Effects of Selective Breeding for Voluntary Exercise, Chronic Exercise, and Their Interaction on Muscle Attachment Site Morphology in House Mice
Authors: Castro Alberto A. , McClednon Holland E. , Trivedi Aayushi P. , Schwartz Nicolas L. , Garland Theodore 
Published: April 2020

Skeletal muscles attach to bone at their origins and insertions, and the interface where tendon meets bone is termed the muscle attachment site (or enthesis). Muscle attachment sites function to anchor muscles/tendons to bone and to dissipate stress (force per unit area) resulting from force transmission. Mechanical stresses at the muscle/tendon‐bone interface are proportional to surface area of the bony attachment sites, so a larger muscle attachment site will distribute loads over a wider surface area. Muscles that are frequently active and/or are of larger size should cause muscle attachment sites to hypertrophy (training effect), so paleontological studies have often used characteristics of attachment sites as indicators of locomotor behavior.In studies of humans, muscle attachment sites have been used to infer physical activity of fossil specimens, with larger attachment sites assumed to indicate larger muscles and/or habitual use. For example, muscle attachment site morphology was examined in prehistoric Andean fishing and agricultural populations. From this data, anthropologists concluded that the fishing population had more muscular upper bodies while the agricultural population had more muscular lower bodies. Such differences in muscle attachment site morphology have been attributed to differences in physical activity levels and loading history of limb bones experienced by individuals during their lifetime. However, bone performance during loading is governed by both genetic factors and loading history, but also by age, sex, and epigenetic factors. Thus, controversy arises when paleontologists attempt to infer the past loading history (i.e., activity levels) of individuals from bone morphology.We studied 4 replicate lines of high runner (HR) mice that had been selectively bred for 57 generations based on the average number of wheel revolutions on days 5 & 6 of a 6‐day period of wheel access. Four additional lines have been bred without regard to wheel running and serve as controls (C). On average, mice from HR lines voluntarily run ~3 times more than C mice on a daily basis. We housed 100 mice with or without wheels for 12 weeks starting at weaning. We quantified the evolved differences in muscle attachment site morphology between HR and C mice as well as plastic changes resulting from chronic exposure to exercise by measuring the cortical cross‐sectional areas, linear dimensions, and total surface area for muscles that attach on the femur and humerus. Sites chosen for subsequent analysis included the femoral third trochanter, an insertion point for the quadratus femoris, and the humerus deltoid crest, an insertion point for the spinodeltoideus, superficial pectoralis, and acromiodeltoideus.With body mass as a covariate, HR mice had significantly larger humerus deltoid tuberosities than C, with no significant training effect. Morphology of the femoral third trochanter did not differ between HR and C mice and was unaffected by chronic wheel access. Our results suggest that muscle attachment site morphology is largely determined by genetic background rather than exercise during ontogeny.3D Model of the Femoral Third Trochanter of a MouseFigure 1