Increased non-linear locomotion alters diaphyseal bone shape.

Comparative studies of vertebrate morphology that link habitual locomotor activities to bone structural properties are often limited by confounding factors such as genetic variability between groups. Experimental assessment of bone's adaptive response to altered activity patterns typically involves superimposing exercise onto a normal locomotor repertoire, making a distinction between qualitative changes to locomotor repertoires and quantitative increases in activity level difficult. Here, we directly tested the hypothesis that an increase in turning activity, without the application of exercise per se, will alter femoral cross-sectional shape. Thirty day-old female BALB/cByJ mice (n=10 per group) were single-housed for 8 weeks in custom-designed cages that either accentuated linear or turning locomotion or allowed subjects to freely roam standard cages. Consistent with a lack of difference in physical activity levels between groups, there were no significant differences in body mass, femoral length, midshaft cortical area, and individual measures of mediolateral (ML) and anteroposterior (AP) bending rigidity. However, the ratio of ML to AP diaphyseal rigidity, an indicator of cross-sectional shape, was significantly greater (P<0.05) in turning subjects than in linear or control subjects. Considering that across all groups mice were genetically identical and had equivalent levels of bone quantity and physical activity, differences in femoral shape were attributed to qualitative differences in locomotor patterns (i.e. specific locomotor modes). These data indicate that increased turning can alter distribution of bone mass in the femoral diaphysis, and that turning should be considered in efforts to understand form-function relationships in vertebrates.