Comparison of the arm-lowering performance between Gorilla and Homo through musculoskeletal modeling

Objectives: Contrary to earlier hypotheses, a previous biomechanical analysis indicated that long-documented morphological differences between the shoulders of humans and apes do not enhance the arm-raising mechanism. Here, we investigate a different interpretation: the oblique shoulder morphology that is shared by all hominoids but humans enhances the arm-lowering mechanism. Materials and methods: Musculoskeletal models allow us to predict performance capability to quantify the impact of muscle soft-tissue properties and musculoskeletal morphology. In this study, we extend the previously published gorilla shoulder model by adding glenohumeral arm-lowering muscles, then comparing the arm-lowering performance to that of an existing human model. We further use the models to disentangle which morphological aspects of the shoulder affect arm-lowering capacity and result in interspecific functional differences. Results: Our results highlight that arm-lowering capacity is greater in Gorilla than in Homo. The enhancement results from greater maximum isometric force capacities and moment arms of two important arm-lowering muscles, teres major, and pectoralis major. More distal muscle insertions along the humerus together with a more oblique shoulder configuration cause these greater moment arms. Discussion: The co-occurrence of improved arm-lowering capacity and high-muscle activity at elevation angles used during vertical climbing highlight the importance of a strong arm-lowering mechanism for arboreal locomotor behavior in nonhuman apes. Therefore, our findings reveal certain skeletal shoulder features that are advantageous in an arboreal context. These results advance our understanding of adaptation in living apes and can improve functional interpretations of the hominin fossil record.

Julia van Beesel, John R. Hutchinson, Jean-Jacques Hublin, Stephanie Melillo