The knee joints of one hind limb of male Wistar rats (250–300 g) were immobilised in a plaster cast for up to 3 weeks with the limb in either the fully extended or flexed position Serial frozen sections of
vastus intermedius muscle were stained histochemically for myosin ATPase, succinic dehydrogenase, phosphorylase and capillaries. Muscle fibres were classified as either slow twitch oxidative (SO), fast twitch oxidative glycolytic (FOG) or fast twitch glycolytic (FG).
Fibre area of SO fibres (3019
2) decreased by 50% following 3 weeks' immobilisation in the stretched (1532
2) and shortened (1517
2) positions. Atrophy of FOG fibres (2456
2) was greater in muscles immobilised in the stretched (750
2) than shortened (1185
2) positions. Evidence has been obtained that muscle fibre number remained unchanged following immobilisation. Control muscles contained 70.4% SO fibres, 29.6% FOG fibres and >0.5% FG fibres. Immobilisation produced an increased proportion of high myosin ATPase staining fibres. The percentage occurrence of FOG fibres increased to 65% and 83% respectively, in muscles immobilised for 3 weeks in the stretched and shortened positions. Mechanisms for the transformation of muscle fibre types and the influence of muscle length on the properties of immobilised muscle are discussed.
Key words Skeletal muscle - Muscle fibre types - Limb immobilisation