The relative growth of muscle fibre types as distinguished by the myosin ATPase reaction at pH 9.5 in m. semitendinosus, m. diaphragma and m. pectoralis transversus of growing Thoroughbreds and other horses ranging in age from perinatal life to adulthood was investigated.
The growth rate of the area of myosin ATPase high-reacting fibres in m. semitendinosus of both types of horse and in m. pectoralis transversus of Thoroughbreds was greater than that of low reacting fibres. Myosin ATPase low reacting fibres increased in area faster than high reacting fibres in the diaphragm of both types of horse and m. pectoralis transversus of other horses. These differential growth rates result in myosin ATPase high reacting fibres being larger than myosin ATPase low reacting fibres in m. semitendinosus and m. pectoralis transversus in adults of both types however, low-reacting fibres are larger than high reacting fibres in m. diaphragma of adults of both types. These results suggest that the greater relative muscle mass of the proximal limbs of Thoroughbreds and associated greater mean fibre area associated are with relatively faster growing high reacting fibres in their muscles than those of other horses.