Item

Abstract

Objectives:Contrary to earlier hypotheses, a previous biomechanical analysis indi-cated that long-documented morphological differences between the shoulders ofhumans and apes do not enhance the arm-raising mechanism. Here, we investigate adifferent interpretation: the oblique shoulder morphology that is shared by all homi-noids but humans enhances the arm-lowering mechanism.Materials and methods:Musculoskeletal models allow us to predict performancecapability to quantify the impact of muscle soft-tissue properties and musculoskeletalmorphology. In this study, we extend the previously published gorilla shoulder modelby adding glenohumeral arm-lowering muscles, then comparing the arm-loweringperformance to that of an existing human model. We further use the models to dis-entangle which morphological aspects of the shoulder affect arm-lowering capacityand result in interspecific functional differences.Results:Our results highlight that arm-lowering capacity is greater inGorillathan inHomo. The enhancement results from greater maximum isometric force capacitiesand moment arms of two important arm-lowering muscles, teres major, andpectoralis major. More distal muscle insertions along the humerus together with amore oblique shoulder configuration cause these greater moment arms.Discussion:The co-occurrence of improved arm-lowering capacity and high-muscleactivity at elevation angles used during vertical climbing highlight the importance of astrong arm-lowering mechanism for arboreal locomotor behavior in nonhuman apes.Therefore, our findings reveal certain skeletal shoulder features that are advanta-geous in an arboreal context. These results advance our understanding of adaptationin living apes and can improve functional interpretations of the hominin fossil record