Osteoporosis: How Much Exercise Is Enough for Bone Health?

In addition to the extensive body of literaturethat supports medical therapiesfor osteoporosis and for preventionof postmenopausal fractures, numerousstudies have explored thebenefits of exercise in both premenopausaland postmenopausal women.¹The results of these studies indicatethat weight-bearing exercise and resistancetraining may play a role in preventingbone loss and increasing bonemass.

How much physical activity is requiredto promote bone health? Andwhich types of exercise are most beneficial?Here, we offer answers to thesequestions and offer some specific recommendationsbased on an analysis ofthe literature.

PATHOPHYSIOLOGYBone metabolism. Osteoporosisis characterized by bone loss accompaniedby qualitative changes in bonemicrostructure. These changes compromisethe integrity and strength ofbone, which results in an increasedrisk of fracture.² Bone strength is determinedby bone mineral density(BMD) and bone quality. BMD correlateswith the magnitude of peak bonemass-which is attained in mostwomen by the age of 30 to 35 years-and subsequent loss of bone mass.Bone quality is determined by bone architecture,turnover, mineralization,and the amount of damage accumulatedin the form of microfractures. Afracture results when a sufficient failure-inducing load, such as a fall, is appliedto an osteoporotic or compromisedbone.¹

Osteoporosis occurs as a result ofan imbalance between bone formationand bone resorption. In young persons,new bone is formed at a rateequal to that at which it is resorbed.Loss of BMD in older persons iscaused by an increase in osteoclastbone resorption activity and a concomitantdecrease in osteoblast-mediatedbone formation. In postmenopausalwomen, this imbalance is furtherexacerbated by a decrease incirculating estrogen, the hormone thatis believed to regulate the activity ofbone remodeling cells through cytokinemodulation. Laboratory studies have demonstrated that estrogen mayaffect osteoclast apoptosis and osteoblastproliferation and thereby promotepositive bone modeling.^3,4

Effect of strain on bone. In additionto the relative activity of osteoblastsand osteoclasts, it has beentheorized that bone remodeling-bothlocally and throughout the skeleton-is continually influenced by the leveland distribution of the functional strainacting on bone. According to Wolff'slaw, remodeling depends directly onthe mechanical strain placed on boneand the functional requirements ofbone.⁵ Strain induction-the normaldeformation that occurs under loading-may cause a greater level of boneformation and an inhibition of resorptionwithin the normal remodelingcycle.⁶ Laboratory studies have demonstratedthat imposed mechanicstrains can produce osteogenic stimulicapable of increasing bone mass.⁷ Exposureto intermittent forces decreasesmineral resorption in part throughdecreased osteoclast activity.⁸ Therefore,induced strains may affect bonemass through the inhibition of osteoclastresorption and stimulation of osteoblastgrowth.² In older persons, arelative decrease in weight-bearing activitiestends to promote further boneloss.

EXERCISE AS THERAPY
The rationale for exercise as ameans of reducing or preventing postmenopausalbone loss is based on thephysiologic effects of mechanicalstrain and loading on bone, the resultingdecreased rate of bone loss, andthe actual increase in bone mass. Anumber of studies have explored theeffects of different interventions, rangingfrom simple low-intensity weightbearingexercises to variable resistanceexercises, on various measuresof BMD. Weight-bearing exercise mayhelp to increase BMD by imposingstresses through repetitive impactloading. Resistance training may helpby exposing bone to varying loads andrates of strain.

Exercise performed at significantintensity and frequency in the settingof adequate calcium and vitamin D intakeprobably has a modest effect onslowing the decline in BMD in postmenopausalwomen.¹ Most studiesdemonstrate a reduction in the ratesof bone loss at the spine, wrist, andfemur in these women compared withpostmenopausal controls who did notexercise. The rate of bone loss maybe as much as 3% to 7% per year in the7 to 10 years around the onset ofmenopause and 1% to 2% per year aftermenopause.³ Since each standard deviationdecrease below the average infemoral neck BMD increases the ageadjustedrisk of hip fracture 2.6 times,a decrease in the rate of bone loss,such as may be achieved with exercise,would reduce the risk of osteoporoticfracture.⁹

Weight-bearing exercise. Studiesof weight-bearing exercise in elderlypostmenopausal women have primarilyinvolved brisk walking, low-impactaerobics, and occasionally jogging--rather than more intense activities-because of the lower levels of fitnessand increased risk of falls in this agegroup. These studies-which are summarizedin Table 1-suggest that increasedfrequency, duration, and intensitymay significantly affect the resultsof a weight-bearing exercise programon BMD.^10-15

Resistance training. Many postmenopausalwomen have preexistingfractures and are at an increased riskfor falls. Resistance training is a viableexercise alternative to combat boneloss in these women as well as an importantadjunctive therapy in womenwho are at lower risk for falls. Likeweight-bearing exercise, resistance exerciseseems to require a threshold intensityto have a positive impact onbone modeling. Several studies haveshown that bone mass in postmenopausalwomen is unaffected by a low-intensityprogram. Moreover, resistanceexercises may selectively affect theBMD of specific bones by targeting themuscles that support them or that originatein or insert on them. Progressivestrengthening routines, in which theload is increased as strength increasesto maintain the intensity of stimulus,may result in significantly lower ratesof bone loss. In addition to the reductionin the rate of bone loss through resistancetraining, the improvement inmuscle strength may provide a basisfor better coordination, balance, andstrength that may ultimately protectagainst falls and fractures.¹⁶

