Concentric muscle contraction (shortening): In weight training, this is the lifting phase of an exercise. ...
Eccentric muscle contraction (lengthening): In weight training, this is the phase in which the muscle returns to the starting position of the exercise.
our muscles are made of bundles of muscle fibers that contain thousands of smaller structures called myofibrils, where the actual contraction occurs. There are two types of protein filaments within myofibrils—actin and myosin. Actin forms thin filaments. Myosin forms thick filaments and is a "molecular motor" that converts chemical energy into mechanical energy, which creates force and drives motion.
