OSTEOPOROSIS is defined by the NIH Consensus Development Panel on Osteoporosis as "a skeletal disorder characterized by compromised bone strength predisposing a person to an increased risk of fracture."1
The WHO Working Group has defined osteoporosis according to the measurement of bone mineral density (BMD) using dual-energy X-ray absorptiometry (DEXA). A T-score value of 2.5 standard deviation or more below the young adult mean is considered diagnostic for osteoporosis.2
Bone mineral density at any time in life depends upon the peak bone density achieved during the development and growth period (especially at the time of puberty) and the subsequent adult bone loss. Regular physical activity done during childhood increases the peak bone mass to its maximum as the human skeleton before puberty is quite sensitive to mechanical stimulation elicited by physical activity. A study revealed that weight bearing exercise for six months increased bone parameters by 5.5 % approximately around puberty. This fell to a maximum of 1.9 % in late puberty.3
High volumes of exercise are not required to achieve notable osteogenic effect, rather just three hours of participation in sports is enough.4
Girls who exercise athletically during years of peak bone acquisition have higher bone mass than their sedentary counterparts and this appears to have lasting benefits for lumbar spine and proximal femoral bone mass density later in life.5
Exercise done during late adulthood increases the bone mass minimally if at all, its main purpose being to maintain the bone mass. During old age the bone loss is rapid. This loss starts at around the age of 40 in both sexes and is initially 0.3 - 0.5% of the peak bone mass per year, gradually increasing as the age progresses, though some women may have a very rapid loss after the menopause, up to 5 or 6%, resulting in a rapid reduction in bone mass. Exercise done at this age decreases the rate of bone loss as well as improves muscle strength thereby providing balance, preventing falls and subsequent fractures.6
In view of the increase in life expectancy in our population and increased incidents of multiple fractures in middle aged and elderly subjects the effect of exercise on osteoporosis is reviewed.
Factors affecting Osteoporosis
Age related loss in bone mineral content occurs in both the compact and trabecular bones, however, the mass of trabecular bone does not change due to an overall enlargement of trabecular area as the age advances.7
Besides aging, several conditions can exaggerate the loss of mineral content which include nutritional, endocrinological, and exercise-related factors. The role for dietary calcium in influencing bone mineral content is well established especially in the first three decades of life, however, its intake decreases as a person grows older.8
In women, the most important determinant of bone mineral content is the presence of circulating estrogen. Any condition that reduces estrogen concentrations influences bone remodeling. After menopause the rate of trabecular mineral loss can increase up to 6% a year, with a greater loss in first 5 years. The rate of loss is large compared to the average loss before menopause of less than 1% a year. Hormone replacement therapy given at this time increases the BMD and affects bone remodeling but not as much as when it is combined with an exercise program.9
Classification of subjects according to BMD
The WHO Study Group10
has classified osteoporosis according to BMD as follows:
A. Normal: BMD within 1 standard deviation (SD) of the young adult reference mean.
B. Osteopenia: Mild bone loss - BMD more than 1 SD below the young adult mean but less than 2.5 SD below this value.
C. Osteoporosis: BMD 2.5 SD or more below the young adult mean.
D. Severe osteoporosis (or established osteoporosis): BMD 2.5 SD or more below the young adult mean in addition to one or more fragility fractures.
Exercise prescribed for a younger person with a normal BMD would not be suitable for a frail elderly with osteoporosis, having an imminent risk of fracture. And the goals for one group may not be achievable in another group.
How exercise effects bone mass
The stimulus for bone to maintain its structural and functional strength is the load exerted upon it on account of gravity and mechanical stress applied by the muscles. It has been observed that weight bearing activity stimulates bone remodeling whereas lack of mechanical stimulation as seen in space travelers or in paralyzed limbs can lead to osteoporosis. To evaluate the role of exercise in osteoporosis it is important to understand how mechanical signals are converted into biochemical response. Osteocytes are the primary bone mechanosensors embedded within the bone matrix that sense mechanical signals via 'stress-generated fluid flow' inside the bone. This is evident by the increase of both intra-cellular and extra-cellular chemical mediators after skeletal strain such as the formation of cyclooxygenase-2 (COX-2) and release of prostaglandins.11,12
However, the exact mechanism by which osteocytes and osteoblasts respond to fluid flow is not known.
Recently, it has been shown that the bones respond not only to increased magnitude of strain but to increased frequency of strain as well. This finding is particularly advantageous to frail population who are unable to do strenuous exercise due to the risk of fracture.13,14
What type of exercise should be advised?
