Absolute and relative growth of rat skeletal muscle.

Change in body weight and lower hindlimb length as physical properties, muscle weight and length as whole-muscle properties, and fiber numbers, diameters, and numbers of proliferating cells as intramuscular properties, were measured in 2- to 60-week-old Wistar male rats. Results showed that the rapid increase in muscle weight (muscle mass) in the postnatal "growing phase" depended on the longitudinal and transverse growth. The increase in muscle length is due to longitudinal growth of muscle fiber, which may be derived from an increase in the number of sarcomeres. The increase in muscle girth depends on the increase of individual fiber diameter (muscle fiber hypertrophy) and fiber numbers (hyperplasia). The remaining increase in muscle mass in the "steady phase" (after the 10th week) is caused entirely by transverse growth, depending mainly on the muscle fiber hypertrophy (but may include increase of connective tissues). In conclusion, to study the effects of exercise and/or various overloads on muscle hypertrophy or hyperplasia, especially in the case of the plantaris muscle in Wistar rats, they should be 10-30 weeks old, and over 300 g in body weight, because in this period their characteristics remain most constant.