In this study, we compare rotator cuff muscle architecture of typically used animal models to humans and quantify the scaling relationships of these muscles across mammals. The four muscles that correspond to the human rotator cuff: supraspinatus, infraspinatus, subscapularis, and teres minor, of 10 commonly studied animals were excised and subjected to a series of comparative measurements. When body mass among animals was regressed against PCSA (physiological cross sectional area), muscle mass, and normalized fiber length, the confidence intervals suggested geometric scaling but did not exclude other scaling relationships. Based on the architectural difference index, a combined measure of fiber length-to-moment arm ratio, fiber length-to-muscle length ratio, and the fraction of the total rotator cuff physiological cross-sectional area contributed by each muscle, chimpanzees were found to be the most similar to humans (ADI=2.15), followed closely by capuchins (ADI=2.16). Interestingly, of the eight non-primates studied, smaller mammals such as mice, rats, and dogs were more similar to humans in architectural parameters compared to larger mammals such as sheep, pigs, or cows. The force production vs. velocity trade-off (indicated by fiber length-to-moment arm ratio) and the excursion ability (indicated by fiber length-to-muscle length ratio) of humans was also most similar to primates, followed by the small mammals. Overall, primates provide the best architectural representations of human muscle architecture. However, based on the muscle architectural parameters of non-primates, smaller rather than larger mammals may be better models for studying muscles related to the human rotator cuff.

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