What type of contraction occurs in muscles that are shortening under tension?

The type of contraction that occurs in muscles that are shortening under tension is concentric contraction. During a concentric contraction, the muscle fibers generate enough force to overcome the external load, which allows the muscle to shorten as it contracts. This is typically seen in movements like lifting weights or performing a bicep curl, where the bicep muscle shortens as the forearm is raised.

In contrast, eccentric contractions occur when the muscle lengthens under tension while it is still actively contracting, which often happens when lowering a weight. Isometric contractions involve muscle tension without any change in muscle length, such as holding a weight in place without moving it. Static is not a recognized contraction type in this context, as it generally refers to movements that do not involve muscle shortening or lengthening. Hence, the defining characteristic of concentric contractions is the shortening of muscle fibers while generating tension, making this the correct answer.