Adaptations in muscle fibre characteristics induced by physical activity in pigs

Abstract The influence of regular exercise training and physical activity on distribution of muscle fibre types, fibre cross-sectional areas and the number of adjacent capillaries per fibre type and per fibre (capillarization) was studied in five muscles from 48 female and 48 entire male pigs. In the growth interval from 30 to 100 kg, the pigs were subjected to one of three treatments: individual housing in pens of 2·5 m 2 (treatment C), individual housing and treadmill training for 15 minlday at a speed of 4 kmlh, 5 days/week for a period of 70 days (treatment T), and housing in large pens (36 m 2 , 40 pigs per pen) allowing for spontaneous physical activity (treatment F). In m. longissimus dorsi, treatment F increased the ratio of fast-twitch oxidative (FTa-) to fast twitch glycolytic (FTb-) fibres, elevated the mean fibre cross-sectional area and the number of capillaries per fibre. In m. biceps femoris (BF) from female pigs, the only adaptation found was a marked training-induced (treatment T) increase in the cross-sectional area of the slow-twitch (ST-) fibres. In m. semitendinosus and BF from male pigs, treatments T and F increased the ratio of FTa- to FTb-fibres. Both training and spontaneous activity increased the proportion of ST-fibres in m. trapezius thoracis (0·48 in treatment C, 0·53 in T and 0·52 in F). Conversely in m. psoas major, treatment F increased the proportion of FTa-fibres (0·15 in C and 0·19 in F) at the expense of ST- and intermediate FTc-fibres. Spontaneous activity induced ST-fibre hypertrophy in the five muscles. For several muscles, the mean fibre cross-sectional area was significantly higher in female than in male pigs.