Bone Adaptations

When most of us see a hunchbacked elderly woman at the mall or on the street, we unconsciously pull our shoulders back and straighten our spines. We take our calcium pills and eat our yogurt dutifully in the hope that we won't one day end up like that old lady, too fragile and bent over to function as we would wish. Osteoporosis is an entirely preventable disease that is not only disfiguring and painful but can also lead to life-threatening situations. As women, we owe it to ourselves and to those we love to become well versed on the topic of bone health and what we can do to preserve it. Fortunately, strength training has an incredibly positive effect on our bones. It stimulates new bone growth so that our bones grow strong, and it is one of the most effective ways to avoid osteoporosis.

Imagine bone as a living and breathing tissue, just like your heart or lungs. Although we usually think of bone as being hard, it is really viscoelastic — sticky, gummy, filled with thick fluid, and bendable. Bone is constantly working, whether it's protecting internal organs, providing movement through joints and muscles, or acting as a reservoir of calcium. It gives your body its shape and allows white blood cells to flourish. It also is adept at reinventing itself, either through the ongoing natural process of remodeling, whereby it removes old bone cells and forms new ones, or through mechanotransduction, which is how bone cells respond to physical activity. Bone is composed of thousands of cells, some tightly packed and some loosely packed.

Osteoblasts are young bone cells that lay the groundwork for the osteocytes, which are mature cells. Osteoclasts are bone cells that remove old bone. Collagen is a protein that the osteoblasts manufacture and secrete to connect the cells together and form the strong geometric mazes of the bone matrix. Some of the collagen in the matrix mineralizes into salts like calcium, phosphate, magnesium, and sodium. Bone mineral density (BMD) is how dense your bones are, or how much mass they have. Researchers have now proven that peak bone mineral density is reached by late adolescence. After adolescence it is difficult to build more bone mass, but if we eat well and train correctly and in moderation, we can maintain our bone mass and prevent the bone loss than can result in fractures and osteoporosis. Osteoporosis occurs over time when the amount of bone broken down greatly exceeds the amount of bone replaced by new bone cells. At this point bone mineral density decreases, which decreases the bone mass. The results are bones that become more porous or brittle and have an increased risk of fracture.

A popular saying, "Form follows function," couldn't be truer than when describing bone. The most widely accepted theory on bone formation comes from German anatomist Julius Wolff's classical theory, called Wolff's law. He proposed that bone formed and changed formation depending on the force of muscular tension and the stress of gravity placed on it. Because bone is living tissue, it adapts to the stress placed on it and can increase and decrease in size as muscular tissue does. His theory led biomechanists to devise a load-deformation relationship for bone, in which load is defined as stress (force per unit area), and deformation is defined as strain (change of length). A stress-strain curve was developed to determine how much stress and strain is beneficial for a bone and how much is detrimental, resulting in bone loss and fracture.

When we consider the body's anatomy, we can understand why increasing muscular stress can increase bone tissue. Examine any place on the body where many muscles or particularly strong muscles attach to the bone and you will find bony protuberances. Just place your hand below your kneecap on the part of your leg that you always cut shaving and you'll understand. That bony protuberance is where your quadriceps or patella tendon attaches to the tibia. It develops when the quadriceps muscles contract through physical activity and the tendon pulls on its attachment to the bone. When a muscle places the right amount of stress on a bone, the bone grows to support the stress. Muscle also adds weight to the bone, and the bone responds by increasing its mass so that it can support the weight. Many studies have shown that bone density decreases when muscles atrophy (waste away) through inactivity and in weightless environments like bed rest and space travel. Many highly competitive swimmers have low bone densities similar to those of sedentary people because of the weightless environment of the pool water. Just remember that strong muscles performing work against gravity make strong bones, and weak muscles that don't challenge gravity make weak ones.

We know that moderate-impact loading-weight-bearing physical activity like walking, jogging, and jumping-has a positive effect on bone mineral density by mechanically loading and stressing the skeleton. As you walk, jog, or jump, you transport your entire body weight a certain distance against gravity, which produces an impact on your heel that travels up through your entire skeleton. Of course the most pronounced effect is in your heel, but the rest of your bones get some benefit too. With each heel strike (the moment at which your foot hits the ground) you are basically tearing down some bone tissue to remodel it and replace it. The body senses the cell deformation that comes from the impact, resorbs the damaged cells, and sends in new cells to replace the old.

Here's a warning for those of you who follow the more-is-better philosophy: When you perform activities that include a great amount of impact or heel strikes, then the greater their number and frequency, the more likely you are to decrease bone mass rather than increase it. Your body just can't produce enough new bone cells to replace the old ones. Bone physiology requires a delicate balance. For example, marathon runners are at risk for fractures and osteoporosis, given the high number of heel strikes required in training for and running a 26-mile race. They lose bone faster than their bodies can replace it. Although they may supplement with calcium or vitamin D, the body can't produce osteoblasts as quickly as the stress and strain of training for a marathon breaks down bone cells. Ballet dancers are also known for having fractures and osteoporosis for a number of reasons (including poor nutrition and low body weight), but a primary factor is overtraining. They perform thousands of heel strikes (or toe strikes) and allow themselves little to no rest. Bone simply needs more time to rebuild. Clearly you must err on the side of moderate walking, running, and jumping to stay ahead in the bonedensity race.

When you think of impact-loading, weight-bearing activity, you usually think of typical cardiovascular endurance exercises like walking, jogging, and jumping, because they emphasize impact loading and weight bearing in the traditional sense. But did you know that the National Osteoporosis Foundation's Physician's Guide to Prevention and Treatment of Osteoporosis (1998) recommends regular muscle-strengthening exercise, both for osteoporosis prevention and for general health? Why is strength training so good for your bones and overall health? Because you typically do it standing (or otherwise bearing your own body weight) while also holding additional weights. Strength training also causes a stress to be applied to bone, and if the stress is significant enough, the bone becomes strained. When the bone is strained, the fluid within the bone moves around in a ripple effect, communicating with all of the cells. The fluid provides nutrition to the bone cells and information that a stress has occurred and must be repaired. Intracellular calcium is released to help the existing osteoblasts rebuild, and secondary messengers stimulate new osteoblast formation. Therefore, strength training is a valuable tool to prevent osteoporosis and fractures.

The strength training that is best for preventing osteoporosis, however, specifically uses free weights instead of machines. Research about the benefits of strength training with machines for bone mineral density suggests that although women can improve their muscular strength and size with machines, they do not increase their bone mineral density. The primary reason for this outcome is that sitting in machines to strength train does not provide your body with the benefit of bearing the weight against gravity, as you do when using free weights. Furthermore, allowing the machine to assist and guide you throughout an exercise doesn't provide enough stress and strain on the bone. You can think of strength training with free weights as a weight-bearing activity.

Not only does strain affect the building of bone, but also the magnitude of that strain, otherwise known as strain magnitude; and how fast this strain is applied and released, otherwise known as strain rate. For bone maintenance, the mechanostat theory states that a minimum effective strain (MES) is necessary. To increase bone tissue, however, the strain has to generate an overload. Thus activities that increase strain magnitude, like heavy strength training, increase your bone mass. The strain on the bone is significantly more than that produced by just standing around or doing normal daily activities. In addition to strain magnitude, it appears that strain rate and unusual strain patterns may play an even more important role than the number of strain cycles, or how often the activity is done. Performing free-weight exercises vigorously in different planes (as in some functional training programs) and jumping instead of running increase both strain rate and magnitude, which is the most beneficial combination for bone building. Check out the section titled Bone-Building Program in chapter 7.