Which phase of muscle contraction involves muscle shortening?

The phase of muscle contraction that involves muscle shortening is the concentric phase. During a concentric contraction, the muscle generates force while it is actively shortening. This occurs, for example, when lifting a weight towards the body, where the biceps muscle shortens as it pulls the forearm up. This type of contraction is crucial for movements such as lifting, jumping, and running, where the muscle needs to produce force to overcome resistance.

In contrast, during the eccentric phase, the muscle lengthens while still under tension, which typically occurs while lowering a weight. The isometric phase happens when the muscle exerts force without changing its length, like holding a weight in a fixed position. Isokinetic contraction refers to muscle contractions that occur at a constant speed, which can involve both shortening and lengthening phases but doesn't specifically focus on just shortening.