Muscle contractions during exercise can be divided into three categories; isotonic (meaning same tension throughout the contraction), isometric (meaning same tension), also known as a static contraction and isokinetic muscle contractions which are performed with a constant speed throughout the movement. Here we explain these in more detail including in which types of exercise they occur.
Isotonic contractions are those which cause the muscle to change length as it contracts and causes movement of a body part. There are two types of Isotonic 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 muscle. Concentric contractions are the most common type of muscle contraction and occur frequently in daily and sporting activities.
Eccentric contractions are the opposite of concentric and 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 of contraction puts a lot of strain through the muscle and is commonly involved in muscle injuries. Plyometric training exercises (hopping and bounding) involve a lot of eccentric muscle contractions and can lead to severe muscles soreness (DOMS) if you overdo it too soon.
Isometric contractions occur when there is no change in the length of the contracting muscle. This occurs when carrying an object in front of you as the weight of the object is pulling your arms down but your muscles are contracting to hold the object at the same level. Another example is when you grip something, such as a tennis racket. There is no movement in the joints of the hand, but the muscles are contracting to provide a force sufficient enough to keep a steady hold on the racket.
The amount of force a muscle is able to 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.
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 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.