During childhood and adolescence, bone is built up by cells called osteoblasts and broken down by cells called osteoclasts. Both groups of cells work in unison to promote an increase in bone density and size as a result, bones are at their strongest during the late twenties and early thirties. This effect balances out to allow the same amount of bone to be replaced as is broken down by late thirties . At the age of around 30 years the bone mass ceases to increase and the function of healthy bone is to maintain the existing bone mass as long as possible. Following this, bone density begins to be gradually lost due to the osteoclasts cells breaking down more bone than is replaced leading to osteoporosis .
Numbers of reasons have been offered to explain why bone loss occurs with aging. Since milk is the major source of calcium in our diet, and since, traditionally, adults drink less milk than children, calcium intake decreases. Furthermore, as aging proceeds, the gastrointestinal tract becomes less efficient in absorbing calcium, and the kidneys in conserving it. Activity levels generally decrease with age, and a recently published paper has suggested that some bone can be preserved through increased physical activity. As weight bearing decreases, there is less stress on bone, less muscle contraction, decreased cardiac output, and a reduced blood flow to bone .
Several risk factors are correlated with the occurrence of osteoporosis and contribute to the probability of an individual suffering from osteoporosis. There are several risk factors that are controllable, such as sex hormones, anorexia nervosa, dietary intake of calcium and vitamin D, consumption of medications such as longterm corticosteroids, life style, smoking, consumption of alcohol and coffee, and hyperparathyroidism. On the other hand, gender, age, body size, ethnicity, and family history are uncontrollable risk factors for osteoporosis .
There are two types of osteoporosis, type I due to a decrease in cumulating estrogens which affects trabecular bone (especially vertebral bone) and affects females more than males in a ratio of 1:6. Type II, senile osteoporosis, which is age related and occurs in cortical and trabecular bone, affects females and males in a ratio of 2:1One in three women over age of 50 years will develop the disease during their lifetime .
2. Bone mineral density
Low bone mass can only be diagnosed by measuring bone mineral density (BMD) by various techniques, of which the gold standard is DEXA (Dual energy X-ray Absorptiometry) BMD assessment confirms diagnosis, detects disease in asymptomatic state, predicts chances of future fractures, and is also useful for monitoring response to therapy. Low bone density has been used to predict risk for fractures as well as to diagnose osteoporosis [51, 52].
A World Health Organization working group proposed that osteoporosis should be diagnosed in epidemiologic studies when bone mineral density is 2.5 standard deviations (SDs) or more below the mean for healthy young adult women at the spine, hip, or wrist (corresponding to a T-score of ?-2.5). For every 1 standard deviation below the mean, the fracture risk roughly doubles [53, 54].
3. Exercise and osteoporosis
Results of studies on medical methods to stop osteoporosis indicate that, in spite of their preventive efficiency, these methods in the long run may induce heart diseases, breast cancer and heart attack, while physical activities have positive effects on preventing the reduction of bone mineral density with no side effects. Thus, exercise therapy has been recently considered by a great number of researchers .
It is generally agreed that exercise should be recommended to patients with osteopenia or osteoporosis (Turner et al., 2003). The physical activity can be an important instrument in prevention and acting as an auxiliary treatment to osteoporosis, not only by reducing the chances of fracture due to bone loss maintenance but also by increasing neuromuscular capacity, improvement of core stability thus to aid in preventing falls and reducing its impact .
Since the pathogenesis of osteoporosis is complex and multifactorial . A decrease in physical activity may lead to an increased loss of bone mineral density (BMD) and an increase in the incidence of fragility fractures [58, 59], therefore, it is commonly accepted that weight-bearing activities provide an osteogenic stimulus to the bones . Yet, there are some exercise prescriptions that use body weight maintenance as main factor for gain of BMD without making a difference between equipment and methodology that should be applied in physical activity .
In 2009, De Matos et al.,  conducted a study to analyze the effect of a specific program of weight training exercise with closed kinetic chain in bone mineral density in postmenopausal women with osteopenia or osteoporosis where a total of 59 postmenopausal women with osteoporosis or osteopenia were included in this prospective study. This protocol of weight training exercise did not significantly improve bone mineral density in postmenopausal women with osteopenia or osteoporosis, but in comparison to the control group, the results showed the importance of practicing the specific exercise program for maintenance of bone health in postmenopausal women. .
In 2012, Saarto et al.,  studied the effect of supervised and home exercise training on bone mineral density among breast cancer patients. The study included 573 breast cancer survivors aged 35�68 years randomly allocated into exercise or control group after adjuvant treatments, 498 (87%) were included in the final weekly supervised step aerobic- and circuit-exercises and similar home training. Bone mineral density (BMD) at lumbar spine and femoral neck were measured by dual energy X-ray absorptiometry. Physical performance was assessed by 2-km walking and figure-8 running tests, and the amount of physical activity was estimated in metabolic equivalent-hours/week. Their results revealed, among premenopausal breast cancer survivors, a 12-month exercise intervention completely prevented bone loss at the femoral neck, whereas no exercise effect was seen at lumbar spine or at neither site in postmenopausal women .
Similarly, a study was conducted to investigate the effect of a 12-week selective aerobic exercise trial in water on bone density in obese postmenopausal women. Twenty obese post-menopausal women participated in this study. Subjects were randomly divided into an experimental and control group. The experimental group performed selective aerobic training in water for 3 months. The results of this study indicated that there was a significant increase in femoral bone density in experimental group compared with control group in end of study period. Also, the lumbar spine bone density was increased moderately, but not significantly, in experimental group .
Also in 2012, Andreoli et al.,  conducted a retrospective study to determine the long-term effect of exercise on bone mineral density (BMD), bone mineral content (BMC) and body composition (BC) in post-menopausal women who were elite athletes during their youth compared with sedentary controls concluding that the high level of physical activity observed in female athletes is associated with improved muscle mass, BMD and BMC, and physical activity during youth seems to have a beneficial effect on bone mass and helps to prevent bone loss due to aging .
In the same manner, One hundred and twelve postmenopausal women with low bone mineral density (BMD) and forearm fractures were randomized to physical training or control group. After one year the total hip BMD was significantly higher in the women in the physical training group. The results indicate a positive effect of physical training on BMD in postmenopausal women with low BMD .
A recent study was conducted to evaluate the effect of pulsed electro-magnetic therapy and exercise training on bone mineral density (BMD) in elderly women with osteoporosis. A total of 30 elderly women with osteoporosis aged from 60 to 70 years old were randomly divided into two groups: A magnetic group consisting of 15 women who received pulsed electro-magnetic therapy at a frequency of 33 Hz and an intensity of 50 gauss for 50 min per session and an exercise group consisting of 15 women who practiced active exercises that included treadmill walking and selected exercises for hip and back muscles for 50 min per session. Both interventions were applied three sessions per week for three months at a physical therapy clinic. This study concluded that pulsed electro-magnetic therapy and exercises can increase BMD at the neck of the femur and the lumbar spine in elderly women. Physical therapists could apply pulsed electro-magnetic therapy or exercise training to increase BMD in elderly women .