when two equally strong teams meet in a rugby scrum or judo competition.

It also happens during resistance training when an individual reaches the sticking point in an exercise – the participant cannot perform another repetition, but still continues to try.

It is essential when performing an isometric contraction that the breath is not held, due to the increase in blood pressure and the potential dangers of the valsalva manoeuvre.

Isometric contractions only strengthen the muscle at the joint angle worked.

Isokinetic contraction

This involves the muscle shortening and increasing in tension while working at a constant speed against a variable resistance.

Normally when a muscle contracts the angular velocity of the muscle shortening or lengthening varies throughout the contraction. However, with the use of specialist hydraulic machines, it is possible to keep the speed at which the muscle shortens or lengthens constant, but not necessarily the resistance applied. The speed of the movement cannot be increased. Any attempt to increase the velocity results in equal reaction force from the machine.

Click to enlarge