Biomechanical models and simulations of musculoskeletal function rely on accurate muscle parameters, such as muscle masses and lines of action, to estimate force production potential and moment arms. These parameters are often obtained through destructive techniques (i.e., dissection) in living taxa, frequently hindering the measurement of other relevant parameters from a single individual, thus making it necessary to combine multiple specimens and/or sources. Estimating these parameters in extinct taxa is even more challenging as soft tissues are rarely preserved in fossil taxa and the skeletal remains contain relatively little information about the size or exact path of a muscle. Here we describe a new protocol that facilitates the estimation of missing muscle parameters (i.e., muscle volume and path) for extant and extinct taxa. We created three-dimensional volumetric reconstructions for the hindlimb muscles of the extant Nile crocodile and extinct stem-archosaur Euparkeria, and the shoulder muscles of an extant gorilla to demonstrate the broad applicability of this methodology across living and extinct animal clades. Additionally, our method can be combined with surface geometry data digitally captured during dissection, thus facilitating downstream analyses. We evaluated the estimated muscle masses against physical measurements to test their accuracy in estimating missing parameters. Our estimated muscle masses generally compare favourably with segmented iodine-stained muscles and almost all fall within or close to the range of observed muscle masses, thus indicating that our estimates are reliable and the resulting lines of action calculated sufficiently accurately. This method has potential for diverse applications in evolutionary morphology and biomechanics.

Musculoskeletal computer models allow us to quantitatively relate morphological features to biomechanical performance. In non‐human apes, certain morphological features have long been linked to greater arm abduction potential and increased arm‐raising performance, compared to humans. Here, we present the first musculoskeletal model of a western lowland gorilla shoulder to test some of these long‐standing proposals. Estimates of moment arms and moments of the glenohumeral abductors (deltoid, supraspinatus and infraspinatus muscles) over arm abduction were conducted for the gorilla model and a previously published human shoulder model. Contrary to previous assumptions, we found that overall glenohumeral abduction potential is similar between Gorilla and Homo. However, gorillas differ by maintaining high abduction moment capacity with the arm raised above horizontal. This difference is linked to a disparity in soft tissue properties, indicating that scapular morphological features like a cranially oriented scapular spine and glenoid do not enhance the abductor function of the gorilla glenohumeral muscles. A functional enhancement due to differences in skeletal morphology was only demonstrated in the gorilla supraspinatus muscle. Contrary to earlier ideas linking a more obliquely oriented scapular spine to greater supraspinatus leverage, our results suggest that increased lateral projection of the greater tubercle of the humerus accounts for the greater biomechanical performance in Gorilla. This study enhances our understanding of the evolution of gorilla locomotion, as well as providing greater insight into the general interaction between anatomy, function and locomotor biomechanics.

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.

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