The relationship of physical activity to bone mineral content in postmenopausal women

The purpose of this study was to compare the bone mineral content (BMC) of two groups of healthy postmenopausal women. The two groups differed on their physical activity profile as established by the Simons-Morton scale. Subjects (33) were French Canadian, resident in the same geographical area and aged between 60 and 75 years. Subject selection criteria were as follows: no history of oestrogen supplements, osteoporosis, premature menopause, no medication related to bone metabolism and exemption from factors predisposing to secondary osteopathy. Subjects were categorized as very active (VA) or sedentary (S) based on their physical activity profile established over a 6-month period. The VA group was composed of 20 women (62.1+/-2.8) and the S group of 13 other females (68.0+/-4.7). Subject intake of calcium, coffee and alcohol was evaluated. BMC of the lumbar spine and wrist were measured by dual and single photon absorptiometry. In order to validate the different physical activity levels, general fitness parameters including functional exercise capacity, abdominal and arm muscle endurance and forearm strength were also measured. A significant difference (p<0.018) was observed between mean values measured at the lumbar spine between the VA group (0.76 g+/-0.09) and the S group (0.65+/-0.12) g hydroxyapatite per cm(2). However no significant difference was found between the VA and the S group for the wrist measure, with mean values of 34.0 unit length (UL)+6.3 and 31.3 UL+/-6.4 respectively. The observations that the subjects of the VA group were only involved in weight-bearing aerobic activities which mainly exert a mechanical load to the axial skeleton and that no difference was observed between both groups for the forearm strength suggest that the positive effect of exercise on BMC is limited only to the bone structure submitted to sufficient mechanical forces.