An eccentric contraction is defined as a contraction where the muscle is lengthening, as opposed to a concentric contraction where the muscle is shortening, or an isometric contraction where there is no change in the muscle length.
There is nothing magical about eccentric muscle strengthening in the role of alleviating tendon problems. The key is the controlled progression of exercise, which elicits an adaptation in the tendon that produces increased tensile strength. Research has shown that eccentric muscle contractions generate more tension than isometric or concentric contractions. By using just eccentric work this variable can be controlled.
It is important to note that the load and number of repetitions must be carefully recorded and progressively increased. This ensures that the overload on the tendon is carefully controlled and gradually increased. Finally, the speed of the exercise is increased, making the exercises more sports specific.