Exercise in Youth and Long-Term Effects on Bone and Joints
Abstract: Partly due to an ageing population, the number of osteoporosis-related fractures and osteoarthritis (OA)-related hip and knee arthroplasty procedures is increasing. The individual suffering and the burden on society for these conditions is immense. Genetic contribution for the variance in bone mineral density (BMD) is estimated at around 60–80%, and for OA 50%, leaving a considerable proportion to environmental factors of which exercise is known to influence both. Most intervention studies have shown exercise-induced BMD benefits in young, mid, and old age groups, though all have been short-term. Most studies that have followed changes in BMD with up to ten years of reduced activity level have shown a higher BMD loss than expected by age. Retrospective studies of fracture incidence and OA prevalence have been inconclusive. Little is known about the association between types of exercise in youth and prevalence of total hip arthroplasty (THA) or total knee arthroplasty (TKA) in old age. We evaluated bone traits in two studies. The first was a 39-year prospective cohort study that followed 46 active male athletes into a mean 29 years of retirement and 24 age-matched controls. The second used a cross-sectional mixed model design that included 329 measurements in 193 active and former male soccer players and 450 measurements in 280 controls, all aged between 18 and 85 years. The first study measured BMD by single-photon absorptiometry (SPA) on both occasions, and with dual energy X-ray absorptiometry (DXA), peripheral computed tomography (pQCT) and quantitative ultrasound (QUS) at follow-up. The second study used DXA. Fracture incidence was evaluated in two retrospective cohort studies, one including 397 former male soccer players and 1368 age-matched controls and the second 709 former male athletes in a variety of sports and 1368 age-matched controls. The prevalence of hip and knee OA and arthroplasty was also evaluated in the second study. In the first study we found no changes in relative leg BMD levels Δ Z-score 0.0 (95% CI –0.4, 0.4) in athletes from activity into retirement. At follow-up, former athletes still had 0.5 to 1.2 standard deviations (SD) higher BMD, bone size, and strength index than controls (all p<0.05). In the second study, total body BMD, leg BMD, and femoral neck area were 0.3 to 0.5 SD higher in 30 or more years retired athletes than in controls (all p<0.05). Former male athletes had a 50% RR 0.5 (95% CI 0.3, 0.9) and former soccer players had a 60% RR 0.4 (95% CI 0.2, 0.9) lower risk respectively of any fragility fracture incidence than controls (both p<0.05), and two times higher age-adjusted risk of a hip and/or knee OA OR 1.9 (95% CI 1.5, 2.3), as well as a hip and/or knee arthroplasty OR 2.2 (95% CI 1.6, 3.1). After adjustment for differences in BMI, occupational load, and previous soft tissue knee injury, there was no increased risk of knee OA in former impact athletes while the risk remained in non-impact athletes OR 3.2 (95% CI 1.5, 6.9). This thesis infers that exercise-associated benefits in BMD and bone size remain long-term after sports, and that former athletes are at lower fracture risk than controls. In contrast, the risk of hip and/or knee OA and THA and/or TKA is higher in former athletes than in controls. The higher risk of knee OA in former athletes is associated with a knee injury in impact but not in non-impact athletes.
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