What is the shortening phase of isotonic muscle action called?

The shortening phase of isotonic muscle action is referred to as the concentric phase. During concentric muscle contractions, the muscle fibers shorten while producing force, which occurs when the muscle is activated to perform work against resistance. This phase is commonly observed in movements such as lifting weights, where muscles contract to overcome the load and bring the body segment closer to the center of the body or to the starting position. The process allows for dynamic movements that are essential in various physical activities and workouts.

The other options do not accurately describe this specific phase. Isometric refers to muscle contractions that occur without any change in muscle length, meaning the muscle does not shorten or lengthen. Eccentric refers to the phase wherein the muscle lengthens while under tension, typically as it controls or resists a load. Static is not a term typically used in this context and could lead to confusion with isometric actions, which do not involve dynamic muscle shortening.