What Makes Bone Adapt: Gravitational Forces or Muscle Loading
Len Kravitz, Ph.D.

Exercise professionals regularly exclaim the benefits of exercise towards maintaining and improving bone mineral density and bone health. One of the major tenets of exercise, as we project and plan for our aging exercise enthusiasts, is osteoporosis prevention from physically active lifestyles. Personal trainers regularly describe the benefits of gravitational weight-bearing (i.e., ground impact activities such as walking, running and jumping) and muscle loading activities such as weight training. However, scientists are currently debating which exercise intervention provides the more beneficial bone health adaptations. Are the impact forces from the gravitational force activities (i.e., contact of the body with the ground) the key factor that stimulates bone growth, or is it the bone-loading mechanisms from muscle (i.e., the contractile forces from muscle through the tendon to bone) that provide bone the better stimulus for enlargement.

No study has unequivocally demonstrated that either gravitational or muscle forces impart the dominant growth promoting effect on bone. Recently, Medicine & Science in Sports & Exercise published a series of articles exploring this debate, which will be highlighted in this column, as well as some descriptive details how bone builds, remodels and breaks down.

Bone Basics 101
Bone is clearly one of the most essential, intricate and interesting organs of the human body. It is affected dramatically by physical activity, nutrition, and life changes such as menopause. It serves as a reservoir for calcium, with 99% of the body's calcium in the bones and teeth. The remaining 1% of calcium is found in the blood and interstitial (liquid found between the cells of the body) fluids (Taylor and Johnson, 2008). The 206 bones in the adult human skeleton provide structural support for the muscles to attach and protection for all vital organs including the spinal cord, brain, heart and lungs.

Bone in human and other mammal bodies is generally classified into two types: 1) trabecular bone, also known as cancellous bone and 2) cortical bone, also known as compact bone. The trabecular bone makes up the interior of bone and has a spongy construction. It has many blood vessels and contains red bone marrow, where the production of red blood cells occurs. Cortical bone, or compact bone is the other of the two types of osseous (bone) tissue that forms bones. The cortical bone is the outer shell of bone and is a much denser, stronger and stiffer tissue than the trabecular bone.

Bone renews itself through a process called bone remodeling which involves two phases, formation and resorption. Bone tissue formation is referred to as ossification. Ossification is the process cartilage, which is relatively soft, transforms into hard bone during infant and child development. Bone resorption is the process by which osteoclasts (specialized cells that secret enzymes that dissolve bone) break down bone and transfer calcium, magnesium, and phosphate products from bone fluid to the blood. Bone resorption can also be the result of disuse and the lack of stimulus for bone maintenance, thus one of the major physiological reasons for promoting regular exercise. During childhood, bone formation exceeds resorption, but as the aging process occurs, resorption exceeds formation.

How Does the Body Regulate Bone Health?
Vitamin D, which is primarily absorbed from sunlight through the skin, becomes biologically activated in the kidneys to form calcitriol, which helps the bones absorb calcium from the blood. Calcium binds with phosphorus to form calcium phosphate, a mineral that helps to form and strengthen bones. The kidneys also excrete phosphorus (a dietary mineral), which helps to balance calcium levels in the blood. Thus, if kidney function becomes impaired, bones may not get enough calcium either because they allow too much phosphorus build up in the blood or the kidneys are failing to turn vitamin D into calcitriol. The parathyroid gland (which is actually four small glands located posterior to the thyroid gland) regulates how much calcium is secreted by the kidneys as well how much calcium your bones will store. For instance, if blood calcium levels drop below a certain point, specialized calcium-sensing receptors in the parathyroid gland are activated to direct the kidneys to release more calcium. Estrogen helps the parathyroid glands keep calcium levels in balance. The drop in estrogen levels at menopause leads to greater bone resorption, and loss of bone tissue in females. In men, testosterone plays an important role in bone maintenance and bone formation through a complex interaction between testosterone and other hormonal receptors. Thus for men, a low testosterone level is a risk factor for developing osteoporosis.

