Development of a comparative chimpanzee musculoskeletal glenohumeral model: implications for human function

Abstract

Modern human shoulder function is affected by the evolutionary adaptations that have occurred to ensure survival and prosperity of the species. Robust examination of behavioral shoulder performance and injury risk can be holistically improved through an interdisciplinary approach that integrates anthropology and biomechanics. Coordination of these fields can allow different perspectives to contribute to a more complete interpretation of biomechanics of the modern human shoulder. The purpose of this study was to develop a novel biomechanical and comparative chimpanzee glenohumeral model, designed to parallel an existing human glenohumeral model, and compare predicted musculoskeletal outputs between the two models. The chimpanzee glenohumeral model consists of three modules – an external torque module, musculoskeletal geometric module, and an internal muscle force prediction module. Together, these modules use postural kinematics, subject specific anthropometrics, a novel shoulder rhythm, glenohumeral stability ratios, hand forces, musculoskeletal geometry, and an optimization routine to estimate joint reaction forces and moments, subacromial space dimensions, and muscle and tissue forces. Using static postural data of horizontal bimanual suspension task, predicted muscle forces and subacromial space were compared between chimpanzees and humans. Compared to chimpanzees, the human model predicted a 2mm narrower subacromial space, deltoid muscle forces that were often double those of chimpanzees and a strong reliance on infraspinatus and teres minor (60-100% maximal force) over other rotator cuff muscles. These results agree with previous work on inter-species differences that inform basic human rotator cuff function and pathology.

Anatomical and molecular analyses of the deltoid muscle in chimpanzees (Pan troglodytes) and modern humans (Homo sapiens): Similarities and differences due to the uses of the upper extremity

“…In conclusion, the different functional uses of the upper limb in chimpanzees and humans (McLean and Dickerson, 2020) moderately affect the anatomical characteristics of the deltoid but have no impact on the expression patterns of MHC isoforms. Of the differences proposed in our initial hypothesis, only those regarding the muscle architecture of the AD have been borne out by our study, with significantly higher nMFL values in humans, an adaptation to manipulative activities, and significantly higher nPCSA values in chimpanzees, an adaptation to both their locomotor behavior, especially vertical climbing and brachiation (Thorpe et al, 1999) and to manipulative activities, given the higher percentage of body mass located in their upper extremities…”

“…In chimpanzees, the AD had significantly higher aPCSA and nPCSA values (Figure 4), indicating a greater capacity of force generation in chimpanzees than in humans (Kikuchi, 2010; Michilsens et al, 2009; Thorpe et al, 1999; Zajac, 1992). This greater capacity could be related to the important role of the AD in different phases of brachiation and vertical climbing (Kikuchi et al, 2012; Larson & Stern, 1986; MacLean & Dickerson, 2020; Michilsens et al, 2009; Tuttle & Basmajian, 1978b) and in knuckle‐walking (Larson & Stern, 1987; Tuttle & Basmajian, 1978a). In addition, the greater capacity of force generation in the AD of chimpanzees may be related to its participation in the abduction of the glenohumeral joint during manipulative tasks (Larson & Stern, 1986), where chimpanzees need to generate a greater force than humans because 16% of their body mass is in the upper extremity, compared to only 9% in humans (McLean and Dickerson, 2020).…”

“…The anatomical structure of the deltoid as a whole is similar in chimpanzees and humans despite the different uses they give to the upper limb (Cartmill & Smith, 2009; Lewis et al, 2001; McLean and Dickerson, 2020; Veeger & Van der Helm, 2007). In fact, all primates of the Hominoidea superfamily have certain anatomical characteristics of the deltoid in common, including its high degree of development, the almost total fusion of its different portions, and its relatively distal insertion into the humeral diaphysis (Ashton & Oxnard, 1963; Dunham et al, 2016).…”

“…Unexpectedly, however, the greater participation of the upper limb in locomotor behavior in chimpanzees did not translate into significant differences in the expression patterns of MHC isoforms in any of the three portions of the deltoid (Table 2). The greater development of the upper limb muscles in chimpanzees (McLean and Dickerson, 2020; Myatt et al, 2012; Thorpe et al, 1999; Zihlman, 1992) would be expected to accompany an increase in type I muscle fibers, which mostly express the MHC‐I isoform (Scott et al, 2001), as occurs in the deltoid of human athletes involved in activities like weight lifting (Mandroukas et al, 2010; Tesch & Larsson, 1982). However, our analysis did not identify any significant differences in the expression patterns of MHC isoforms between humans and chimpanzees.…”

“…We hypothesized that the three portions of the deltoid in Pan troglodytes would present architectural and molecular parameters related to a greater capacity for force generation as an adaptation to knuckle‐walking, brachiation, and vertical climbing, while the three portions of the deltoid in Homo sapiens would present parameters related to a greater contraction speed that would facilitate the manipulative functions of the upper limb (McLean and Dickerson, 2020). Since we had access to a relatively large number of chimpanzee specimens, a rare opportunity in comparative anatomy studies of hominoid primates, and we were able to carry out the architectural and molecular analyses in the same specimens, we believe that our findings on the functional anatomy of the deltoid will enhance our knowledge both of human adaptations to manipulation and chimpanzee adaptations to locomotion.…”