Which muscle action indicates an isometric contraction?

An isometric contraction is characterized by muscle tension being produced without any change in muscle length. This means that during an isometric contraction, the muscle generates force but does not shorten or lengthen, allowing it to maintain a constant position against resistance. This type of muscle action is often utilized in various exercises where a stable posture is required, such as holding a plank position in Pilates.

The other options depict different muscle actions that describe varying lengths and movements of muscles. Muscle shortening without movement refers to a contraction type but typically includes movement, which is not the case in isometric contractions. Muscle lengthening with resistance describes an eccentric contraction, while muscle relaxing against resistance involves a different muscular response entirely. Together, these options illustrate other muscle actions, yet they do not define the nature of an isometric contraction, which is solely characterized by the tension in the muscle without any change in length.