Weight training is a common type of strength training for developing the strength and size of skeletal muscles. It uses the force of gravity (in the form of weighted bars, dumbbells or weight stacks) to oppose the force generated by muscle through concentric or eccentric contraction. Weight training uses a variety of specialized to target specific muscle groups and types of movement. Weight training differs from bodybuilding, Olympic weightlifting, powerlifting, and strongman, which are sports rather than forms of exercise. Weight training, however, is often part of the athlete's training regimen.