An eccentric contraction is a muscle contraction that is caused by external force greater than the force of the muscle itself, causing the muscle to lengthen. During the movement, the tension in the muscle remains unchanged.
About.com continues by explaining that common exercises that can cause this to occur are push-ups or pull-ups, lowering weights and running uphill. It can also happen during any sport that requires the individual to use controlled movements. About.com suggests that eccentric contractions are related to the early stages of delayed muscle soreness, where the muscles begin to feel painful a few days after the exercise has occurred.