Three types of muscle contractions allow athletes to apply force in sports: concentric, eccentric, and isometric actions. Concentric and eccentric actions are dynamic muscle actions, where isometric actions are static. Knowing how these actions occur in sport movements tells you the type of training activities that will simulate those specific movements.
A concentric action occurs when a muscle's force is greater than the resistance, so the muscle shortens such as in the upward phase of a bicep curl. Concentric muscle contractions are essential for overcoming the force of gravity and performing lifting movements.
Concentric actions are required for skills that involve quick reactions. When an external force causes the desired movement without any muscular action, but too slowly, concentric muscle action produces the desired speed.
An eccentric action occurs when the force generated by the muscle is less than the resistance, so the muscle actively lengthens. Eccentric actions are often used when muscles have to slow down body parts, control movements, or oppose external resistances. The downward phase of a biceps curl, for example, requires eccentric action of the biceps muscle. The muscle exerts force to control the speed of the downward movement, but its length increases.
An isometric action is a static action where the muscle generates force but there is no movement. When a barbell is held at the midpoint of the bicep curl, the biceps exert force but do not change in length. As in the graphic where the athlete places resistance against the curling movement, the biceps neither overcome the resistance, nor are they overcome by the resistance. The muscle length does not change even though it is contracting.
Isometric muscle contractions are essential for matching the force of gravity and maintaining given joint positions, as in push ups when the trunk muscles hold the body straight. During isometric contraction the contractile part of the muscle shortens, but the elastic connective tissue lengthens proportionately.
During all resistance exercise involving the arms or legs, the trunk muscles are activated isometrically to stabilize the trunk and help prevent injury during many movements. Muscles may also act isometrically to stabilize or prevent an undesirable body segment movement.
An isokinetic muscle action is a dynamic movement where the speed is constant. These actions are rare in sport and are mainly performed under artificial conditions, such as when using a machine called a dynamometer. A dynamometer accommodates counter-resistance based on the amount of torque (rotational force) produced. This allows movement at a constant velocity. Because isokinetic actions rarely occur in sports, the extent to which these contractions transfer to sport movements is questionable.
This video explains how exercise physiologists use isokinetic testing using the Biodex System 4.
Muscle actions used when lifting weights have traditionally been called isotonic actions, the term used to describe both concentric and eccentric actions. During such isotonic actions a muscle generates a constant amount of force through a range. However, force production is almost never constant because the joint angle changes. Thus, the new term now used is dynamic constant external resistance (DCER) because it better describes the notion that resistance remains constant, but not muscle tone, when lifting weights.
In summary, according to Chandler and Brown (2013), the three types of muscle contractions are DCER, isometric, and isokinetic. Isokinetic and DCER muscle actions may be performed concentrically or eccentrically.
Brooks, C. (2013). Become an successful personal trainer online lecture. Cocoa Beach, FL: Self.
Chandler, T. J. & Brown, L.E. (2013). Conditioning for strength and human performance (2nd ed.). Philadelphia: Lippincott, Williams, & Wilkins.