Physical activity and musculoskeletal health

Population ageing is unprecedented and enduring. By 2050, the proportion of those aged over 65 would have increased from 13% to 26%. As the population ages, the prevalence of chronic diseases that disproportionately affect the elderly will markedly increase. Osteoarthritis and osteoporosis, including resulting fractures are two such diseases. Physical activity (PA) is an important risk factor for both of these diseases but many questions remain unanswered. This thesis examines how PA and exercise interacts with features of falls, osteoarthritis and osteoporosis. In a population based sample of 50-80 year olds (n=850) followed over 2.6 years, self reported functional ability and pain, and to a lesser extent stiffness (but not radiographic osteoarthritis) have modest but independent associations with physiological predictors of falls risk suggesting it is symptoms not radiographic changes that increase falls risk. In addition, ambulatory activity is positively associated with hip bone density in both sexes which appears most important in those aged over 65 years. However, the associations for spine bone density are both weaker and inconsistent. In a convenience sample of 26-61 year olds (n=325) followed over two years, we found knee cartilage volume and tibial plateau area are dynamic structures that can respond to physical stimuli. Greater muscle strength and endurance fitness, especially in women, protects against cartilage loss, but also results in a maladaptive enlargement of subchondral bone in both sexes, suggesting PA may have both good and bad effects on the knee. 1,434 children, aged 7-15 years, were measured in 1985 and approximately 20 years later. We found childhood fitness levels, particularly in females and in the early pubertal years, predicts adult bone mass, while BMI predicts bone mass in males only. These results suggest that increased skeletal loading in childhood leads to an increase in peak bone mass independent of current loading. In 183 children examined at age 8 and 16, bone mass measured by dual x-ray absorptiometry (DXA) is a good predictor of upper limb fracture risk during puberty. DXA measures track moderately to strongly from childhood to adolescence. Tracking is independent of linear growth and sex indicating bone mineralisation and growth are under largely separate mechanistic control. Body composition is the main predictor of altered tracking but environmental factors, such as having been breastfed, sports participation, fitness and inhaled corticosteroid use also appear important. In conclusion, this series of related studies shed considerable insight onto the role that PA and exercise play in preventing osteoarthritis and osteoporosis. In particular, childhood appears the most opportune time to prevent osteoporosis but later life is also important while for osteoarthritis, results remain less certain for structural change.