The suggestion from our results that moderate activity levels induce a greater skeletal response compared with light activity as defined by movement frequency and intensity is consistent with previous observations of a positive relationship between strain rate and periosteal bone formation in animal models.(27) However, strain rate was positively related to bone formation across the physiological dose range in the latter studies, with which our observation that vigorous activity elicited a weaker skeletal response compared with moderate activity would seem to be at odds. A likely explanation for this apparent discrepancy is that our accelerometer recordings may have misclassified a substantial proportion of vigorous as moderate activity. For example, activity recordings were averaged out over 1-minute epochs, which may have reduced the detection of short bursts of vigorous activity sustained for 2.5 Hz to exclude nonhuman movement, which may have limited the detection of certain high-intensity activities. The observation in our previous calibration study that playing hopscotch is categorized as moderate activity in contrast to jogging, which is classified as vigorous activity (CG Mattocks, S Leary, AR Ness, K Deere, J Saunders, J Kirkby, K Tilling, SN Blair, C Riddoch, unpublished data, 2006), despite the fact that hopscotch consists of intermittent high-impact jumping, is consistent with this interpretation. In contrast, upper limb bone mass showed a greater response to vigorous compared with moderate activity, which may reflect a stronger tendency for activities classified as vigorous, such as jogging, to involve the upper limbs.