Osteoporosis is a disorder in which bone mineral density decreases, thereby enhancing the risk of bone fracture. The age-corrected incidence of osteoporotic fractures is continually increasing in Europe. Within the past 20–30 years, the incidence of spinal fracture has increased three- to fourfold in women and more than fourfold in men (Mosekilde, 2001). The incidence of hip fracture has also increased two- to threefold, with the increase being most pronounced in men (Obrant et al., 1989). Due to the accelerating loss of bone density around menopause, osteoporosis has been considered a women's disease. However, this mechanism cannot explain: (1) the large age-corrected increase in osteoporotic fractures over the past 30 years (Obrant et al., 1989), (2) the large intra-European differences in the incidence of hip fracture (Kanis, 1993), (3) the large intra-European differences in the hip fracture–gender ratio (Kanis, 1993) and (4) the fact that the fracture incidence is increasing more rapidly in men than in women (Obrant et al., 1989).
The maximum bone mass, which is attained at the age of 20–25 years, is termed the peak bone mass and is primarily genetically determined. Intake of calcium and vitamin D is also important for protection against osteoporosis, and dietary supplements of vitamin D and calcium effectively reduce the incidence of fracture (Fairfield & Fletcher, 2002). Other factors of importance for the development of osteoporosis are smoking, early menopause and lack of exercise (Mosekilde, 2001).
A lack of weight-bearing exercise in children prior to puberty is a factor of great importance (McKay et al., 2000). A Dutch longitudinal study in which young people were followed for a 15-year period showed that lumbar and hip bone mineral density at the age of 28 years was significantly related to daily physical activity during adolescence and young adulthood (Kemper et al., 2000).
The “physiological window” within which the bones are affected is wide, and it is reasonable to differentiate between physical inactivity as understood to mean sedentary work/no exercise, and actual immobilization such as in paralysis, strict bed rest or travel in space.
Loss of bone during immobilization is due to an acceleration of the remodelling process caused by an enhanced negative balance/replacement unit (Krolner & Toft, 1983). The clinical consequences of immobilization are considerable. One study thus showed that immobilization due to tibia fracture led to pronounced loss of hip bone on both the fractured side and on the contralateral side (Van der Wiel et al., 1994). In a follow-up study it was shown that the bone mineral density on the fractured side had still not normalized 5 years later (van der Poest et al., 1999). In addition, a meta-analysis has shown that 3 weeks of bed rest doubles the risk of hip fracture during the following 10 years (Law et al., 1991).
Excessive physical activity can have unintentional negative effects, including on the bones. Girls with exercise-dependent secondary amenorrhea thus lose bone and are (reversibly) sterile with reduced libido (Helge, 2001).
Osteoporosis research, prevention and treatment have previously focused on hormonal factors (especially the cessation of estrogen production around the menopause), but epidemiological, clinical and bone biology studies now indicate that mechanical factors (physical activity) play a prominent role in the health of bones. Generally speaking, the decreasing level of physical activity among the population is probably a major cause of the general increase in the incidence of hip fracture over the past 30 years.
Evidence for physical training
Evidence exists that aerobic training can enhance bone mineral density, while combined strength conditioning and balance training reduce the risk of falls and fractures in the elderly.
A 2002 Cochrane Review (Bonaiuti et al., 2002) based on 18 randomized-controlled trials (Chow et al., 1987; Sinaki et al., 1989; Prince et al., 1991; Grove & Londeree, 1992; Lau et al., 1992; Smidt et al., 1992; Hatori et al., 1993; Martin & Notelovitz, 1993; Revel et al., 1993; Nelson et al., 1994; Preisinger et al., 1995; Prince et al., 1995; Pruitt et al., 1995; Bravo et al., 1996; Kerr et al., 1996; Lord et al., 1996; Ebrahim et al., 1997; Mayoux-Benhamou et al., 1997) encompassing 1423 post-menopausal women assessed the effect of aerobic training or strength conditioning on bone mineral density. The study did not differentiate between women with and without osteoporosis. Aerobic training and strength conditioning both improved bone mineral density of the spine, with the weighted mean difference for the combined effect of aerobic training and strength conditioning being 1.79 (95% CI 0.58–3.01). Moderate training in the form of walking improved bone mineral density of both the spine and the hip, while aerobic training only increased bone mineral density of the wrist.
