Validation of a physical activity questionnaire to measure the effect of mechanical strain on bone mass

Most of the questionnaires available to estimate the daily physical activity levels of humans are based on measuring the intensity of these activities as multiples of resting metabolic rate (METs). Metabolic intensity of physical activities is the most important component for evaluating effects on cardiopulmonary fitness. However, animal studies have indicated that for effects on bone mass the intensity in terms of energy expenditure (metabolic component) of physical activities is less important than the intensity of mechanical strain in terms of the forces by the skeletal muscles and/or the ground reaction forces. The physical activity questionnaire (PAQ) used in the Amsterdam Growth and Health Longitudinal Study (AGAHLS) was applied to investigate the long-term effects of habitual physical activity patterns during youth on health and fitness in later adulthood. The PAQ estimates both the metabolic components of physical activities (METPA) and the mechanical components of physical activities (MECHPA). Longitudinal measurements of METPA and MECHPA were made in a young population of males and females ranging in age from 13 to 32 years. This enabled evaluation of the differential effects of physical activities during adolescence (13-16 years), young adulthood (21-28 years), and the total period of 15 years (age 13-28 years) on bone mineral density (BMD) of the lumbar spine, as measured by dual-energy X-ray absorptiometry (DXA) in males (n = 139) and females (n = 163) at a mean age of 32 years. The PAQ used in the AGAHLS during adolescence (13-16 years) and young adulthood (21-28 years) has the ability to measure the physical activity patterns of both genders, which are important for the development of bone mass at the adult age. MECHPA is more important than METPA. The highest coefficient of 0.33 (p < 0.01) was between MECHPA measured over the total period of 15 years (13-28 years) and lumbar BMD at age 32 years. Only during adolescence (12-16 years) was METPA more important with regard to lumbar BMD at age 32 years, with a beta of 0.21 (p < 0.01). The relative validity of the PAQ was established by comparing PAQ scores during four annual measurements in 200 boys and girls with two other objective measures of physical activity: movement counters (pedometers) and heart rate monitoring. These showed significant (p < 0.01) correlations in both genders, varying between 0.16 and 0.20. The small variation indicates, however, that all three instruments measure different aspects of physical activity. The results from the PAQ, with respect to MECHPA, validated in humans the results from animal studies in which bone adaptation during skeletal growth and development continuously adjust skeletal mass and architecture to changing mechanical stimuli caused by physical activity.