Exercise strengthens muscles, increases flexibility, and improves coordination and balance. The importance of exercise in relation to bones is evident after disuse which results in a decrease in bone mineral density and thinning of cortical bone at the diaphysis. Bones being live tissue, respond to the stress placed by the environment. When that stress is removed in the form of lack of physical activity, the bones loose their density. A paralyzed limb loses > 10% of bone mass in a year - equivalent to 20 years of bone loss.15
How much exercise is needed to prevent this loss is yet to be determined as longitudinal studies are underway. Presently what is needed is to prescribe the type and amount of activity on an individual basis keeping in mind the goals to be achieved for that person. A person with normal BMD or osteopenia could be advised a more vigorous exercise programme in order to prevent the development of osteoporosis. These people because of their bone structure can withstand the stress applied during such exercises. On the other hand persons having osteoporosis already have fragile bones and should not be prescribed strenous exercises. Persons with established osteoporosis, i.e. those people who have severe osteoporosis and already have had a fracture have their activity restricted because of the fear of falling and over here the basic aim is to promote balance thereby preventing further falls.16
The beneficial effect of exercise on BMD is greatest among those who were previously sedentary as compared to very active individuals who will see less of an effect. To be effective, in terms of compliance and body fitness, all exercise programs should be started from a lower level of performance and build up progressively. An ideal programme should include activities that increase muscle strength, balance, flexibility and coordination.
Types of exercises
Two types of exercises are generally recommended for osteoporosis:
1. Weight-bearing exercises
2. Strength training or resistance exercises
Weight-bearing exercises are those in which the muscles and bones bear the weight of the person and work against gravity. This includes any exercise in which the arms, feet and legs are bearing the weight, such as walking, jogging, stair climbing, playing tennis, volleyball and similar sports, performing aerobics, tai chi and dancing. Persons having osteoporosis should start with a gentle exercise such as walking on level ground that has been shown to increases BMD17
as other forms of exercises may increase their chances of fall.
Low impact aerobic exercises such as swimming and cycling do not have a significant effect on the bones as regards osteoporosis, however, due to their beneficial influence on the heart, they should be part of a regular fitness regimen.
Strength training or Resistance exercises are activities in which the body is moved through its range of motion against some kind of resistance. The resistance can be in the form of free weights, machines, tubing, or one's own body weight. Lifting weights, push-ups, and squats apply resistance against the bones by contracting the muscles and pulling upon the tendons. This has a direct effect upon bone formation and remodeling which is site specific and associated with high BMD in young and older individuals.18,19
Strengthening the muscles helps in daily tasks such as walking, standing from a sitting posture and helping improve balance thus preventing falls.20
Postural exercises that strengthen the back muscles prevent against the development of compression fractures of the spine.21
Persons with established osteoporosis have a fear of fall due to their lack of balance and coordination in their limbs and thus become more and more dependent on others. In this group, the above exercises will put a further strain on the bones and can lead to more fractures. The aim therefore should be for a targeted exercise programme that would improve their balance and increase muscle power. Exercises like yoga and tai chi are low impact weight bearing exercises involving slow, controlled movements that improve balance, muscle strength and help in alleviating the fear of falling.22
In old age, pain is a prominent factor, restricting activity of any kind. Hydrotherapy consists of exercises performed in warm water. The temperature of the water reduces the pain thus allowing patients to move their limbs. In addition, buoyancy relieves the body of its weight and allows certain movements that would not have been possible otherwise.23
Whole body vibrations is a new form of exercise consisting of vibrations of low amplitude and high frequency applied to the whole body while the subject is in a comfortable position. It has been shown to be a better alternative than walking in improving both bone mineral density and balance of elderly frail subjects.14
How much exercise is recommended?
A consensus on the optimal dimensions of exercise (type, frequency, duration and intensity) recommended for a person has still not reached as it would also depend on the age of the person and their bone health.24
However, an exercise programme that includes both weight bearing and resistance exercises done for 30 - 40 minutes daily, three to four times each week appears to achieve the purpose of increasing bone mass. Although the recommended intensity of training is between 70 and 80% of the functional capacity or maximal strength but this has yet to be decided.25
Extreme levels of exercise such as marathon running may in fact cause osteoporosis besides having other deleterious effects on the health. Of course each person has to tailor the exercises according to his or her own capabilities, keeping in mind that some exercise is better than no exercise at all. It should be realized, however, that any skeletal benefit acquired during exercise will not be maintained if the person returns to his previous sedentary lifestyle.26
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