Digital muscle reconstructions have gained attraction in recent years, serving as powerful tools in both educational and research contexts. These reconstructions can be derived from various 2D and 3D data sources, enabling detailed anatomical analyses. In this study, we evaluate the efficacy of surface scans in accurately reconstructing the volumes of the rotator cuff and teres major muscles across a diverse sample of hominoids. Additionally, we investigate whether muscle origin area, as a dissection-based observation, can reliably predict muscle volume. Our findings reveal that surface scans provide sufficient coverage to accurately reproduce the in situ volumes of the rotator cuff muscles. However, the volume of the teres major was estimated less reliably, suggesting that muscles with less distinct skeletal boundaries may present challenges for accurate reconstruction. Future studies will explore whether such muscles can be reconstructed with greater precision. Furthermore, we identify a significant correlation between the origin area and muscle volume for the supraspinatus, infraspinatus, and subscapularis muscles. These results suggest that muscle origin area can serve as a reliable predictor of muscle volume, offering a skeletal indicator for estimating muscle size in both extant and extinct hominoids. These insights are particularly valuable for paleontological reconstructions, where direct soft tissue evidence is often lacking. By establishing a relationship between skeletal traits and muscle volume, our study provides a framework for evaluating the accuracy of soft tissue reconstructions in hominoid species. This approach not only enhances our understanding of hominoid anatomy but also offers new avenues for exploring the functional morphology of extinct taxa.

Chimpanzees are knuckle walkers, with forelimbs contacting the ground by the dorsum of the finger's middle phalanges. As these muscular apes are given to high-velocity motions, the question arises of how the ground reaction forces are buffered so that no damage ensues in the load-bearing fingers. In the literature, it was hypothesized that the finger flexors help buffer impacts because in knuckle stance the metacarpophalangeal joints (MCPJs) are strongly hyperextended, which would elongate the finger flexors. This stretching of the finger flexor muscle-tendon units would absorb impact energy. However, EMG studies did not report significant finger flexor activity in knuckle walking. Although these data by themselves question the finger flexor impact buffering hypothesis, the present study aimed to critically investigate the hypothesis from a biomechanical point of view. Therefore, various aspects of knuckle walking were modeled and the finger flexor tendon displacements in the load-bearing fingers were measured in a chimpanzee cadaver hand, of which also an MRI was taken in knuckle stance. The biomechanics do not support the finger flexor impact buffering hypothesis. In knuckle walking, the finger flexors are not elongated to lengths where passive strain forces would become important. Impact buffering by large flexion moments at the MCPJs from active finger flexors would result in impacts at the knuckles themselves, which is dysfunctional for various biomechanical reasons and does not occur in real knuckle walking. In conclusion, the current biomechanical analysis in accumulation of previous EMG findings suggests that finger flexors play no role in impact buffering in knuckle walking.