Depending on the relationship that an individual establishes with external resistances, muscle activation can lead to three different contractions:
CONCENTRIC: With shortening of muscle fibers. Overcoming external resistance. The external force acts in the opposite direction to the movement.
ECCENTRIC: With lengthening of muscle fibers. Assignment to external resistance. The external force acts in the same direction as movement.
ISOMETRIC: The magnitude of muscle tension is equal to the force caused by external resistance, so the length of the muscle does not vary and there is no mechanical work. However, there are internal variations with respect to the state of rest: the only thing that remains the same is the angle at which the muscle tension is occurring, but the action of the muscle is to shorten fibers and stretch the connective tissue (tendon).