What happens to the muscle during isometric tension development?

Shortening describes a concentric contraction, where internal muscle force exceeds the external load.

Lengthening describes an eccentric contraction, where the external load exceeds the internal force and the muscle elongates under tension.

During isometric tension development, the muscle generates force by activating cross-bridge cycling between actin and myosin filaments, but the external load equals the internal force produced, so the muscle neither shortens nor lengthens. This static contraction type is distinct from dynamic contractions and is commonly used in rehabilitation and stabilization training.

Relaxation implies a reduction or absence of motor unit recruitment and tension, which is the opposite of active tension development.