Bone Health and Incidence of Osteoporosis
Bone is a dynamic tissue that is constantly undergoing remodelling activity, which is carried out by specialised bone cells. During this remodelling activity, old bone tissues is removed and new bone tissue formed by the activity specific bone cells, termed osteoclasts and osteoblasts, respectively. Osteoporosis is a disease characterized by minerals loss, low bone mass and micro-architectural deterioration of bone tissue leading to increased bone fragility and a consequent increase in fractures risk. The individuals afflicted by osteoporosis typically experience reduced mobility, pain, loss of independence, and psychological distress associated with postural disfigurement and the fear of additional fractures. Hip fractures are the most severe fracture since they carry the highest incidence of morbidity and mortality. The postural abnormality associated with vertebral fractures results in reduced cardiovascular capacity and affects other internal organs due to compression of the chest and abdominal regions. In order to define effective preventive strategies, it is important to determine the lifestyle factors that influence fracture risk. The two primary determinants of fracture risk are low bone mass and falls.
A number of studies showed that the increase in physical activity represents an important strategy to reduce fractures by increasing bone mass and by preventing falls through improved functional ability. Although the mechanism by which exercise increases bone mass is not fully understood, it is believed that it influences bone density directly through mechanical forces (loading) transferred to the bone. In essence, when a force is applied to a bone, the bone temporarily is deformed, and the body responds to such mechanical loading, resulting in enhancement of bone strength and bone mineral density (BMD). Such responsive adaptations are more evident in the specific are of the skeleton, which is exposed to the loads. In other words the skeleton greatly benefits from the effect of the forces occurring during physical activity, as this leads to improved bone health.
Conversely, if the skeletal system is not exposed to mechanical loading (forces), often due to hypokinesia (lack of physical activity), this may result in a progressive decrease in BMD, and diminished bone capability to resist forces. This means that the lack of physical activity increases significantly the likelihood of mineral loss within the bones, which become brittle, weak and prone to fractures. The most striking examples of marked bone loss occurs in the absence of weight-bearing activity, such as that observed in space travel and prolonged bed rest. In contrast, many investigations have shown that bone mass among physically active individuals and athletes is significantly higher compared to their non-active and non-athletic counterparts. Some studies have exposed the bones of participants to significant mechanical forces, using physical exercise as an intervention. The results demonstrated a positive effects on bone mass. Furthermore, investigations also showed that individuals such as athletes, who train at higher intensity have even stronger and healthier bones. Therefore, this demonstrates that an appropriate exercise programme represents an effective intervention to promote bone health and prevent osteoporosis.
Evidence is accumulating to suggest, that exercise also increases muscle strength, mass, and power, which may provide the best osteogenic stimulus, encouraging bone forming and repair. In addition to providing skeletal protection, physical activity also can help improve coordination and functional ability, and thus reducing in the older adults the likelihood of falls, which are highly related to the incidence of fractures, particularly at the hip.
Physical Activity and Bone Mass
This section discusses in more details the underlying mechanisms and effects of physical activity on bone health. Physical activity transmits loads to the skeleton in two ways: by muscle pull and by gravitational forces from weight-bearing activity. It is generally assumed that a high level of activity corresponds to a high level of mechanical loading. For instance, the activity of swimming brings about different health and fitness-related benefits, but the increase in BMD is more limited, compared with other forms of exercise. In fact, activities that require full support of body weight (i.e., those that are performed on the feet) are recommended if increase in BMD is a desired outcome of exercise participation. Sports with unilateral activity, such as tennis, continue to provide the best representation of the positive effects of exercise on bone in humans. Indeed, studies have demonstrated a greater BMD in the dominant playing arm versus the non-dominant arm across different age groups. However, most forms of activity are not characterized by such specific, localized loading patterns.
Other indicators that physical activity exerts a positive influence on the skeleton are the finding that certain measures of physical fitness are correlated with BMD. Specifically, body composition and muscular strength exhibit positive associations with bone mass. Mechanical forces are directly applied to bones by muscular attachments, and individuals with high muscle strength are able to generate larger forces during contraction. Thus, muscle strength is a measure of physical fitness that has been studied with respect to skeletal health. Research has shown that the relationship between muscle strength and bone health is site-specific. Indeed, strength of the hip muscles has been related to hip BMD, while, grip strength has been associated with forearm BMD. The contribution of muscle strength to BMD in various cross-sectional studies has ranged from 9 to 38% in non-athletic adults. Since approximately 60% of bone mass is estimated to be genetically determined, thus, the relationship between muscle strength and bone density is not trivial and again points to the importance of the muscular system in promoting bone health.
Furthermore, research has demonstrated that male and female athletes who participate in sports that require muscular strength and power (e.g., weight lifting, gymnastics, wrestling) exhibit higher bone mass than those whose sports involve primarily muscular endurance (e.g., distance running, triathlon). Information on the loading characteristics of various activities suggests that walking and slow running provide loads equal to or slightly higher than body weight alone at the spine. In comparison, forces at the spine have been estimated to be five to six times the body weight while weight lifting. While, jumping associated with gymnastics training, may elicit forces as high as 10 to 12 times body weight.
Investigations focusing on athletes and the size of the load for a specific sport suggests that the skeletons’ response to mechanical loading depends on the magnitude of the force. In practical terms, the skeleton must encounter forces that are greater than those it experiences on a day-to-day basis. For instance, even though walking is a weight-bearing activity, its ability to evoke increase in BMD is limited in the older adults who were previously bedridden and unable to ambulate for a period of time. On the other hand, those who perform activities of daily living without assistance will be in a weight-bearing posture much of the day. For this person, walking as an exercise will not exceed the loading threshold of daily activities and therefore will not improve bone mass, although it may aid to prevent mineral loss.
However, it is important to bear in mind that any form of physical exercise brings about a number of physiological and psychological benefits. Also, it is important to consider that exercise programmes prescribed by exercise physiologists and experts represent effective tools to improve fitness, health and performance, as well as promoting BMD. Nonetheless, low intensity exercise, such as walking, to an extent also contributes to improving health and wellbeing.
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