There are two types of muscle contraction, concentric contraction and eccentric contraction. In concentric contraction, muscle tension is sufficient to overcome the load, then you can lift the load. For example, when you lift dumbbell, the muscle contraction is concentric contraction. And in eccentric contraction, the tension generated while isometric is insufficient to overcome the external load, and the muscle fibers lengthen as they contract. For instance, the action of lowering the dumbbell back down from the lift is the eccentric phase. In that process, the muscle contraction is eccentric contraction. Eccentric training is proofed that can lead to stronger muscle, faster muscle repair and increasing metabolic rate. It is particularly good for casual and high performance athletes or elderly and patients.
According to experiment, eccentric exercise can decrease 1/4 loading more than traditional concentric exercise. It would make elders and weak people to training recovery more easily. Moreover, eccentric exercise could reduce Sarcopenia of elder people for less fall down happened. And by test, in the same period and same intensity, eccentric training could strengthen more muscle than traditional concentric training.