With regard to duration, it is likelythat bone needs to be exposed tostrain for a significant period-say, 45minutes twice a week-to have the desiredeffect on BMD. Studies of resistancetraining are summarized inTable 2.^16-23

A FEW CAVEATS
Despite some encouraging findings,a number of factors make it difficultto quantify the impact of exerciseimpact on BMD in postmenopausalwomen. Very few longitudinal studieshave examined the effectiveness of exercisein reducing fracture rates. Furthermore,the studies cited here reflectsignificant variability in types ofexercise, age of participants, and measurementsof initial BMD. This makesit difficult to generalize results to allpostmenopausal women.

In the studies that involvedyounger women, initial bone densitiesmay have been in the normal range.Normal bone may be less responsive tolower-intensity exercises.²⁴ The sameexercises may be more effective in a differentpopulation. Moreover, because avariety of exercises were studied, it isdifficult to draw conclusions about theeffectiveness of a particular exercise interventionfor a given population.

Another confounding factor is theconcomitant use of pharmacotherapy.Because many of the studies involved women who were using hormone replacementtherapy (HRT), calcium, orvitamin D, it is difficult to quantify howmuch of an effect exercise alone hason BMD.^{11-13,16,18,20} In one study of postmenopausalwomen with low BMD,exercise alone was far less effectivethan exercise plus HRT or calciumsupplementation in slowing bone lossor increasing BMD.²⁵ However, in amore recent study that controlled forvarious factors that could affect BMD(including dietary dairy intake andHRT), women who engaged in prolongedmoderate-intensity physical activityhad higher BMD than matchedcontrols who did not exercise.26 Futurestudies that control for pharmacotherapy,as this one did, may help elucidatethe impact of exercise on BMD.

Compliance is a significant problemwith long-term exercise programs,especially those involving resistancetraining.^18,21,25 Studies with higher ratesof compliance (90% to 100%) generallyincluded trainer supervision or involvedhighly motivated participants.^16,19 Thus, more highly motivatedpatients may have better success witha structured exercise program, especiallyone that includes resistancetraining. However, a recent study thatevaluated long-term physical activityfound that even moderate weight-bearingexercise was associated with higherBMD.²⁶ Although a regimen that includesboth weight-bearing exercisesand resistance training is likely to havethe most significant impact on BMD,a simple weight-bearing program performedat moderate intensity may stillprove beneficial and may encouragecompliance.

Further studies involving largerstudy populations and different levelsof intensity are needed to help define adose-response relationship before anydefinitive conclusions can be made therapy for osteoporosis. Longitudinalstudies that examine how exercise affectsfracture rates would be especiallyhelpful.

BENEFITS BEYOND BONE
Beyond the effect on BMD, exercisehas many additional benefits inpostmenopausal women. A number ofstudies^11,24,25 have shown that regularexercise can:

• Improve psychological health.
• Increase muscle mass, strength, andendurance.
• Reduce musculoskeletal complaints.
• Prevent chronic disease.
• Decrease the risk of falls through improvedagility and balance.
• Promote functional independence.

RECOMMENDATIONSFOR YOUR PATIENTS
Include exercise in the treatmentregimen of motivated patients withpostmenopausal osteoporosis. An exerciseprogram that involves both brisk weight-bearing activity and progressiveresistance training can easilybe supervised within the primary caresetting.

An effective strength-training programuses both large and small musclegroups; exercises-such as bicepscurls-are performed through a fullrange of motion. Commercially availablefree weights or household itemssuch as cans may be used as resistanceobjects. Instruct patients to startby attempting to perform 2 or 3 sets of8 to 12 repetitions at 70% to 80% of 1repetition maximum twice per week;weights should be increased graduallyevery few weeks. A good weight-bearingprogram should include 30 to 45minutes of brisk activity, such as walking,4 or 5 times per week. Each sessionshould be preceded by 10 to 15minutes of gentle stretching exercises.In addition, patients can incorporateexercises that help strengthen themuscles that support the spine (seethe Health Guide on page 834).

You may wish to refer patientswith a history of vertebral compressionfractures or concomitant musculoskeletalconditions, such as degenerativejoint disease, to a physiatrist, whocan help initiate a monitored exerciseprogram.

