There is a strong positive relationship between physical activity and bone health throughout life. This is because the musculoskeletal system is sensitive to weight-bearing physical activity as a stimulus to maintaining and building bone, as well as for the prevention of muscle wasting. There is evidence that exercise, especially during childhood and adolescence, may influence bone structure and geometry, such as greater diameter of bones and stronger trabecular architecture, which may reduce the risk of fracture later in life [1]Bass, S.L., et al., The effect of mechanical loading on the size and shape of bone in pre-, peri-, and postpubertal girls: a study in tennis players. J Bone Miner Res, 2002. 17(12): p. 2274-80.

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Being active benefits bone and muscle strength regardless of age [2]Bonaiuti, D., et al., Exercise for preventing and treating osteoporosis in postmenopausal women. Cochrane Database Syst Rev, 2002(3): p. CD000333.

[3]Feskanich, D., W. Willett, and G. Colditz, Walking and leisure-time activity and risk of hip fracture in postmenopausal women. JAMA, 2002. 288(18): p. 2300-6.

 as increased muscle strength leads to an improved maintenance of bone mass by stimulating bone formation and decreasing bone resorption, which reduces fracture risk, as well as improving confidence and coordination, which contributes to the prevention of falls [4]Girgis, C.M., Integrated therapies for osteoporosis and sarcopenia: from signaling pathways to clinical trials. Calcif Tissue Int, 2015. 96(3): p. 243-55.

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Clinical studies that compare individuals who exercise with groups who do not have demonstrated significantly higher bone mineral density (BMD) in those who exercise regularly [5]Kemmler, W., et al., Benefits of 2 years of intense exercise on bone density, physical fitness, and blood lipids in early postmenopausal osteopenic women: results of the Erlangen Fitness Osteoporosis Prevention Study (EFOPS). Arch Intern Med, 2004. 164(10): p. 1084-91.

. Exceptions occur with high-intensity non-weight-bearing activities, such as swimming, and in amenorrhoeic athletes (hormonal changes due to high-intensity sportive activities), who may have a BMD similar or worse than controls. Measurable differences in fracture risk are also observed between those habitually active (non-athlete) and sedentary individuals [6]Pfeifer, M., et al., Musculoskeletal rehabilitation in osteoporosis: a review. J Bone Miner Res, 2004. 19(8): p. 1208-14.

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Studies show a beneficial association between greater lifetime physical activity and preservation of BMD, as well as a lower risk of hip, humerus and vertebral fracture, in older age [3]Feskanich, D., W. Willett, and G. Colditz, Walking and leisure-time activity and risk of hip fracture in postmenopausal women. JAMA, 2002. 288(18): p. 2300-6.

[7]Gregg, E.W., et al., Physical activity and osteoporotic fracture risk in older women. Study of Osteoporotic Fractures Research Group. Ann Intern Med, 1998. 129(2): p. 81-8.

. In stark contrast, immobilization of the skeleton (in the form of bed rest, casting or spinal cord injury) leads to bone loss, muscle wasting, and increased susceptibility to fracture within just a few weeks [8]LeBlanc, A.D., et al., Skeletal responses to space flight and the bed rest analog: a review. J Musculoskelet Neuronal Interact, 2007. 7(1): p. 33-47.

[9]Vico, L., et al., Effects of long-term microgravity exposure on cancellous and cortical weight-bearing bones of cosmonauts. Lancet, 2000. 355(9215): p. 1607-11.
 

. Immobilization causes more bone to be resorbed than formed. At the cellular level this translates to an increase in osteoclastic resorption linked to a decline in osteoblastic formation [10]Pocket Reference to Osteoporosis, S. Ferrari, Roux, C., Editor 2019, Springer International Publishing

. Read more about bone biology and pathophysiology of osteoporosis.

While evidence from large trials that test exercise in the prevention of fractures is lacking, several studies confirm a benefit of exercise on BMD, muscle strength, and the prevention of falls [2]Bonaiuti, D., et al., Exercise for preventing and treating osteoporosis in postmenopausal women. Cochrane Database Syst Rev, 2002(3): p. CD000333.

