Isometric Contraction

An isometric contraction is a muscle contraction in which force is produced without an appreciable change in the overall length of the muscle-tendon unit or movement of the associated joint. Because there is no visible shortening or lengthening, it is sometimes called a static contraction. Examples include holding a load in a fixed position or pushing against an object that does not move .^1,2

During an isometric contraction, the muscle is nevertheless physiologically active. Motor neurons activate muscle fibers, calcium enables interactions between actin and myosin, and cross-bridge cycling generates force. Although the overall muscle-tendon unit can remain at approximately constant length, its internal components do not necessarily remain at constant lengths. Muscle fibers or fascicles can shorten while elastic structures such as tendons lengthen .¹

Isometric contractions can be performed at different muscle lengths, joint angles, intensities, and durations. Repeated isometric resistance training can increase muscular strength, with improvements often greatest at or near the joint angles used during training. Isometric training can also produce changes in muscle size, neural activation, and tendon properties, depending on how the contractions are performed .²

Isometric contraction is distinct from concentric and eccentric contraction. During a concentric contraction, an active muscle shortens while producing force. During an eccentric contraction, an active muscle lengthens while producing force against an external load. Isometric contraction instead produces force while overall muscle-tendon length and joint position remain approximately constant .^1,2