The loss of bone density is becoming a major health concern in industrialized societies. Increasing bone density during puberty and young adulthood is considered the best option for preventing the negative health consequences associated with osteoporosis, even in middle aged and older adults an exercise program can increase bone density. While low volume impact oriented aerobic activities like running have been shown to be effective at increasing bone density excessive endurance training has been linked to low bone density. Strength training remains the best option for adults wishing to increase bone density. A regular program of high load (60-85% 1RM) training three or more times per week using a variety of exercises that challenge all major muscles has been shown to significantly increase bone density even in elderly adults.
Key Words: Bone Density, Exercise, Osteoporosis, Training
Osteoporosis, which has been defined as bone mineral density (BMD) more than 2.5 standard deviations below the young adult mean value (14), is a growing health problem for both men and women. In developed and developing countries, the incidence of osteoporosis is increasing at a rate faster than what would be predicted by the aging of the population alone (15). In the U.S., it has been estimated that by 2025 the number of hip fractures attributed to osteoporosis will double to nearly 2.6 million with a greater percentage increase in men than in women (12).
Epidemiological evidence suggests that genetic factors are the most important cause of osteoporosis (20) and can account for as much as 80% of the variability in bone density in the population (6), but a variety of environmental factors have been linked to bone density including: negative energy balance, low calcium intake, lack of fruit and vegetable consumption, low body mass index, strength, and hormone levels (13,22,9,7,23) - all of which may influence the ability to develop or maintain bone density.
A well designed exercise can have a tremendous impact on bone, increasing density, size, and mechanical strength (23) and may be one of the keys to preventing complications associated with osteoporosis. If bone density and maximum tensile strength are increased before osteoporosis sets in, subsequent complications could be minimized (21). Unfortunately many adults wait to start an exercise program once they are diagnosed with low bone density.
For middle aged and older adults one of the primary health goals of an exercise program is to maintain bone density. Without an exercise intervention, after the age of 40, bone mass decreases by about 0.5% per year, regardless of sex or ethnicity (15). Whether appreciable increases in bone density can occur for this age group is equivocal (15) and dependant on the duration of the exercise program, age, dietary factors, and history of physical activity. A variety of different types of exercise have been used in bone building programs middle aged or older adults.
Although not all studies have shown improvement in bone density with strength training (15), strength training, if done with a high enough intensity for a prolonged period of time, seems to be effective for improving bone density in middle aged and older women who have low bone density (16). Programs that have been successful at increasing bone density have several common characteristics; training intensity above 70% 1RM, programs that last more than 12 months, and training frequency greater than two times per week.
Endurance training can be an acceptable form of exercise for maintaining or increasing bone density in middle aged or older adults provided there is sufficient impact. Stuart and Hannan (2000) examined the effects of cycling, running, or both on bone density in recreational male athletes. They found that runners had greater total and leg BMD than controls and that those athletes participating in both cycling and running had greater total and arm BMD - whereas the cyclists had decreased spine BMD compared to controls. The lack of impact involved in cycling may explain the lack of change in BMD even though all groups performed equal volumes of work throughout the study period. Walking programs, because of their low impact, tend to show only modest or no effects on BMD (3,18). Rowing, because of the high compressive and shear forces placed on the spine (4.6 times body weight) has been shown to increase lumbar spine BMD but not at other areas (17). Moderate training volumes seem to be more effective for increasing bone density. Running mileage of 20-30 km per week has a positive effect on bone, particularly lower leg and distal femur, but training volumes greater than this may cause a chronic increase in cortisol that negatively impacts bone (4) as running 92 km per week has been shown to result in bone density lower than sedentary controls (2).
Although effective and popular in school based programs for increasing bone density in younger people jump training does not appear to be as effective in middle aged and older women. In a study comparing the effects of 12 months of vertical jumping on spine and proximal femur BMD in a group of pre and post menopausal women, Bassey, Rothwell, Littlewood and Pye (1998) found that 50 jumps six days per week increased BMD in the pre-menopausal group but not in the post menopausal group compared to group specific controls. Interestingly, the lack of change occurred even though the ground reaction forces and rate of force development on landing were higher in the post menopausal group resulting in a greater strain overload than in the pre menopausal group.
While a variety of exercise modalities have proven to be effective at maintaining bone density in adults, there are some basic principles that should be considered when designing a long-term program for people with osteoporosis:
Increase resistance and intensity progressively. This is necessary because for bone to form it requires a minimum amount of strain. Once a bone adapts to a given strain level, the stimulus for bone to form is removed and a higher strain level becomes necessary for it to adapt further (10).
Mechanical loading of bone has an osteogenic effect only if the loading is dynamic and variable, as static loading of bone does not trigger an adaptive response (23). Impact and rapid changes of direction can be particularly effective because ground reaction forces tend to be highest during these activities. Jumps, running, and more explosive or dynamic strength training activities should make up the majority of exercise in a bone-building program. In adults with advanced osteoporosis, more explosive exercises should be phased in gradually as their conditioning and bone strength improves.
Bone adaptations occur primarily at insertion and origin points where muscles attach to the bones. Ryan et al. (1994) suggest that increased BMD from strength training and explosive activities is related to the load placed on the muscles that act as prime movers. A wide variety of exercises, which change every 2-4 weeks, exercising the whole body will help ensure that all bones receive stimulus to increase BMC or BMD.
As with most training, there is a minimum level of intensity that is needed to stimulate increase in BMD. For strength training activities there is a linear relationship between weight lifted and improvements in bone density (5). Chilibeck, Sale and Webber (1995) suggest that for strength training intensities of at least 60%, 1RM are needed to increase BMD, with faster and greater increases in bone density coming as intensity climbs (16). For impact activities like running and jumping, ground reaction forces of greater than two times body weight can increase bone density with higher forces having a greater effect.
Improvements in BMD can occur with relatively short training sessions if high impact activities like jumping are the core of the program. However, there is a need to perform these sessions frequently. Studies of jump training have found that where three or more sessions per week are sufficient to increase bone - two sessions per week has negligible effect on bone density (11).
Consistency is one of the keys to long term bone health. Like other tissues, bone undergoes both adaptation to training and detraining during periods of decreased activity. The bone remodelling cycle lasts four to six months (8); this is the minimum period of time needed for BMD to change significantly. Training programs need to be designed so that they offer the variety and adaptability for people to make them a year round part of lifelong fitness regime.
Decreased bone density is a growing problem in modern societies. Exercise remains one of the most potent alternatives to drug treatments for maintaining or improving bone density. An intensive program, three or more times per week featuring a variety of exercises that considers the individual needs of each person and promotes long term compliance can have a positive impact on bone density.
Over the past years, adults have become more and more active in age group sports, particularly in the endurance sports like running, cycling, and triathlon. The inclusion of an intensive strength training program will not only improve their performance, but will help offset the decrease in bone density that often accompanies aging and higher volumes of aerobic training.
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Ed McNeely is the senior physiologist at the Peak Centre for Human Performance and a partner in StrengthPro Inc. a Las Vegas based sport and fitness consulting company he is also a National Faculty member of the United States Sports Academy
Ed McNeely, MS: firstname.lastname@example.org