A meta-analysis (Wallace & Cumming, 2000) published in 2000 that identified 35 randomized-controlled trials of aerobic training and strength conditioning, but that also included studies of pre-menopausal women, concluded that both aerobic training and strength conditioning had a positive effect on lumbar spinal bone loss in both pre- and post-menopausal women. Aerobic training probably had a positive effect on the neck of the femur, but there were too few trials to enable conclusions to be drawn regarding the effect of strength conditioning on the neck of the femur.
We have also identified a number of other systematic reviews on this topic that will not be mentioned here, either because there is considerable overlap with those already mentioned, or because it is not possible to identify the randomized trials (Ernst, 1998; Kelley, 1998a, b, c; Wolff et al., 1999; Espallargues et al., 2001; Falkenbach, 2001; Kelley et al., 2001b).
In a randomized-controlled trial (de Jong et al., 2003, 2004), 309 patients with rheumatoid arthritis were assigned to a 2-year intensive exercise program. The intervention group participated in two weekly training sessions lasting 75 min. Each session consisted of endurance training on bicycle, strength conditioning in the form of circuit training and weight-bearing sports in the form of volleyball, football, basketball or badminton. The effects of the training program were assessed every 6 months for 2 years.
The intensive exercise program, which included weight-bearing sports activities, inhibited bone mineral loss (de Jong et al., 2004), a finding that is in accordance with an earlier study of rheumatoid arthritis reporting a modest but positive effect of dynamic training on bone mineral content (Westby et al., 2000). Strength conditioning alone does not appear to affect bone mineral content in patients with rheumatoid arthritis (Hakkinen et al., 1999; Hakkinen et al., 2001b). Moreover, a recent meta-analysis found no evidence for an effect of resistance exercise in increasing or maintaining lumbar spine and femoral neck bone mineral density in premenopausal women (Kelley & Kelley, 2004).
In a randomized-controlled trial (Carter et al., 2002) of women aged 65–75 years diagnosed with osteoporosis, 93 women were randomized to a 20-week gymnastics program consisting of two 40 min sessions weekly of balance training and strength conditioning. Training improved both balance and muscle strength, but bone mineral density was not measured at the end of the study. On the other hand, 10 weeks of the same balance training and strength conditioning was not effective (Carter et al., 2001).
In another randomized-controlled trial of postmenopausal women with osteoporosis (Iwamoto et al., 2001), the subjects were randomized to control (n=20), 2 years of training (n=8) or 1 year of training followed by 1 year without training (n=7). The training consisted of daily walking and gymnastics. Bone mineral density improved significantly in the exercise groups, but reverted to the level in the control group after the year without training.
In a recently published Australian study (Day et al., 2002), 1090 persons aged 70–84 years living at home were assigned to one of eight groups defined by the presence of one or more of the following three interventions: (1) group-based exercise, (2) home hazard management aimed at preventing falls and (3) vision improvement. The training exercises were designed to improve flexibility, leg strength and balance (which improved significantly in the exercise groups). The group-based exercise reduced the fall rate ratio to 0.82 (95% CI 0.70–0.97; P<0.05) relative to no intervention. With all three interventions combined, the corresponding fall rate ratio was 0.67 (95% CI 0.51–0.88; P<0.004).
In a 2002 meta-analysis (Robertson et al., 2002) encompassing 1016 women aged 65–97 years, strength conditioning combined with balance training was found to reduce both falls and fall-related fractures by 35% (incidence rate ratio 0.65 (95% CI 0.57–0.75) and 0.65 (95% CI 0.53–0.81), respectively). The training program was equally effective in men and women and in persons with and without a previous fall, but participants aged 80 and older benefited most from the program. An American meta-analysis arrived at the same conclusions (Province et al., 1995).