Your body is comprised of three types of muscles; skeletal, smooth, and cardiac. Each type of muscle serves a different function. Resistance training/ weight lifting utilize the skeletal muscles. Skeletal muscles attach to the bones in your body. The muscle groups must work together to perform successful movements without injury. One muscle group must contract while the reciprocal muscle group relaxes to allow the movement to complete.
There are two types of muscles contractions used during weight lifting; concentric contractions and eccentric contractions. When the muscle shortens that is considered a concentric contraction. If the muscle shortens and lengthens simultaneously, that is an eccentric contraction. The concentric contraction occurs usually when you begin a repetition of an exercise. As you begin an exercise, your muscles must contract to allow you to complete the movement. A contraction causes the muscle to shorten.
When you finish the exercise, you must slowly return the weight to the starting position. To accomplish this, your muscles must lengthen. But, at the same time, the muscles have to contract to continue the slow movement to return the weight to the starting position. This is an eccentric contraction. The eccentric contraction is also referred to as negative work. The movement can require more force and strength than the concentric contractions.
Because the eccentric contraction can require additional effort, the contraction can cause muscle soreness. Since the eccentric contraction utilizes two movements and can use greater force, your form during the movement should be precise and controlled. The negative phase of the repetition is just as important, if not more, as the beginning of the movement.