Muscle Movement

Every time a nerve impulse stimulate a muscle, the muscle contracts. However, a contracting muscle isn't necessarily getting shorter along its whole length. A concentric contraction, also called a positive contraction, shortens the muscle and develops tension. An eccentric contraction or negative contraction develops tension while actually lengthening the muscle. In either case, the muscles are moving against external resistance. If the force of the contraction is greater than the external resistance, it's a concentric contraction. If the force of the contraction is less than the external resistance, it's an eccentric contraction.