Elevated activity levels do not influence extrinsic fiber attachment morphology on the surface of muscle‐attachment sites

Abstract

Extrinsic fibers (EFs) are a type of penetrating collagenous fiber, closely related to the periodontal ligament, which help anchor soft tissue into bone. These fibers are associated with muscle attachment sites (entheses). Their size and grouping patterns are thought to be indicative of the loading history of the muscle. EFs are of particular significance in anthropology as potential tools for the reconstruction of behavior from skeletal remains and, specifically, entheses. In this study, we used a mouse model to experimentally test how activity level alters the morphology of EF insertion sites on the bone surface of a fibrocartilaginous enthesis, the biceps brachii insertion. Further, we adapted surface metrological techniques from studies of dental wear to perform automated, quantitative and non-destructive analysis of bone surface histology. Our results show that experimentally increased activity had no significant effect on the quantity or density of EF insertions at the enthesis, nor on the size of those insertions. Although EF presence does indicate muscle attachment, activity did not have an observable effect on EF morphology. Exercise and extrinsic fiber insertion, C. M. Turcotte et al. 828 Exercise and extrinsic fiber insertion, C. M. Turcotte et al. 831

Muscle attachment sites and behavioral reconstruction: An experimental test of muscle‐bone structural response to habitual activity

“…Interestingly, both Zumwalt (2006) and Rabey et al (2015) observed a positive effect of exercise on bone formation rate, suggesting that a bone's internal microstructure may be more informative than macroscopic surface observations. However, analysis of the distribution and morphology of extrinsic fibers, the collagenous fibers directly anchoring muscle into the bone, demonstrated no clear variation between experimental activity groups in mice also used in the present study (Turcotte et al, 2020).…”

“…When considering internal structure, Djukic et al (2015) found no link between trabecular or cortical bone architecture and surface morphology at enthesis sites of the lower limb in human archaeological samples. But this would be expected if surface morphology is not responsive, in a straightforward manner, to the local strain environment of the individual enthesis as the experimental studies suggest (Rabey et al, 2015;Turcotte et al, 2020;Wallace et al, 2017;Zumwalt, 2006).…”

“…,Rabey et al (2015; mice),Turcotte et al (2020; mice), andWallace et al (2017; turkeys), considered model organisms but found no effect of activity level on the surface complexity or linear dimension of muscle attachment sites. In contrast,Karakostis et al (2019, b) was able to distinguish individuals of different activity level groups when considering pooled 3D surface morphologies from several entheses using multivariate methods.…”

Evaluating bony predictors of bite force across the order Carnivora

“…The quantification of muscle attachment sites has been widely used to estimate muscle size and function in archeological populations, even serving as a basis from which to infer occupational intensity (Karakostis et al, 2017) or the practice of specific activities such as harpoon launching (Hawkey & Merbs, 1995). However, neither direct measurements of attachment size (e.g., see Hawkey & Merbs, 1995;Henderson, 2013;Henderson et al, 2013;Milella, 2014) nor calculations of optical complexity (Wallace et al, 2017;Zumwalt, 2006) are supported consistently by experimental studies (see Rabey et al, 2015;Turcotte et al, 2020;Wallace et al, 2017;Williams-Hatala et al, 2016;Zumwalt, 2006). For example, Rabey et al (2015) demonstrated a clear division in the mass and internal architecture of the F I G U R E 4 Phylogenetic generalized least squared (PGLS) regressions of residuals of bony proxies against masseter mass (left) and masseter PCSA (right), following adjustment for body mass.…”

“…Moreover, muscle attachment areas have been shown to be of varying value for predicting muscle size and strength within other skeletal regions. While correlations between muscle size/force and entheseal areas have been questioned in several postcranial regions (e.g., Turcotte et al, 2020; Wallace et al, 2017; Zumwalt, 2006), some morphometric approaches have suggested a relationship between muscle force potential (i.e., PCSA) and external bony morphology (Harbers, Neaux, et al, 2020; Harbers, Zanolli, et al, 2020). Thus, the predictive value of muscle attachment sites for the estimation of myological properties remains contentious.…”