There are actually three phases of each rep: the concentric (the “up” portion), the eccentric (the “lowering” portion), and the isometric (the “holding” portion). You probably know this.

Objective: To investigate the acute effects of isolated eccentric and concentric calf muscle exercise on Achilles tendon sagittal thickness. Design: Within-subject, counterbalanced, mixed design.

An eccentric contraction provides the control of a movement on the downward phase and it works to resist the force of gravity. Figure caption, Left: Concentric contraction - the biceps are ...