Basic science
The effect of age on rat rotator cuff muscle architecture

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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.

Section snippets

Methods

We performed a cross-sectional study of muscle architectural dimensions across the rat lifespan. Thirty healthy Sprague-Dawley rats were euthanized under a protocol approved by the Institutional Animal Care and Use Committee of the University of California, San Diego. The animals were weighed and divided into 6 approximate age groups based on total body mass: approximately 3 weeks (51-70 g, n = 4), approximately 1 month (100-108 g, n = 4), approximately 2 months (229-263 g, n = 7),

Results

Muscle mass increased linearly with body mass in both the supraspinatus and infraspinatus muscles (Figure 2, Figure 3, A). However, normalized fiber bundle length increased nonlinearly (logarithmically) with total body mass (Figure 2, Figure 3, B). There was a weak relationship in the supraspinatus and no relationship in the infraspinatus between total body mass and sarcomere length, which remained constant at approximately 2.4 μm (Figure 2, Figure 3, C) in both muscles. The development of PCSA

Discussion

Our results suggest that the architecture of the rat rotator cuff, in general, develops proportionally with total body mass. In particular, the mass of the muscle dominates the PCSA, and therefore, we would expect that force-generating capacity increases linearly with body mass. Perhaps most interestingly, normalized muscle fiber bundle length increased nonlinearly with total body mass, allowing sarcomere length to be conserved. When these relationships were further explored, we observed that

Conclusion

The development of muscle architectural features in the rat rotator cuff muscles follows linear and nonlinear growth trajectories. By use of these data in the body mass range of 400 to 600 g, as frequently seen in experimental models of rotator cuff tendon injury, a 30% increase in supraspinatus and infraspinatus muscle mass was observed. Similarly, a 7% increase in fiber length, a 27% increase in PCSA, and nearly constant sarcomere lengths were observed. These data indicate that investigators

Disclaimer

The sources of funding for this study were as follows: NIH R01 AR057836, and NIH R24 HD050837.

The authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.

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