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An optimal model for prevention of osteoporotic fractures includes maximisation and maintenance of bone strength, and minimisation of trauma (18). Regular physical activity can contribute to each of these determinants, but with different outcomes throughout the lifespan. Physical activity is a determinant of peak bone mass (2, 41). Sufficient exercise during childhood and adolescence, particularly the prepubertal years, is more effective for increasing bone mass and strength than exercise in adulthood (3, 5, 13, 20, 26, 31). Whether benefits achieved before puberty are sustained into adulthood remains to be determined by appropriate longitudinal studies. Conversely, the primary benefit of exercise on the bones of adults is conservation, not acquisition (13, 17, 24, 27, 31, 38). In elderly individuals, exercise can reduce the rate of bone loss (33, 35, 37), and improved fitness and muscle strength contributes to prevention of falls (4, 14, 40) and a lower risk of fracture (7,15, 30).

Disuse results in a loss of bone mass from the skeleton (22, 23, 32). The minimum amount of activity needed to minimise such loss is unknown. Precise prescriptions of exercise in relation to osteoporosis must await the outcome of well-designed, longitudinal studies that include fracture efficacy as an outcome. Based on available evidence, general recommendations for physical activity can be made according to the goal of the activity program and the fracture risk of the individual. For example, asymptomatic individuals with normal BMD have a low risk of fracture and could be directed to more vigorous exercise to help maintain bone mass. Patients with osteoporosis and/or a history of atraumatic fracture are at high risk. There is no evidence that vigorous weight-bearing exercise will correct this condition, and it could theoretically cause more fractures (10, 11). In this group, modified physical activity will be necessary with a primary focus on minimising trauma, rather than building bone mass.

Exercise and Osteoporosis
Primary goals should focus on improved fitness, muscle strength, and posture. Physical inactivity, postural instability and muscle weakness are independent contributors to the risk of fracture (7, 29). Among older community-dwelling women, greater physical activity is associated with a lower risk for hip fractures (7, 15, 30). Moreover, improvements in muscular strength and endurance, balance and stability, reaction and movement time (19, 34, 36) decrease the predisposition to fall (4, 14, 29, 40). A combination of aerobic and resistance (strength training) exercises provides a balanced program which is safe (1). Activities such as walking, tai chi, dance routines or exercise tapes for 20 to 30 min, 2-3 times per week can improve fitness, muscle strength and balance6, 9, 19, 36). Free weights attached to the limbs, or rubber tubes attached to a secure object can be used for muscle training. For strength training, a single-set program of 8-10 exercises, performed a minimum of two times per week is recommended over multiple-set programs because it is less time-consuming, more efficient and produces most of the health and fitness benefits (12, 16, 27).

Targeted exercise programs have a greater impact than general programs for preventing falls (10, 14), and they can significantly improve the quality of life and level of daily function (25). To this end, postural exercises to increase back extensor strength, to correct forward head postures, and maintain and improve shoulder range of motion and trunk stability should be considered on an individual basis (9, 25). Individuals who are frail, severely kyphotic, or suffer from pain or poor balance may benefit from water exercise (hydrotherapy) or home-based activities of low intensity (36). Due to increased skeletal fragility, exercises should be chosen to avoid adverse events. Patients with a diagnosis of osteoporosis should avoid dynamic abdominal exercises (trunk flexion) and exercise that requires twisting, explosive, or staccato movements (11).

Exercise to Maximise or Maintain Bone Mass
In healthy adults, vigorous exercise programs and resistance training of moderate to high intensity can preserve bone density (17, 37), or result in modest (1-3%) increases in bone mineral content at clinically relevant sites (5, 8, 16, 24, 27, 35, 38). The positive effects of physical activity are site-specific (3, 20, 21), and may also depend on moderate to high calcium intakes (greater than or equal to 1000 mg/d) (28, 39), but this is yet to be proven in randomised controlled clinical trials. Weight-bearing physical activity is important for maintenance of bone mass (13, 31, 42), and activities that increase muscle strength are also safe and beneficial, particularly for bones of the upper limb (16, 21, 27). An optimal exercise program should include activities for increasing strength, balance, flexibility and coordination of the upper and lower limbs and trunk (1).

To influence BMD, physical activity undertaken 2-3 times per week and maintained for 20 to 60 minutes has been found to be helpful (16, 26, 27). Training intensities between 70 to 80% of functional capacity, or maximum strength (5, 8, 16, 27, 37) can preserve bone density, but it remains to be determined whether these are optimal for influencing BMD. Low intensity exercise such as walking has minimal effect on BMD (28). In adults, any skeletal benefit accrued from an exercise program will not be sustained if an individual returns to a sedentary lifestyle (8, 13).

Conclusion
In conjunction with advice to increase dietary calcium, exercise plays a significant part of a lifestyle prescription for reducing fractures in later life. In postmenopausal women, it is less effective than oestrogen for maintaining BMD (33), and should be regarded as part of an overall treatment strategy.

Acknowledgments
The position statement was prepared by Dr Mark R. Forwood PhD, Department of Anatomical Sciences, The University of Queensland and Judy A. Larsen MAPA, Hydrotherapy Consulting and Training, St Andrews Hydrotherapy Centre, Brisbane. Members of ANZBMS and the Medical and Scientific Advisory Board of Osteoporosis Australia read and commented on early drafts of the manuscript.

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