Exercise that puts the "best" kind of mechanical load to strengthen bones, especially during childhood and adolescence, Turner says, involves impact or high rates of load such as running or jumping, as opposed to swimming or biking. Growing bones are most responsive to the strengthening effects of running/jumping, which have the additional benefit that these types of exercise don't affect longitudinal growth, Turner says.
Activities like "serious weight-lifting, however, aren't recommended for children because overloading growing joints can stunt longitudinal bone growth," and consequently stunt overall limb growth and height, he adds.
Turner says that the strengthening effect of exercise is very efficient because the cellular mechanosensors within bone direct osteogenesis (new bone growth) to where it is most needed to improve bone strength and hence bone mass.
Editors note: Turner is reporting his findings at the American Physiological Society's 2004 Intersociety Meeting, "The Integrative Biology of Exercise," Oct. 6-9 in Austin.
The schedule for the exercise meeting can be found at (http://www.
Arrangements for on-site interviews, or telephone interviews during the meeting can be arranged through APS Communications Officer Mayer Resnick (cell: 301-332-4402, email@example.com) or through Stacy Brooks, APS Communications Specialist, 301-634-7253). From Oct. 6 (2 p.m.) - Oct. 9, the onsite phone number in Austin is 512-482-8000, room 602, or direct dial: 512-682-2950.
The meeting is cosponsored by APS, the American College of Sports Medicine and the Canadian Society for Exercise Physiology. Additional support through unrestricted educational grants came from: the National Aeronautics and Space Administration (NASA), the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMSD), Gatorade Sports Sciences Institute, Pfizer Inc. and the U.S. Army Research Institute of Environmental Medicine (USARIEM).
Mechanosensors, desensitization under study
Though the cellular mechanosensors are very efficient, Turner noted that the biological processes involved in bone mechanotransduction are poorly understood, "yet several pathways are emerging from current research." These include ion channels in the cell membrane, ATP (adenosine triphosphate) signaling, and second messengers such as prostaglandins and nitric oxide. Specific targets of mechanical loading include the L-type calcium channel (alpha 1C isoform), a gadolinium-sensitive stretch-activated channel, P2Y2 and P2X7 purinergic receptors, EP2 and EP4 prostanoid receptors, and the parathyroid hormone receptors.
"One characteristic of the mechanosensing apparatus that has only recently been studied is the important role of desensitization," Turner notes. "Experimental protocols that insert 'rest' periods to reduce the effects of desensitization can double anabolic responses to mechanical loading," he adds. Again, it's unclear how desensitization of bone cells occur, but it's an area ripe for further study.
A recent paper with his colleague, Alexander G. Robling, "Designing exercise regimens to increase bone strength," dealt with desensitization and age-related effects of exercise, among many other topics, including development of an exercise "osteogenic index" or OI. The paper appeared in the "Exercise and Sport Sciences Reviews." Among the OI observations were: (1) "short intense exercise bouts build bone most effectively, hence short sprints should build more bone than a long jog," (2) "OI is best improved by adding more exercise sessions per week rather than lengthening the duration of individual sessions," (3) "to reduce exercise time, it is far better to shorten each session than to reduce the number of sessions," and (4) "the osteogenic potential of exercise can be increased further when daily exercise is divided into two shorter sessions separated by 8 hours."
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