Shoulder to Shoulder
Dr. Susan Larson shares her research on primate scapular morphology with the NYCEP, Hunter College community
For any novice to physical anthropology the expanse of bones, their features, muscles, species, and functions can be overwhelming. Once the basics are down pat, graduate students then struggle to understand the complexities of individual joints, integrated units, and the interaction of each morphological unit within any given taxon. As a current graduate student it is both intimidating and inspiring to hear an advanced researcher describe these complexities not only in one – but many – species.
Dr. Susan Larson (Professor, Stony Brook University), who has dedicated much of her career to studying the shoulders of primates, gave a talk to the Hunter College Physical Anthropology Discussion Group on November 14, 2014. The room was filled with an eclectic audience composed of academics and non-academics alike: undergraduates, graduate students, faculty, and interested members of the public.
In her talk, Revisiting Functional Interpretations of Primate Scapular Morphology, she shared her most recent research incorporating functional, electromyography (EMG), and morphological data of the rotator cuff musculature into considerations of scapular shape across a wide sample of primates.
Interestingly, Dr. Larson’s study found that the rotator cuff muscles were changing in their fiber pinnation angles (orientation of muscle fibers extending out like vanes on a feather) instead of their size based on demands associated with locomotor behavior. Increased pinnation angle allows more muscle fibers, and therefore force output, without expanding the muscle itself. Through changing the architecture of the muscle instead of the volume, species are able to build up necessary muscles for their given locomotor repertoire without changing scapular morphology.
These results indicate that scapular shape was a constraint for the muscles, forcing a change in the architecture of the fibers in response to increased muscular needs. Why? Dr. Larson concluded that the overall scapular shape was predominately related to the leverage of the muscles involved in scapular rotation. The final stages of arm elevation is done by rotating the scapula itself, not by the action of the shoulder muscles.
This complex study and detailed presentation by Dr. Larson was incredibly insightful for me as a graduate student interested in the evolution of the upper limb. Specifically, the importance of considering both hard (bone) and soft (muscle) tissues when studying functional morphology. The body has many morphological options when engaging in a given behavior. In order to understand the evolution of a particular morphological unit it is important to incorporate multiple lines of evidence including consideration of the broader functions of a particular bone. These results highlighted the complexity of the shoulder joint specifically, but also exposed the numerous questions that need to be asked and answered with a variety of data to fully understand functional morphologies and evolutionary change.
Many members of the audience, NYCEP affiliated and not, had questions following Dr. Larson’s presentation. A lively discussion ensued about the applicability to other, non-anthropoid primate groups, future directions, and details about the methodology behind her muscle data collection. Both sides of the discussion seemed to benefit through exchanging ideas from different intellectual backgrounds. Such discourse is important for a fruitful academic community, inspiring future research projects and collaborations. Specifically at NYCEP, opportunities to bring in outside scientists supports the variety of research interests of the students and faculty.
With many NYCEP students pursuing questions in biomechanics, functional morphology, and soft tissue evolution we truly gained a new perspective on these topics. I join the Physical Anthropology Discussion Group, the Hunter College Department of Anthropology, and NYCEP in thanking Dr. Larson for sharing and explaining her research results. Many of my NYCEP colleagues and I benefitted greatly from personal conversations with Dr. Larson at Hunter College and during dinner thereafter. She gave us all advice and guidance in moving forward with projects related to the hard and soft tissue morphology of primate upper limbs.
Many of the results presented in Dr. Larson’s talk are discussed in her most recent publication here.
The Physical Anthropology Discussion Group meets bi-weekly at Hunter College.
Kristen is a Ph.D. student at CUNY/NYCEP concentrating on the evolution of the upper limb in the genus Homo