Which type of muscle contraction involves muscle shortening?

Concentric contraction is a type of muscle contraction where the muscle fibers shorten while generating force. This occurs when the muscle overcomes resistance, allowing the joint to move in the direction of the muscle's contraction. For example, during a bicep curl, as you lift the weight towards your shoulder, the bicep muscle shortens, demonstrating concentric contraction.

In contrast, isometric contraction occurs when the muscle generates force without changing its length, such as holding a weight in a fixed position. Eccentric contraction involves the muscle lengthening under tension, often while controlling the descent of a weight, like lowering the weight back down in a bicep curl. Isotonic contraction is a broader term encompassing both concentric and eccentric actions; however, it does not specifically refer to the muscle shortening. Thus, concentric contraction is the correct answer as it specifically describes the shortening of muscle fibers during a contraction.