Swan MA, Sato E, Galatz LM, Thomopoulos S, Ward SR.
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.
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).
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
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
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