Over the past two centuries there have been documented secular changes in stature, weight, body proportions, and skeletal maturation rates in the United States. These changes along with a more sedentary lifestyle are likely reflected in femur morphology. Here we examine secular changes in diaphyseal cross-sectional size, shape, area, robusticity, and rigidity at midshaft and subtrochanteric of the femur using 395 adult White females and males from the United States born between the 1850s and the 1970s. The effect of secular change was controlled for an age effect. We also examine the relationship between femur length (proxy for stature) and femur head diameter (proxy for body weight) on measurements of diaphyseal size and biomechanical properties. The femur morphology of Americans born in the 20 th century reflects the combination of changes in stature, body build, and activity levels. Both sexes show significant changes in femur midshaft shape due primarily to a decrease in the mediolateral diameter. There are no significant changes at subtrochanteric in size or biomechanical properties in either sex after controlling for age variation. The results suggest that the change in femur midshaft shape are primarily associated with a decrease in activity. The stability of the subtrochantericPre-print version. Visit http://digitalcommons.wayne.edu/humbiol/ after publication to acquire the final version.dimensions and femur anteroposterior diameter may reflect a combination of decreased activity with a corresponding increase in femur length (moment arm) and a decrease in body breadth.Keywords secular trend, mechanical loading, femur, sexual dimorphism, platymeric, pilastricThere is a significant body of literature demonstrating that long bones such as the femur respond to mechanical loading by altering diaphyseal geometry and structure (Pearson and Lieberman 2004). However, lower limb bone biomechanical structural properties (diaphyseal size, shape, robusticity, and strength) are affected by the interaction of numerous factors including mechanical usage, body mass, body shape, and bone length (Ruff 1984, Agostini and Ross 2011, Demes et al. 1991, Gruss 2007, Moore 2009, Pearson and Lieberman 2004, Ruff and Larsen 2014, Wescott 2006, 2014. In many industrialized populations, especially the United States, there have been significant secular trends in biological variables such as stature, weight, body proportions, long bone lengths, and skeletal maturation primarily due to better nutrition and health (Fogel 2004, Wescott and Jantz 2005, Fredriks et al. 2000, Danubio and Sanna 2008, Driscoll 2010, Floud et al. 2011, Jantz and Meadows Jantz 2000, Wescott and Jantz 2005, Godina 2011, Harrington and Wescott 2015. There has also been a significant decrease in daily physical activity levels due to technological advances in transportation and in leisure-time activities resulting a more sedentary lifestyle. (Dollman et al. 2005, Nelson et al. 2006, Sandercock et al. 2010, Sigmundova et al. 2011, Jekauc et al. 2012, S...