concentric, eccentric, and isometric contraction

Pick up a heavy bag and your arm muscle bulges and shortens; lower the same bag slowly and that same muscle is still working hard, but now it is lengthening under control; hold the bag still in mid-air and the muscle is straining without changing length at all. Same muscle, same load, three completely different jobs — these are the three types of contraction. (Note that 'contraction' here means the muscle is active and generating force, even when it is not actually getting shorter.)

Concentric contraction is the familiar one: the muscle shortens while it pulls, producing movement and lifting the load — your biceps shortening as you curl a weight up. Eccentric contraction is the muscle generating force while it lengthens, acting as a brake to control a movement against gravity — your biceps paying out length as you lower the weight slowly. Isometric contraction is the muscle tensing without changing length and without movement — your biceps holding the weight steady at a fixed angle. A useful detail: muscle can produce the most force eccentrically and the least concentrically, which is why you can lower a weight you could never lift; and eccentric (braking) work is the kind most likely to leave you sore the next day.

All three are tools in therapeutic exercise, chosen on purpose. Isometric work builds strength without moving a painful or freshly repaired joint, which makes it valuable very early in recovery. Eccentric, controlled-lowering exercise is a mainstay for stubborn tendon problems and for the everyday skill of safely lowering oneself — controlling the descent into a chair, or the step down a kerb, is eccentric work, and weakness there shows up as 'collapsing' rather than 'can't lift'. Picking the right type lets a therapist load a muscle while protecting whatever must not yet be moved.