Muscle fibre characteristics in four muscles of growing bulls - I. Postnatal differentiation

Mammalian muscle fibres maturation follows a distinct programme lineage (I --> IIC --> IIA --> (IID/X) --> IIB). Changes in morphology, protein content and metabolic characteristics are related to several factors, age being one of the most crucial for muscle fibre type. Similarly, muscle fibre type affects overall muscle growth rate. The objective of this study was to characterise the profile of muscle fibre types and metabolic enzyme activities in various muscles [semitendinosus (ST), longissimus dorsi (LD), biceps femoris (BF), triceps brachii (TB)] of growing Montbeliard bulls. Groups of four or five animals were slaughtered at 4, 8, 12 and 16 months of age. Results indicated that contractile muscle fibre type maturation occurred similarly across all muscles investigated. The percentage of slow myosin heavy chain 1 (MHC1) diminished between 4 and 8 months of age (26.3% vs. 19.0%) and steadily increased until 16 months (34.4%; P < 0.001). The evolution of metabolic enzyme activities in the muscles studied was related to muscle-specific growth patterns: e.g., TB showed enhanced (P < 0.05) glycolytic metabolism during the final period of increased allometric growth rate (1.23). These data show that both contractile fibre categories are involved in postnatal muscle fibre differentiation of cattle, which might contribute to alterations in meat tenderness with increasing age. (C) 1998 Elsevier Science B.V.