1Center for the Advanced Study of Hominid Paleobiology, George Washington University, 2Unit of Human Anatomy and Embryology, University of Barcelona, 3Department of Anatomy, Howard University
March 26, 2015 8:00, Grand Ballroom E/F/G
The evolution of primate locomotor strategies is a subject of considerable debate. Bones survive into the fossil record but muscles are critical for producing locomotor behavior. It is therefore essential to understand how muscle anatomy varies with locomotion. This study adds to the critically low sample sizes of previous work, integrating anatomical data with muscle physiology. We hypothesized that muscles requiring high force-generating capacity due to locomotor function would exhibit high PCSA values. Additionally, postural muscles are predicted to have higher proportions of Type I slow-twitch myosin than muscles responsible for short-burst high force actions, which would have more myosin Type II concentration.
We dissected out six appendicular muscles from nine primates, including great apes and monkeys representing a range of locomotor modes. Mass, fascicle length and pennation angle were measured in each muscle to calculate physiological cross-sectional area (PCSA). Furthermore, biopsies from chimpanzee, gorilla, gibbon and grivet muscles were subject to mRNA extraction to assay myosin isoforms.
Results support the myosin type hypothesis. For example, the long head of the triceps brachii expresses higher concentrations of Type I myosin in brachiators (70.5%) than quadrupeds (56.2%). Triceps brachii also exhibits the highest PCSA value in all samples, demonstrating this muscle’s importance across species, including brachiators. In contrast, the quadrupeds exhibit both high PCSA values and Type II concentration in the gastrocnemius and the soleus. These results indicate that muscle architecture varies with locomotor mode and that these relationships may be useful for contextualizing activity-dependent changes observed in hominin fossil bone.
Funding support was provided by the National Science Foundation (Graduate Research Fellowship), NSF-IGERT 0801634 and the Warren Foundation.