Foot arch height, arch stiffness, and intrinsic muscle size are not strongly associated with daily physical activity levels in a cross-sectional study of adults in the United States.

The feet of people in non-industrial societies often have higher, stiffer longitudinal arches (LAs) and larger intrinsic muscles than those of many people in post-industrial societies. The prevailing explanation for this phenomenon is that people in post-industrial societies commonly wear shoes that restrict foot mobility, while people in non-industrial societies are often habitually barefoot or minimally shod. However, people in post-industrial societies also tend to be less physically active than in non-industrial societies, and it is possible that this, too, is a major determinant of their foot form and function. Here, we test the hypothesis that among people in post-industrial societies, lower physical activity levels are associated with lower, less stiff LAs and smaller intrinsic muscles. In a cross-sectional analysis of 40 adults in the United States, none of whom were habitually barefoot or minimally shod, we measured daily physical activity using accelerometry, LA height and static stiffness using photography, LA dynamic stiffness using kinematic and kinetic data, and intrinsic muscle size using ultrasound. Using Bayesian models, we found very low probabilities of positive associations between physical activity (step count, time spent in moderate-to-vigorous activity) and LA height, LA static stiffness, and muscle size. For LA dynamic stiffness, we found small to moderate probabilities of positive associations with physical activity variables. These findings suggest that physical activity is unlikely a major determinant of variation in LA and intrinsic muscle form and function among post-industrial societies. It remains possible that physical activity affects LA and intrinsic muscle traits, but perhaps primarily among people who are habitually barefoot or minimally shod.