BACKGROUND: Understanding rotator cuff muscle function during disease development and after repair is necessary for preventing degeneration and improving postsurgical outcomes, respectively. The rat is a commonly used rotator cuff animal model; however, unlike humans, rats continue to grow throughout their lifespan, so age-related changes in muscle structure may complicate an understanding of muscle adaptations to injury.
METHODS: Infraspinatus and supraspinatus muscle mass, fiber length, pennation angle, sarcomere length, and physiological cross-sectional area (PCSA) were measured in Sprague-Dawley rats (n = 30) with a body mass ranging from 51 to 814 g (approximately 3 weeks to approximately 18 months).
RESULTS: Both the supraspinatus and infraspinatus showed a striking conservation of sarcomere length throughout growth. There was linear growth in muscle mass and PCSA, nonlinear growth in muscle length and fiber bundle length, and a linear relationship between humeral head diameter and fiber bundle length, suggesting that muscle fiber length (serial sarcomere number) adjusted according to skeletal dimensions. These muscle growth trajectories allowed sarcomere length to remain nearly constant.
DISCUSSION: During the typical rat rotator cuff experimental period (animal mass, 400-600 g), muscle mass will increase by 30%, fiber length will increase by 7%, and PCSA will increase by 27%, but sarcomere lengths are nearly constant. Therefore, these normal growth-induced changes in architecture must be considered when muscle atrophy or fiber shortening is measured after rotator cuff tears in this model.

Full text (pdf)