What phase of a lift is characterized by the muscle shortening and generating force?

The concentric phase of a lift is defined by the shortening of the muscle fibers as they contract to generate force. During this phase, the muscle actively contracts against a resistance, resulting in movement through the joint. For example, when performing a bicep curl, as the lifter lifts the weight towards their shoulder, the bicep muscle contracts concentrically, causing the muscle fibers to shorten, which allows the arm to flex.

In contrast, the eccentric phase involves the muscle lengthening while still under tension, typically as the weight is being lowered. The isometric phase represents a static hold where the muscle generates force without changing length, and the recovery phase usually refers to the rest or recuperation time after the lift has been completed. Understanding these phases is crucial for structuring effective resistance training programs and ensuring proper mechanics during exercises.