Quantifying the relationship between mechanical loading and the skeletal response in pre- and early-pubertal girls

Abstract

PURPOSE Before exercise prescription for bone health can be recommended, the relationship between mechanical loading characteristics and the skeletal response need to be quantified. We asked i) does moderate impact exercise result in a greater gain in BMC than low impact exercise, ii) what are the loading characteristics associated with a moderate and low impact exercise program and does this differ from non-structured play?, and iii) does loading history affect the osteogenic response to a moderate or low impact program? METHODS Sixty-eight pre- and early-pubertal girls (aged 8.9 +/- 0.2 yrs) were randomized to take part in a moderate or low impact exercise program 3 times/wk for 8.5 mnths. The number and type of loads associated with the exercise classes and non-structured play (recess) were assessed from video footage. The magnitude of load was assessed using a pedar in-sole mobile system. Hours of moderate and high impact organized sport were assessed from a physical activity questionnaire. RESULTS The moderate and low impact exercise programs consisted of -400 impacts per class, but the jumping, hopping and dynamic activities performed during the moderate impact program produced forces ranging from 2 to 4 times body weight (BW) compared to -1 BW for the low impact program. Moderate impact exercise resulted in a 2.7% greater gain in BMC at the tibia compared to the low impact exercise. The moderate impact exercise program consisted of fewer low impacts (1-2 BW) and a higher number of moderate impacts (2-4BW) compared to those typically performed during non-structured play. There were greater gains in BMC in subjects participating in the moderate versus the low impact exercise programs who participated in 2 to 3 hours of moderate impact sports outside school (2.5% to 4.5%, p CONCLUSION Approximately 400 impacts ranging 2-4 BW, 3 times/wk was enough stimuli to result in an osteogenic response in normally active girls; even in those actively involved in moderate impact sports outside school.