DIFFERENTIAL EFFECT OF BIOMECHANIC AND ENERGETIC ASPECTS OF PHYSICAL ACTIVITY ON BONE MINERAL DENSITY IN 28 YEARS OLD MALES AND FEMALES 881

Abstract

It is widely accepted that physical activity has a beneficial effect on the development and maintenance of bone mineral density. Although positive effects of physical activity are often reported there are still uncertainties about the type, intensity, duration and frequency of these activities that are most effective for (re)modeling bone mass. In a longitudinal study we monitored daily physical activity from age 12 till age 29 in a group of 182 males and females. At the mean age of 28 years bone mineral density (BMD) was measured at 3 sites (lumbar region, femoral neck and proximal radius) by dual X-ray absorptiometry. Physical activity (PA) was scored in two ways: energetics by weighting the intensity (multiples of basic metabolic rate: MET's) and duration (minutes per week) of physical activities. biomechanics by weighting the peak strain (ground reaction forces as multiples of body mass) of physical activities, irrespective of their frequency and duration. The PA score was calculated over three time periods: adolescence (13-17 years), young adulthood (17-22 years) and adulthood (22-29 years). The results show that as the time period over which the PA measurements were taken, came closer to the BMD measurement, the more important became the biomechanical PA. For this biomechanical PA the explained variance of BMD increased from 2% during adolescence to 13% during adulthood. For energetic PA the explained variance on the other hand decreased from 6% during adolescence to 1% during adulthood. These relations were found for both sexes, but only for lumbar BMD and femoral neck BMD. For distal radius BMD there were (as expected) no significant relations with both PA scores. It can be concluded that physical activity measured as peak strain has in both sexes a better osteogenic effect than physical activity measured as metabolic rate.