Contraction is the shortening of muscle as its fibres pull together — in life, the basis of movement; in meat science, a determinant of tenderness. Each myosin head on a thick filament grips a G-actin molecule on a thin filament, forming crossbridges of actomyosin; with ATP present, as in living animals, the complex releases rapidly, but after death actomyosin dominates and rigor mortis sets in. How muscles contract and relax before and after slaughter — chilled too fast, stimulated electrically, hung or stretched — decides whether the meat on the plate is tender or tough.