Biomechanics Of Bone

Unlike muscle, the primary loads experienced by most bones are compressive. The mechanical response of bone to compression, tension, and other complex loads depends on the complex structure of bones. Remember that bones are living tissues with blood supplies, made of a high percentage of water (25% of bone mass), and having considerable deposits of calcium

Application: Osteoporosis

Considerable research is currently being directed at developing exercise machines as counter-measures for the significant bone density loss in extended space flight. A microgravity environment substantially decreases the loading of the large muscles and bones of the lower extremity, resulting in loss of bone and muscle mass.There is also interest in exercise as a preventative and remedial strategy for increasing the bone mass of postmenopausal women. The strong link between the positive stresses of exercise on bone density, however, is often complicated by such other things like diet and hormonal factors. In the late 1980s researchers were surprised to find that elite women athletes were at greater risk for stress fractures because they had the bone density of women two to three times their age. Stress fractures are very small breaks in the cortical (see below) bone that result from physical activity without adequate rest. What was discovered was that overtraining and the very low body fat that resulted in amenorrhea also affected estrogen levels that tended to decrease bone mass.This effect was stronger than the bone growth stimulus of the physical activity. High-level women athletes in many sports must be careful in monitoring training, diet, and body fat to maintain bone mass. Kinesiology professionals must be watchful for signs of a condition called the female athlete triad. The female athlete triad is the combination of disordered eating, amenorrhea, and osteoporosis that sometimes occurs in young female athletes.

salts and other minerals. The strength of bone depends strongly on its density of mineral deposits and collagen fibers, and is also strongly related to dietary habits and physical activity. The loading of bones in physical activity results in greater os-teoblast activity, laying down bone. Immobilization or inactivity will result in dramatic decreases in bone density, stiffness, and mechanical strength. A German scientist is credited with the discovery that bones remodel (lay down greater mineral deposits) according to the mechanical stress in that area of bone. This laying down of bone where it is stressed and reabsorption of bone in the absence of stress is called Wolff's Law. Bone remodeling is well illustrated by the formation of bone around the threads of screws in the hip prosthetic in the x-ray in Figure 4.6.

The macroscopic structure of bone shows a dense, external layer called cortical (compact) bone and the less-dense internal cancellous (spongy) bone. The mechanical

Figure 4.6. X-ray of a fractured femur with a metal plate repair. Note the remodeling of bone around the screws that transfer load to the plate. Reprinted with permission from Nordin & Frankel (2001).

response of bone is dependent on this "sandwich" construction of cortical and cancellous bone. This design of a strong and stiff material with a weaker and more flexible interior (like fiberglass) results in a composite material that is strong for a given weight (Nordin & Frankel, 2001). This is much like a surf board constructed of fiberglass bonded over a foam core. Cortical bone is stiffer (maximum strain about 2%), while cancellous bone is less stiff and can withstand greater strain (7%) before failure. In general, this design results in ultimate strengths of bone of about 200 MPa (29,000 lbs/in2) in compression, 125 MPa (18,000 lbs/in2) in tension, and 65 MPa (9,500 lbs/in2) in shear (Hayes, 1986). This means that an excessive bending load on the femur like in Figure 4.2 would most likely cause a fracture to begin on the lateral aspect that is under tensile loading. Using sports rules to protect athletes from lateral blows (like blocking rules in American football) is wise because bone is weakest under shearing loads.

It is also important to understand that the ultimate strength of bone depends on nutritional, hormonal, and physical activity factors. Research done with an elite power-lifter found that the ultimate compressive strength of a lumbar vertebral body (more than 36,000 N or 4 tons) estimated from bone mineral measurements was twice that of the previous maximal value. More recent studies of drop jump training in pre-pubescent children has demonstrated that bone density can be increased, but it is unclear if peak forces, rates of loading, or repetitions are the training stimulus for the increases in bone mass (Bauer, Fuchs, Smith, & Snow, 2001). More research on the os-teogenic effects of various kinds of loading and exercise programs could help physical educators design programs that help school children build bone mass. The following section will outline the mechanical response of ligaments to loading.

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