The three main types of muscle contraction during exercise are isotonic, isometric, and isokinetic. Also, we further categorise them into concentric and eccentric depending on whether the muscle shortens or lengthens when contracting.
Isotonic muscle contractions
Isotonic muscle contraction types are those where the muscle changes length as it contracts. The load or resistance remains the same. There are two types of Isotonic muscle contraction:
Concentric contractions are those which cause the muscle to shorten as it contracts. An example is bending the elbow from straight to fully flexed, causing a concentric contraction of the Biceps Brachii. Concentric contractions are the most common type of and occur frequently in daily and sporting activities.
Eccentric is a type of muscle contraction opposite to concentric. They occur when the muscle lengthens as it contracts. This occurs when lowering the dumbbell down in a bicep curl exercise. The muscle is still contracting to hold the weight all the way down but the bicep muscle is lengthening.
Another very common example is the quadriceps muscles at the front of the thigh when landing from a jump. As you land the thigh muscles and in particular the quad muscles at the front of the leg are strongly contracting but also lengthening at the same time.
This type puts a lot of strain on the muscle and is commonly involved in muscle injuries. Plyometric training exercises (hopping and bounding) involve a lot of eccentric contractions and can lead to severe muscle soreness (DOMS) if you overdo them too soon.
Isometric muscle contraction
Isometric muscle contractions occur when there is no change in the length of the contracting muscle. For example, when carrying an object in front of you. The weight of the object pulls your arms down, but your muscles contract to hold the object at the same level.
Another example is gripping a tennis racket. There is no movement in the joints of the hand, but the muscles are contracting. They produce a force sufficient enough to keep a steady hold on the racket.
Isometric muscle strength vs length
The amount of force a muscle produces during a maximal isometric contraction depends on the length of the muscle at the time. Each muscle has an optimum length at which the maximum isometric force can be produced. Generally, muscles are stronger when not fully contracted.
Types of muscle contraction – Isokinetic
Isokinetic contractions are similar to Isotonic in that the muscle changes length during the contraction, where they differ is that Isokinetic contractions produce movements of a constant speed. To measure this a special piece of equipment known as an Isokinetic dynamometer is required. Examples of using Isokinetic contractions in the day-to-day and sporting activities are rare. The best is breaststroke in swimming, where the water provides a constant, even resistance to the movement of adduction.