What is the type of muscle contraction where the muscle lengthens under tension, such as when lowering a weight?

The type of muscle contraction in which a muscle lengthens while under tension, such as when lowering a weight, is an eccentric contraction.

During a biceps curl, the lifting phase is concentric: the biceps shortens as it produces force. When the weight is lowered under control, the biceps remains active but lengthens, making the action eccentric. In a squat, the downward phase is another familiar example.

Eccentric contractions can produce high mechanical tension while using less metabolic energy than concentric contractions. That makes them important in strength training, rehabilitation, and studies of muscle growth. They can also cause more exercise-induced muscle damage and delayed-onset muscle soreness, especially after unfamiliar movements.

A common mix-up is isometric contraction, in which muscle length stays essentially constant, as when holding a weight motionless. Isokinetic describes movement performed at a constant speed, usually with specialized equipment; it does not describe whether the muscle shortens or lengthens.