A static (or isometric) contraction occurs when a muscle generates force while the joint angle remains the same. An example would be what happens when a person pushes against a wall. The muscles contract but the joint remains unmoved. A dynamic (or isotonic) contraction occurs when the force generated by the muscle causes movement in the joint. Dynamic contractions occur during most physical activities.
There are two phases to an isotonic contraction. They are known as the concentric phase and the eccentric phase. In the concentric phase, the muscle generates force by shortening and decreasing the joint angle. This happens when an elbow is bent to bring a weight towards the shoulder.
In the eccentric phase, the muscle generates force by lengthening and increasing the joint angle. More weight can be moved in this phase than in the concentric phase. Netfit explains that eccentric contractions help keep motions smooth and normally occur as a braking force against concentric contraction, which allows for the protection of joints and muscles against injury.
Eccentric contractions, also referred to as "negatives," are usually performed in strength training exercises that involve the slow lowering of a resistance or weight. An example of this would be the slow lowering of a bar to the chest during a bench press.