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Physical Activity Guidelines

December 6 - 7, 2007 Advisory Committee Meeting Minutes

Musculoskeletal Health Subcommittee Report

Wendy Kohrt, Ph.D., opened the report on musculoskeletal health. Dr. Kohrt thanked the members of the subcommittee, Miriam Nelson, Jennifer Hootman, Roger Fielding, Nancy Lane and CDC liaisons, David Brown and Jesus Soares. The subcommittee outlined six questions, all of which start with the preface, "is there evidence that physical activity does something?"

  1. With respect to bone, does physical activity reduce the incidence of osteoporotic fractures?

  2. Does physical activity reduce the risk for osteoporosis by increasing or perhaps slowing the decline in bone mineral density?

  3. Does physical activity increase or preserve muscle mass throughout the life span?

  4. Does physical activity improve skeletal muscle quality?

  5. Does physical activity reduce or increase the incidence of osteoarthritis?

  6. Does physical activity have any benefits to individuals with osteoarthritis and other rheumatic conditions?

Dr. Kohrt proceeded in addressing whether physical activity reduces the incidence of osteoporotic fractures? This question was studied through the review of 11 prospective, one retrospective cohort studies, six case controls, one small randomized control trial, a follow-up to a randomized control trial, and two cross-sectional comparisons. In looking at all the different study designs and results there does seem to be enough evidence to support an association between physical activity and reduced fracture risk. However, a limitation of the data is that it does not isolate physical activity as causal in fracture reduction.

Hip fractures are the most frequently examined fracture and the studies are consistent in the beneficial effects of physical activity to reduced hip fracture. From a global skeletal perspective the methods for measuring physical activity may not be adequately capturing or characterizing the potential site specificity of skeletal benefits. It is easier to report a conclusion on a site specific part of the skeleton.

The next issue addressed was sex specificity. The data available included eight women only studies, and surprisingly only five men only studies. All of the studies on women reported a favorable association as well as all the studies on men. Less consistent were the studies that included both men and women. Part of the differences found in combined sex studies may have to do with the different methods of categorizing physical activity.

From a dose-response perspective, both the qualitative and quantitative approaches for discriminating physical activity exposure support an inverse association of level of physical activity with fracture risk, indicating more activity supports better bone health. None of the studies allowed for any conclusions on volume of exercise or intensity.

Dr. Kohrt next addressed the question of whether there is evidence that physical activity reduces the risk of osteoporosis by increasing or perhaps slowing the decline in bone mineral density. Initial conclusions do support that exercise training can attenuate the decrease in bone mineral density. The magnitude of the effect is approximately 1% - 2%. Both endurance training and resistance training can be effective. Long term effects are not yet known. Finally, there is no evidence that has emerged to support a dose-response effect.

Dr. Kohrt introduced Jennifer Hootman who reviewed the evidence on the relationship between physical activity and the reduction of incidence of osteoarthritis and whether physical activity had any benefit to people with osteoarthritis. The group's initial impression is that, in the absence of major joint injury, there is no evidence that regular, moderate or vigorous physical activity increases the risk of osteoarthritis nor does it provide any benefits to individuals that currently have a rheumatic condition. There is moderate evidence that light to moderate activity may be protective for knee and hip osteoarthritis, particularly among women. From the CDC database 35 studies were identified of which 12 were observational, 6 case control and 6 were cohort studies. The self-report studies can be seen as obtained through members of the general population. The control or cohort studies can generally be seen as dealing with elite athletic populations. The studies dealing with elite athletes do show increased incidences of rheumatic conditions but one can conclude the increased incidence from high-level intensity training.

 

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