As inuscle g o c ~ into rigor mortis it contr'icts. If unrt:strainetl it contracts isotonically, that is, it sliortviis. If restrained it contracts isometrically, that is, it develops tension. For miiscle to shorten or dcvelop tension in the living animal or post-mortem, tlie frce C a 2 + ~ conccntration of the sarcoplasm must increase. This applies equally well to thaw-shortening, coldshortening and rigor-shortening. These different kinds of post-mortcain shortening arc illustrated in Figure 1. Thaw shortening is the very rapid shortening that occurs when musclr frozcm pre-rigor is thawed ( Chamhers ancl Hale, 1932; Perry, 1950). Cold-shortening i s the shortening that commences immediately when vnnsclc, is exposed to temperatures below about 14°C within a fcw hoiirs after slaughter (Locker and Hagyard, 1963). Rigor-shortening is the shortening that occiirs during the rapid phasc~ of rigor onset in miiscles which do not cold-shorten either because they are not capable of cold-shortening or because they arc kcpt at temperatures above about 14°C (Rrndall, 1951; Marsh, 1954; Newbold, 19%). Rigor-shortening also occnrs following thaw or cold-shortening ( Bendall, 1973b, 1975; Newbold, 1966). In some muscles, when they are under load, a relaxation phase is apparent after thawor cold-shortening has reached its peak a i d before rigor-shortening commences ( Bendall, 1960, 19'i3n, 1973b, 1975, 1978; Newbold, 1966). This relaxation most likely reflects the recapture of some of the Ca2-b released during the first shortening phase.