Bone mass and exercise in adult premenopausal women

This thesis reports the results of three studies investigating the links between exercise and bone mass in premenopausal women at or beyond the age of peak bone mass. In the first study, bone mass and body composition, historical physical activity, calcium intake and menstrual status were measured in 43 premenopausal swimmers (Australian Union of Senior Swimmers - AUSSI) and 44 controls to test the proposition that swimming has a detrimental effect on bone. In the second study, 152 adult premenopausal women provided data to elucidate important determinants of current bone mass. The third study tested the ability of home-based “impact” exercise programs (~ one year) to improve bone mass in adult premenopausal women identified as having low bone mass for age at the hip (proximal femur), lumbar spine and/or ultradistal radius in the first two studies. Women were randomly assigned to exercise or control groups and subsequently pair-matched on the bases of age, regional bone mass and body mass index. Physical activity, calcium intake and menopausal status were determined retrospectively. The results of the first study indicated that endurance-style swimming in women averaging over two hours per week (current) and ~1.5 hours per week (lifetime) had no negative effect on the skeleton. This conclusion was based upon finding: no significant differences between swimmers and controls for bone mass at any site; significantly more instances of low bone mass in controls; swimmers in the upper quartile for swimming participation had higher bone mass than those in the lowest quartile despite over a four hour per week difference in mean swimming participation (previous three years). The second investigation indicated that lean mass was a stronger independent predictor of bone mass than fat mass, age, weight or body mass index though none produced correlations of more than moderate strength. In terms of Historical physical activity, “Very hard physical activity” (> seven METS) undertaken during the second, third and fourth decades, was the best independent predictor of current bone mass while grip strength was the best predictor of current bone mass of all the functional tests carried out. Maximum oxygen uptake (unadjusted for body weight) was a significant independent predictor of bone mass in the lower extremities. University staff volunteers (20) were used to design the impact exercise programs which involved either dropping from various heights with bilateral or single foot landings or arresting falls onto both or a single hand (forceplate). Two protocols were designed which targeted low bone mass at the hip and spine or the distal radius, both employing unilateral landings. Exercisers achieved significant improvements from baseline and against controls at each of the target-bone sites even with relatively low exercise compliance. It was concluded that this form of exercise is a safe and effective means of improving low bone mass site-specifically with loads of approximately four times body weight (hip and spine) and one body weight (radius)