Effects of divergent selection for body weight on three skeletal muscles characteristics in the chicken

1. Histochemical (fibre type distribution and areas) and biochemical (myosin isoforms) characteristics of three muscles, M. anterior latissimus dorsi, M. pectoralis major and M. sartorius, were compared among male chickens of two lines at 11 and 55 weeks of age.

2. The lines were derived from a divergent selection based on growth rate. Cockerels from the Fast Growing Line (FGL) were 2^‐3 times heavier than those from the Slow Growing line (SGL) when 11 weeks old and 1–7 times at 55 weeks of age. The latter age was chosen as representative of the adult stage and the 11‐week age because, at this time, FGL cocks weighed as much as SGL cockerels at 55 weeks.

3. At both ages, the two lines showed similar fibre type distributions, but the total number in the ALD muscle, and the size (cross‐sectional areas) of fibres in each muscle were higher in the FGL compared with the SGL (14–6% and 33% more at 11 and 55 weeks of age respectively in favour of the FGL birds).

4. The two lines displayed similar myosin isoform patterns when adult muscles were compared (55 weeks). They differed slightly at 11 weeks of age, muscle differentiation being completed only in the FGL.

5 Comparisons of the two lines at the same live weight (i.e. FGL cockerels at 11 weeks of age and SGL cockerels at 55 weeks) showed larger muscle fibres in the SGL and no difference in the isomyosin patterns.