Chapter 5 The Skeletal System 117 Copyright Goodheart-Willcox Co., Inc. Bone Remodeling Although bones grow and change dramatically during childhood and adolescence, living adult bone is also a very active tissue. It is always changing at a micro- scopic level in bone mineral content (and thereby strength) and sometimes in size or shape. This occurs through osteoblast and osteoclast activity during a process called remodeling. Forces such as gravitational force, muscle forces, forces sustained when people push or pull on some- thing, and impact forces created when people bump into something all influence the bones. The remodeling process converts the size and direction of the forces acting on bone into changes in bone mineral density. In some circumstances, the remodeling process also causes changes in the size or shape of bone. Bone mineral content normally peaks in women at about 25 to 28 years of age and in men at about 30 to 35 years of age. Thereafter, bone mass is progressively lost. Because women tend to have smaller bones than men, the loss of bone mass and bone mineral density is generally more problematic for women. SELF CHECK 1. What is the periosteum? Name at least three functions of the periosteum. 2. Where is the endosteum found? 3. What covers the ends of long bones? 4. Describe the functions of osteoblasts and osteoclasts. 5. Describe and analyze the effects of less elasticity on the body as people age. Physical Activity and Bones There are many documented examples of bone remodeling and hypertrophy in response to regular physical activity. Sports that required repeated, forceful use of a certain limb promote not only muscle hypertrophy, but also bone hypertrophy, in the stressed area. For example, clinical case studies have shown increased bone mass, circumference, and mineralization in the dominant forearm of professional tennis players and in the dominant upper arm of baseball players. It also appears that the larger the forces habitually encountered, the more dramatic the effect. In one interesting study, researchers measured the density of the femur among 64 nationally ranked athletes from different sports. The densest femurs were those of the weight lifters, followed by hammer and discus throwers, runners, soccer players, and swimmers. As you might expect from this research, the size of the regularly acting forces on the body is one factor that appears to be related directly to bone mass. The other important factor contributing to bone mineral density is absorption of repeated impacts, such as those routinely encountered during running and landings from jumps. In an investigation involving collegiate female athletes, those participating in high- impact sports (basketball and volleyball) were found to have much higher bone mineral densities than the swimmers, with soccer and track athletes having intermediate values. Another study compared the bone mineral densities of trained runners and cyclists to those of sedentary individuals of the same age. Not surprisingly, the runners were found to have increased bone density, but the cyclists did not. On the whole, the evidence suggests that physical activity involving impact forces is necessary for maintaining or increasing bone mass. One need not be an athlete, however, to exercise for bone health—even vigorous walking generates bone-building impact forces. Competitive swimmers, who spend a lot of time in the water where the buoyant force counteracts gravity, may have even less bone mineral density than that of sedentary individuals. It is important for competitive swimmers to also participate in activities such as weight training and running to maintain normal bone density. Research Notes Pressmaster/Shutt erstock.com Repeated, forceful use of the arms or legs during exercise can result in both muscle and bone hypertrophy.