The Debate: Gravitational Forces or Muscle Loading: What Dominates Bone Loading Better?
A clear scientific answer on what stimulus most favorably affects bone adaptation would help exercise professionals design optimal exercise programs. Beck (2009) observes that a perplexing and unresolved issue with bone is that computer modeling studies 'introducing' and/or 'removing' muscle loading forces during gait activities have both been shown to positively influence bone growth. In other words, in weight-bearing activities, muscles may impart additional loads on the long bones of the legs or counteract the gravitational forces being placed on the bone. Judex and Carlson (2009) state that this controversy between gravitational forces and muscle loading on bone stems from the fact that scientists really doesn't fully understand the series of signals that occur in a skeletal segment to promote bone growth. However, Robling (2009) notes that research does know that it is fluid shear forces (forces of fluid parallel to material in cells) within bone cells and not the mechanical stretch of the bone cells that appears to elicit bone growth from exercise. In addition, Judex and Carlson state that during many exercise movements the forces from gravity and muscle (going through concentric and eccentric contractions) are dynamically changing, and thus separating the contributions of both forces is not possible. The authors add that this controversy is not simply answered by measuring which force provides the greatest load (or strain) on bone. In fact, Judex and Carlson add that bone growth is very sensitive and responsive to different patterns of muscle force recruitment and various speeds of force and contraction from the exercise movement.

Take Home Message to Exercise Professionals
Importantly, from a health perspective, Kohrt, Barry and Schwartz (2009) emphasize that there is convincing evidence that being physically active lowers the risk of hip fracture dramatically (see Side Bar 1 for more). Considering the interdependence of gravitational and muscle loading forces on bone, the magnitude of which is more important may be an unresolved scientific answer. The literature provides favorable evidence for both, muscle forces and gravitational loading, for skeletal bone adaptation. It also appears from this scientific review that there is no ONE specific dose of exercise that appears to be best for bone adaptations. In fact, the research appears to suggest just the opposite. Exercise professionals may best benefit clients' bone strengthening adaptations by incorporating different gravitational activities (walking, running, jumping, stair climbing, etc.), with different muscle loading environments (on land and in water), at various speeds (slow, moderate and explosive), and with varying resistance training load (moderate to high) plans. Alas, incorporating all of the strategies and systems currently being employed in successful periodization programs to aerobic and resistance training design may be the optimal exercise solution for designing exercise programs to improve bone strength and reduce the risk of osteoporosis and hip facture.

Side Bar 1: Ten Facts About Hip Fractures, Falls and Osteoporosis
1) Moderate-to-vigorous physical activity is associated with a hip fracture risk reduction of 45% in men and 38% in women
2) The most common incidents of falls are walking and going up and down stairs
3) 50% of all women and 33% of all men will have a bone fracture during their lifetime
4) Osteoporosis affects more than 200 million women worldwide
5) The annual healthcare costs of osteoporotic fractures in the United States are estimated at 7 to 10 billion dollars
6) Fracture risk is primarily determined by three factors: force of impact of a fall, bone strength, and risk of falling (see below)
7) Established risk factors for falling are: older age, impaired balance, impaired vision, decreased reaction time, weakness in lower-extremity muscles, loss of muscle mass, impaired mobility, and orthostatic hypotension (low blood pressure occurring in some people when they stand up)
8) Sedative medications, alcohol intake, inappropriate footwear, and physical factors in the environment (such as stairs, lighting, and uneven street surfaces), are also risk factors for falling
9) Hip and wrist fractures risk are thought to be influenced by both the tendency to fall and a loss in bone strength
10) Vertebral fractures have not been causally linked to falls and appear to be more related to a loss in bone and muscle strength
Source: Moayyeri, A. (2008). The association between physical activity and osteoporotic fractures: a review of the evidence and implications for future research. Annals of Epidemiology, 18(11), 827-835.

References:
Beck, B.R. (2009). Muscle forces or gravity--What predominates mechanical loading on bone? Introduction. Medicine & Science in Sports & Exercise, 41(11), 2033-2036.

Judex, S. and Carlson, K.J. (2009). Is bone's response to mechanical signals dominated by gravitational loading? Medicine & Science in Sports & Exercise, 41(11), 2037-2043.

Kohrt, W.M., Barry, D.W., Schwartz, R.S. (2009). Muscle forces or gravity: What predominates mechanical loading on bone? Medicine & Science in Sports & Exercise, 41(11), 2050-2055.

Robling, A.G. (2009). Is bone's response to mechanical signals dominated by muscle forces? Medicine & Science in Sports & Exercise, 41(11), 2044-2049.

Taylor, A.W. and Johnson, M.J. (2008). Physiology of exercise and healthy aging. Human Kinetics.


@Bio:Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, Albuquerque, where he won the Outstanding Teacher of the Year award. Len was honored with the 2009 Can-Fit-Pro Specialty Presenter of the Year award and chosen as the ACE 2006 Fitness Educator of the Year. He also received the 2008 Can-Fit-Pro Lifetime Achievement Award.
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