[11]Howe, T.E., et al., Exercise for preventing and treating osteoporosis in postmenopausal women. Cochrane Database Syst Rev, 2011(7): p. CD000333.

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Based on these studies [2]Bonaiuti, D., et al., Exercise for preventing and treating osteoporosis in postmenopausal women. Cochrane Database Syst Rev, 2002(3): p. CD000333.

, moderate to high intensity weight-bearing aerobic exercise (such as brisk walking, hiking, stair climbing or jogging), high intensity progressive resistance training (lifting weights) and high impact exercise (such as jumping or rope skipping) increase BMD by 1-4 % per year in pre- and postmenopausal women [12]Martyn-St James, M. and S. Carroll, Effects of different impact exercise modalities on bone mineral density in premenopausal women: a meta-analysis. J Bone Miner Metab, 2010. 28(3): p. 251-67.

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More vigorous exercise interventions seem to produce greater effects [12]Martyn-St James, M. and S. Carroll, Effects of different impact exercise modalities on bone mineral density in premenopausal women: a meta-analysis. J Bone Miner Metab, 2010. 28(3): p. 251-67.

. It should be noted that casual walking may not reduce fracture risk. However, a large cohort study supports a benefit of brisk walking on reducing the risk of hip fracture (more than 4 hours a week may reduce hip fractures by 41 % [3]Feskanich, D., W. Willett, and G. Colditz, Walking and leisure-time activity and risk of hip fracture in postmenopausal women. JAMA, 2002. 288(18): p. 2300-6.

). Additionally, jumping every day on one leg over 12 months has been linked to an increase in femoral neck cortical thickness [13]Karlsson, M.K., A. Nordqvist, and C. Karlsson, Physical activity, muscle function, falls and fractures. Food Nutr Res, 2008. 52.

 and strengthening back muscles has been shown to reduce the risk of vertebral fractures and kyphosis [14]Itoi, E. and M. Sinaki, Effect of back-strengthening exercise on posture in healthy women 49 to 65 years of age. Mayo Clin Proc, 1994. 69(11): p. 1054-9.

[15]Sinaki, M., Critical appraisal of physical rehabilitation measures after osteoporotic vertebral fracture. Osteoporos Int, 2003. 14(9): p. 773-9.

[16]Sinaki, M., et al., Stronger back muscles reduce the incidence of vertebral fractures: a prospective 10 year follow-up of postmenopausal women. Bone, 2002. 30(6): p. 836-41.

[17]Sinaki, M. and S.G. Lynn, Reducing the risk of falls through proprioceptive dynamic posture training in osteoporotic women with kyphotic posturing: a randomized pilot study. Am J Phys Med Rehabil, 2002. 81(4): p. 241-6.

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Additionally, numerous studies have proven that individuals with better posture, better balance, and greater muscle power are less likely to fall and are therefore less likely to be injured.
Research show for women over 80 years old, an individually tailored exercise regimen that incorporates progressive muscle strengthening, training for balance, and a walking plan, can reduce the overall risk of falling by about 20%, and cut serious injury-sustaining falls by just over 30% [6]Pfeifer, M., et al., Musculoskeletal rehabilitation in osteoporosis: a review. J Bone Miner Res, 2004. 19(8): p. 1208-14.

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The balance aspect of this training is important. A study has revealed patients practicing Tai Chi, a Chinese exercise focused on balance, fall only half as much as their peers [18]Province, M.A., et al., The effects of exercise on falls in elderly patients. A preplanned meta-analysis of the FICSIT Trials. Frailty and Injuries: Cooperative Studies of Intervention Techniques. JAMA, 1995. 273(17): p. 1341-7.

[19]Wolf, S.L., et al., Reducing frailty and falls in older persons: an investigation of Tai Chi and computerized balance training. Atlanta FICSIT Group. Frailty and Injuries: Cooperative Studies of Intervention Techniques. J Am Geriatr Soc, 1996. 44(5): p. 489-97

. Tai Chi can also be beneficial for retarding bone loss in weight-bearing bones in early postmenopausal women [20]Chan, K., et al., A randomized, prospective study of the effects of Tai Chi Chun exercise on bone mineral density in postmenopausal women. Arch Phys Med Rehabil, 2004. 85(5): p. 717-22.

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