Comparison of rotator cuff muscle architecture between humans and other selected vertebrate species

Abstract

In this study, we compare rotator cuff muscle architecture of typically used animal models with that of 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 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 (ADI), 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 with larger mammals such as sheep, pigs or cows. The force production versus 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 were also most similar to those of primates, followed by the small mammals. Overall, primates provide the best architectural representation 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.

Keywords: Architecture; Muscle; Rotator cuff.

Fig. 1.

Fiber length to moment arm ratio for each of the four muscles of the rotator cuff. Human data are represented by the solid line.

Fig. 2.

Fiber length to muscle length ratio for each of the four muscles of the rotator cuff. Human data are represented by the solid line, with dotted lines showing s.e.m.

Fig. 3.

Percent contribution of each muscle to total rotator cuff physiological cross-sectional area (PCSA): supraspinatus, infraspinatus, teres minor and subscapularis.

Fig. 4.

Combined architectural difference index (ADI) for all rotator cuff muscles. A perfect architectural match is an ADI of zero, so low values indicate greater similarity to humans. Humans are used as comparison, and therefore have no ADI (human ADI=0).

Fig. 5.

Individual architectural difference index for each rotator cuff muscle.

Fig. 6.

Log–log plot of the relationship between PCSA and body mass. The points of the line are nearly linear, demonstrating that the scaling of PCSA percentage with respect to animal mass is geometric. Scaling is nearly geometric.

Fig. 7.

Log–log plot of the relationship between muscle mass and body mass. Scaling is nearly geometric. The points of the line are nearly linear, demonstrating that the scaling of PCSA percentage with respect to animal mass is geometric.

Fig. 8.

Log–log plot of the relationship between normalized fiber length and body mass. Scaling is geometric with respect to animal body mass.

Fig. 9.

Representative supraspinatus muscle from each species studied. Images are superior views with the lateral/distal end of the muscle oriented toward the left. Scale bar for all muscles is 10 mm. Average body mass for each species was: mouse=0.03 kg, rat=0.57 kg, rabbit=3.16 kg, dog=17.73 kg, goat=54.09 kg, pig=55.75 kg, sheep=54.55 kg, cow=590.91 kg, capuchin=3.97 kg, chimpanzee=40.90 kg, human=73.41 kg.

Fig. 10.

Chimpanzee rotator cuff muscles. (A) Anterior and (B) posterior view of the chimpanzee scapula with musculature. Representative pennation angle measurements shown on chimpanzee shoulder muscles: (C) subscapularis, (D) infraspinatus, (E) teres minor and (F) supraspinatus.