The bone response to mechanical loading is not yet fully understood. Optimal doses of skeletal loading throughout life may reduce the risk of osteoporosis in the later years. There remains a need to determine what form and amount of mechanical loading comprises an optimal dose. Weight-bearing exercise is an obvious method of exposing the skeleton to mechanical strain, however, issues of practicality and compliance often mar its effectiveness. The aim of this project was to further explore the effect of physical activity on bone. In particular, we set out to determine the effectiveness of an exercise intervention applying previously identified relevant load parameters that could be simply implemented on a community level in order to deliver the greatest widespread impact on osteoporosis incidence.

Measuring bone-relevant physical activity

Current methods of measuring physical activity tend to reflect cardiovascular or metabolic load rather than osteogenic potential. Thus, the first initiative of the project was to develop a physical activity measurement tool of particular relevance to bone. The bone-specific physical activity questionnaire (BPAQ) was constructed and tested for its ability to predict parameters of bone strength in 40 healthy adults in comparison to several common measures of physical activity (e.g. questionnaires, diaries and pedometers) (Chapter 3). In analysing the BPAQ, particular emphasis was placed on biomechanical components of the loads imposed on the skeleton by physical activity. For this reason, we measured the ground reaction forces produced during a large variety of exercises to incorporate into algorithms used to score the BPAQ. We found that the BPAQ was capable of predicting parameters of bone strength at clinically relevant regions of the skeleton in healthy men and women, while other common measures were not.

Influence of physical activity and maturity on bone strength

Current research suggests that the most efficacious approach to reducing the risk of osteoporotic fracture in later life may be to maximise peak bone mass;​ a state that is largely achieved by the age of 20. Thus, early exercise intervention has become a target in the prevention of osteoporosis. Knowledge of the salient factors that influence bone integrity during childhood and more specifically the adolescent years, however, is lacking. For this reason, we studied the relationship of physical activity, sedentary behaviour, and maturational status with bone strength parameters in 99 adolescent boys and girls (Chapter 4). We observed sex-specific differences in bone strength characteristics and discovered, that in this age group, level of participation in physical activity predicted bone strength strongly for boys, while maturational status was a better predictor of bone strength in girls.

Improving bone strength in adolescent boys and girls

While it is known that exercise exerts a positive influence on bone strength, the optimal type and dose that will maximise peak bone mass is unknown. The Preventing Osteoporosis With Exercise Regimes in Physical Education (POWER PE) study was designed to examine the effect of a practical exercise program designed to optimally load the skeleton on the development of bone during adolescence (i.e. prior to the attainment of peak bone mass). POWER PE was an eight-month, randomised, controlled, exercise intervention incorporated as a warm-up activity into the regular high school physical education classes of 81 Year 9 boys and girls (Chapter 5). Activities comprised high-intensity jumping designed to elicit positive bone and muscle responses. Subjects were examined for changes in parameters of bone and muscle strength. We found that children in the intervention group improved lean tissue mass and bone strength parameters at the hip, lumbar spine, heel and whole body significantly more than controls in a sex-specific manner.

Conclusion

In summary, the BPAQ appears to be a valid and useful measure of bone-relevant exercise history in comparison with other traditional measures of physical activity which are not sensitive to bone loading history. The influence of physical activity and physical maturity on bone accrual differs between adolescent boys and girls of the same chronological age. Eight months of brief, twice weekly, high intensity exercise loading of the skeleton incorporated into regular PE classes improves parameters of bone strength in adolescent boys and girls.