Distribution of muscle fibre cross-sectional areas in four sets of 2-day-old PCS piglets, each with a corresponding normal litter mate. At least 10,000 fibres were measured per muscle section and grouped by size in 25 μm2 increments (A). ST and LD muscles of PCS piglets showed a narrow and steep pattern of distribution. In ST muscles, more than 50% of PCS fibres were less than 100 μm2, compared with normal fibres where only 35% were less than 100 μm2. In LD muscles, more than 50% of PCS fibres were less than 75 μm2; compared with normal fibres only about 35% were less than 75 μm2. G muscles of PCS piglets showed a similar but smaller fibre size distribution than their normal litter mates. (B) However, no significant difference in slow or fast fibre type composition was found between normal and affected muscles. The error bars indicate standard error. Norm = normal; PCS = porcine congenital splayleg.