Muscle activation that results in torque that is greater than the resistance encountered, leads to muscle shortening, a so called concentric contraction. During a concentric contraction a muscle performs positive work; it accelerates a load or it imparts potential energy to an object. If however, the load that is applied to a muscle is larger than the torque that it produces, the activated muscle undergoes lengthening and this is called an eccentric contraction. In eccentric contractions, a muscle performs negative work i.e. it decelerates an object or it absorbs potential energy such as during walking downhill. There are several physiological properties that differ notably between concentric and eccentric contractions. The torque that can be produced by muscle tissue can be several times higher during eccentric than during concentric contractions at similar angular velocity. This implies that muscle tissue can be subjected to much higher stress during eccentric than during concentric contractions, potentially leading to muscle damage and delayed onset muscle soreness (DOMS). Furthermore, the metabolic energy required to provide negative work is typically four fold lower than the metabolic energy required to produce the same amount of positive work. We have used eccentric exercise in rehabilitation of patients with limited metabolic performance capacities to subject muscle tissue to high mechanical loads that could not have been achieved by concentric exercise. In a similar vein, eccentric exercise was used in octogenarians in the prevention of sarcopenia. Eccentric exercise further proved effective in preparing competitive alpine skiers to the enormous eccentric muscle loads seen in alpine ski racing.