Decrease in fibre numbers of dog pectineus muscle with age

The total number of muscle fibres, as well as the numbers of Type I, Type II and intermediate muscle fibre types, in transverse sections of the pectineus muscles of dogs of 2 and 12 months of age was determined by an indirect fibre counting method. The total nubmers of fibres in all muscles decreased very significantly (by about 32%) in both sexes between 2 and 12 months of age. The decrease resulted from loss of about 20% of the Type I, 39% of the Type II and 73% of the intermediate muscle fibres.