Definition
Emphasising the lowering phase of a lift, by slowing it down or loading it heavier than the lifting phase.
Muscles are 20 to 40% stronger lowering than lifting, which is why eccentric work can use loads the client could not lift. Methods: slow eccentrics of four to six seconds, eccentric-only reps where a partner lifts and the client lowers, and overload on machines (two limbs up, one down). Nordic hamstring curls are the classic example.
It builds tendon strength, gets a lifter through a sticking point and grows muscle. The cost is soreness and fatigue, so introduce it one exercise at a time, once a week.
Rehab uses it heavily for tendon pain, with slow loaded eccentrics over weeks.