STUDY DESIGN: Controlled laboratory study.
OBJECTIVES: To measure the architectural properties of rat supraspinatus muscle after a complete detachment of its distal tendon.
METHODS: Supraspinatus muscles were released from the left humerus of 29 rats (Sprague-Dawley, mass 400 - 450 g) and animals were returned to cage activity for 2 weeks (n = 12), 4 weeks (n = 9), or 9 weeks (n=8) before euthanasia. Measurements of muscle mass, pennation angle, fiber bundle length (sarcomere number), and sarcomere length permitted calculation of normalized fiber length, serial sarcomere number, and physiological cross-sectional area (PCSA).
RESULTS: Coronal oblique sections of the supraspinatus confirmed surgical transection of the supraspinatus muscle at 2 weeks, with reattachment by 4 weeks. Muscle mass and length were significantly lower in released muscles at 2 weeks, 4 weeks, and 9 weeks. Sarcomere lengths in released muscles were significantly shorter at 2 weeks but not different by 4 weeks. Sarcomere number was significantly reduced at 2 and 4 weeks, but returned to control values by 9 weeks. The opposing effects of smaller mass and shorter fibers produced significantly smaller PCSA at 2 weeks, but PCSA returned to control levels by 4 weeks.
CONCLUSIONS: Release of the supraspinatus muscle produced early radial and longitudinal atrophy of the muscle. The functional implications of these adaptations would be most profound at early time points (particularly relevant for rehabilitation), when the muscle remains smaller in cross-sectional area and, due to reduced sarcomere number, would be forced to operate over a wider range of the length-tension curve and at higher velocities, all adaptations resulting in compromised force generating capacity. These data are relevant to physical therapy because they provide tissue-level insights into impaired muscle and shoulder function following rotator cuff injury.

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