Bone shaft bending strength index is unaffected by exercise and unloading in mice

Anthropologists frequently use the shaft bending strength index to infer the physical activity levels of humans living in the past from their lower limb bone remains. This index is typically calculated as the ratio of bone shaft second moments of area about orthogonal principal axes (i.e. I_max/I_min). Individuals with high I_max/I_min values are inferred to have been very active, whereas individuals with low values are inferred to have been more sedentary. However, there is little direct evidence that activity has a causal and predictable effect on the shaft bending strength index. Here, we report the results of two experiments that were designed to test the model within which anthropologists commonly interpret the shaft bending strength index. In the first experiment, mice were treated daily with treadmill exercise for 1 month to simulate a high-activity lifestyle. In the second experiment, in an attempt to simulate a low-activity lifestyle, functional weight-bearing was removed from the hindlimbs of mice for 1 month. Femoral mid-shaft structure was determined with μCT. We found that while exercise resulted in significant enhancement of I_max and I_min compared with controls, it failed to significantly increase the I_max/I_min index. Similarly, stunted bone growth caused by unloading resulted in significantly diminished I_max and I_min compared with controls, but low activity did not lead to significantly decreased I_max/I_min compared with normal activity. Together, these results suggest that caution is required when the bone shaft bending strength index is used to reconstruct the activity levels of past humans.