Skeletal muscle fibre characteristics in young women

Muscle samples were obtained by needle biopsy from the lateral vastus muscle of 54 healthy female subjects, 18 to 40 years of age. 25 subjects were classified as sedentary, 20 as active, and 9 as endurance trained. In 33 of the subjects, the deltoid muscle was biopsied as well. The lateral vastus muscle of runners and the deltoid muscle of swimmers contained 60% slow twitch fibres (ST) an none or few fast twitch b fibres (FTb). Otherwise the percentage of fibre types was 50% ST fibres, 30% to 35% FTa fibres, and 15% to 20% FTb fibres. In the lateral vastus muscle ST fibres wee larger than FT fibres, whereas in male skeletal muscles FT fibres tend to be larger than ST fibres. The capillarization was linearly related to maximal oxygen uptake, as is also the case in male subjects. The activity of biochemically determined phosphofructokinase (PFK) was the same in both muscles and in all groups; succinate dehydrogenase activity (SDH) was closely related to maximal oxygen uptake. Th activities of PFK and SDH were lower than in male subjects the ratio between the activities indicated that the relative contribution of oxidative metabolism was higher than in males. The morphological and biochemical differences between female and male skeletal muscle are found in sedentary and trained subjects, and may be due both to the influence of sex hormones, and to differences in the daily activity pattern.