The phase of a lift in which the working muscle shortens as it contracts is called the concentric phase.
During the upward portion of a biceps curl, the biceps produces enough tension to overcome the weight, causing the muscle to shorten as the elbow bends. This is a concentric contraction. The same principle applies whenever a muscle actively shortens to create movement against an external load, such as standing up from a squat or pushing a weight overhead.
The lowering portion of a curl is different. The biceps remains active while lengthening under tension, which is called an eccentric contraction. Eccentric work helps control movement and braking, and it can produce more muscle damage and delayed soreness than concentric work. Holding a weight still with no visible change in muscle length is an isometric contraction.
“Isotonic” is a common source of confusion because it describes a broader contraction category involving a change in muscle length. In traditional terminology, isotonic actions can include both concentric shortening and eccentric lengthening, while real-world movements often involve changing tension and speed rather than perfectly constant force.