Chapter 4 The Skeletal System 129 What Research Tells Us …about Physical Activity and Bones There are many documented examples of bone remodeling, and hypertrophy (enlargement), in response to regular physical activity. Sports that require 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 increased bone mineral density is absorption of repeated impacts, such as those routinely encountered during running and landings from jumps (Figure 4.8). 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 the 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 sedentary individuals. It is important for competitive swimmers to also participate in activities such as weight training and running to maintain normal bone density. Taking It Further 1. Explain in your own words the relationship between physical activity, bone remodeling, and hypertrophy. 2. Create a continuum, placing activities that negatively impact bone mass, such as swimming, on one end and those that increase bone mass (for example, basketball and volleyball) on the other end. Plot your daily activities on the continuum. Could you change your routine in a way that would increase your ability to build bone mass? Fernanda Paradizo/Shutterstock.com Figure 4.8 Why is running especially good for bones? Copyright Goodheart-Willcox Co., Inc.