Define "isometric contraction."

An isometric contraction is defined as a contraction where muscle length does not change. This type of muscle contraction occurs when the muscle exerts force without an observable movement of the joint it is associated with. In practical terms, during an isometric exercise, the muscle fibers generate tension and stay at a constant length, despite the effort being applied. This is important in various training scenarios, such as maintaining posture or during certain rehabilitation exercises, where stability is the goal rather than movement or shortening of the muscle.

In contrast, the other options pertain to different characteristics of muscle contractions. A contraction that increases muscle size typically relates to hypertrophy resulting from repeated dynamic contractions, not isometric ones. Movements are related to isotonic contractions, which involve changes in muscle length, either shortening or lengthening. Lastly, contractions that involve multiple muscles would refer to either compound or multi-joint exercises, which differ from the isolated tension present in an isometric contraction. Understanding this allows trainers to better incorporate and explain isometric exercises in strength training and rehabilitation programs.