References:

REFERENCES: 1. NIH Consensus Development Panel on OsteoporosisPrevention, Diagnosis, and Therapy. Osteoporosisprevention, diagnosis, and therapy. JAMA.2001;285:785-795.
2. Allen SH. Exercise considerations for postmenopausalwomen with osteoporosis. Arthritis Care Res.1994;7:205-214.
3. Kenny AM, Prestwood, KM. Osteoporosis: pathogenesis,diagnosis, and treatment in older adults.Rheum Dis Clin North Amer. 2000;26:569-591.
4. Bente JL. Biochemical markers of bone turnover,II: diagnosis, prophylaxis and treatment of osteoporosis.Am J Med. 1993;95:17S-21S.
5. Rubin CT. Response of bone to mechanical stimulation.In: Dee R, ed. Principles of Orthopedic Practice.New York: McGraw Hill; 1989:79-88.
6. Chilibeck PD, Sale DG, Weber CE. Exercise andbone mineral density. Sports Med. 1995;19:103-122.
7. Rubin CT, Lanyon LE. Regulation of bone formationby applied dynamic loads. J Bone Joint Surg.1984;66A:397-402.
8. Klein-Nulend J,Veldhuijzen JP, van Strien ME, et al.Inhibition of osteoclastic bone resorption bymechanical stimulation in vitro. Arthritis Rheum.
9. Cummings SR, Nevitt MC. Falls. N Engl J Med.1994;331:872-873.
10. Krall EA, Dawson-Hughes B. Walking is relatedto bone density and rates of bone loss. Am J Med.1994;96:20-26.
11. Michel BA, Bloch DA, Fries JF. Weight-bearingexercise, overexercise, and lumbar bone densityover age 50 years. Arch Intern Med. 1989;149:2325-2329.
12. Bravo G, Gauthier P, Roy PM, et al. Impact of a12-month exercise program on the physical and psychologicalhealth of osteopenic women. J Am GeriatrSoc. 1996;44:756-762.
13. Dalsky GP, Stocke KS, Ehsani AA, et al. Weightbearingexercise training and lumbar bone mineralcontent in postmenopausal women. Ann Intern Med.1988;108:824-828.
14. Cavanaugh DJ, Cann CE. Brisk walking doesnot stop bone loss in postmenopausal women. Bone.1988;9:201-204.
15. Hatori M, Hasegawa A, Adachi H, et al. The effectsof walking at the anaerobic threshold level onvertebral bone loss in postmenopausal women.Calcif Tissue Int. 1993;52:411-414.
16. Nelson ME, Fiatarone MA, Morganti CM, et al.Effects of high-intensity strength training on multiplerisk factors for osteoporotic fractures: a randomizedcontrolled trial. JAMA. 1994;272:1909-1914.
17. Beverly MC, Rider TA, Evans MJ, et al. Localbone mineral response to brief exercise that stressesthe skeleton. Br Med J. 1989;299:233-235.
18. Mayoux-Benhamou MA, Bagheri F, Roux C, et al.Effect of psoas training on postmenopausal lumbarbone loss: a 3-year follow-up study. Calcif Tissue Int.1997;60:348-353.
19. Hartard M, Haber P, Ilieva D, et al. Systemicstrength training as a model of therapeutic intervention:a controlled trial in postmenopausalwomen with osteopenia. Am J Phys Med Rehabil.1996;75:21-28.
20. Sinaki M, Wahner HW, Offord KP, Hodgson SF.Efficacy of nonloading exercises in prevention ofvertebral bone loss in postmenopausal women: acontrolled trial. Mayo Clin Proc. 1989;64:762-769.
21. Preisinger E, Alacamlioglu Y, Pils K, et al.Therapeutic exercise in the prevention of bone loss.Am J Phys Med Rehabil. 1995;74:120-123.
22. Smidt GL, Lin SY, O'Dwyer KD, Blanpied PR.The effects of high-intensity trunk exercise on bone mineral density of postmenopausal women. Spine.1992;17:280-285.
23. Chow R, Harrison JE, Notarius C. Effect of tworandomised exercise programs on bone mass ofhealthy postmenopausal women. Br Med J. 1987;295:1441-1444.
24. Miszko TA, Cress ME. A lifetime of fitness. Exercisein the perimenopausal and postmenopausalwoman. Clin Sports Med. 2000;19:215-232.
25. Prince PL, Smith S, Dick IM, et al. Preventionof postmenopausal osteoporosis: a comparativestudy of exercise, calcium supplementation, andhormone replacement therapy. N Engl J Med. 1991;325:1189-1195.
26. Hagberg JM, Zmuda JM, McCole SD, et al.Moderate physical activity is associated with higherbone mineral density in postmenopausal women.J Am Geriatr Soc. 2001;49:1565-1567.

FOR MORE INFORMATION:

• Beck TJ, Oreskovic TL, Stone KL, et al. Structuraladaptation to changing skeletal load in the progressiontoward hip fragility: the study of osteoporotichip fractures. J Bone Miner Res. 2001;16:1108-1119.
• Chien MY, Wu YT, Hsu AT, et al. Efficacy of a24-week aerobic exercise program for osteopenicpostmenopausal women. Cacif Tissue Int. 2000;67:443-448.
• Kerr D, Ackland T, Maslen B, et al. Resistancetraining over 2 years increases bone mass in calcium-replete postmenopausal women. J Bone MinerRes. 2001;16:175-181.
• Winters KM, Snow CM. Detraining reversespositive effects of exercise on the musculoskeletalsystem in premenopausal women. J Bone Miner Res.2000;15:2495-2503.