Changes in Mechanical Stress- Bone Adaptations

1. Changes in Mechanical Stress- Bone Adaptations • Wolff's Law (1890s) - Bone remodels according to functional demands • Not tested until the 1960s • Exercise as a health therapeutic agent • Spaceflight

2. BONE and Spaceflight

3. Bone Loss with Spaceflight • Exposure to the spaceflight causes men and women of all ages to lose up to 1% of their bone mass per month due to disuse atrophy. (height increases by 2+ in! - swelling of vertebral discs) • Loss of bone mass may continue as long as a person remains in the microgravity environment? Levels off? Data from Russian Soyuz and MIR missions: only partial recovery of bone mass after 12-18 months of spaceflight

4. Comparing Spaceflight and Inactivity • Changes in Spaceflight rapid - similar initially to immobilization. Slower with inactivity, but progressive. • Decreases in loading, growth factors (IGF-1, TGF), decrease in osteoblast activity (decreased formation), increased osteoclast activity (resorption) • Reduced physical activity is characteristic of aging and ---> loss of bone, but researchers have not yet determined how much of a role disuse plays on Earth.

5. EXERCISE IN SPACE http://www.nasaexplores.com/show2_articlea.php?id=04-202 In order to exercise on this ergometer, the astronaut must be held down with shoulder pads. Strategies • Compression - Exercises • Vibration • Electrical Stimulation • Vitamin D • Ca2+ On the KC-131 (“Vomit Comet”)

6. EXERCISE IN SPACE interim resistance exercise device (iRED) Strategies • Compression - Exercises • Vibration • Electrical Stimulation • Vitamin D • Ca2+

7. Exercise and Bone Health • Wolff's Law - Bone remodels according to functional demands • Exercise increases mechanical stress and strain, growth hormone levels which contribute to increasing bone mass and density • stronger, stiffer, and able to store more energy • However, overtraining, especially in children and older adults is counterproductive.

8. Exercise Training - Animal Studies • Researchers can directly measure: • Material properties • Stiffness, strength, ductility, energy • 6-12 months of training increases bone mass, thickness, density, strength, max. energy absorption

9. Exercise Training - Animal StudiesDr. Savio Woo, 1981 young miniature swine - exercised 12 months

10. Research Factors with Humans These factors should be accounted for when examining the effects of exercise on bone health. • Cross-sectional vs. longitudinal studies • Age • Exercise protocol • type • intensity-weight bearing • duration • specificity • Length of the study

11. Research Factors (continued): • Compliance with established research protocols • Gender • Hormone levels • estrogen • testosterone • cortisol • growth hormone • Are the bone sites measured appropriately? • How is bone size, quality measured?

12. Research Factors (continued): • In humans, bone mass and density are measured INDIRECTLY • X-ray (DEXA or dual x-ray absorptiometry) • Light beams (Dual Emission Photon Absorptiometry) • Computerized tomography (CT) • Ultrasound (sound waves)

13. Research Study 1 • Jones et al., 1977 They found that in tennis players that the thickness of bones in the dominant arm > non-dominant arm. • Sedentary individuals had no difference in bone thickness or strength between the dominant and non-dominant arm.

14. Research Study 1 30% thicker in dominant humerus or tennis players

15. Research Study 2 • Nilsson and Westlin, 1971. • bone density of the femur increases as the fitness level (participation in exercise) of individuals increased. • All exercising groups (whether competitive athletes or not) had higher bone densities than sedentary controls.

16. Research Study 2

17. RESULTS OF EXERCISE TRAINING 1. Exercise training can increase bone density and mass (and energy absorption) in youths and adults (young and middle-age) •strength and stiffness increased 2. In the elderly, exercise can maintain bone mass or slow loss of bone mass

18. RESULTS OF EXERCISE TRAINING 3. Types of exercise that are most effective: • weight bearing (walking/running, stair climbing, etc.) • weight training

19. RESULTS OF EXERCISE TRAINING 4. Exercise increases material properties through mechanical stress and strain: * Piezoelectric effect * Cytoskeleton (nitric oxide synthase, focal adhesion kinase) * Fluid shear (blood and interstitial - NOS) * interaction with hormones (growth hormone, estrogen) * prostaglandins * growth factors (VEGF, IGF-1, TGF)

20. Growth factors - Exercise • IGF-1 responds with level of mechanical stress, exercise intensity • Bone fracture healing increased in rats by IGF-1 + TGF injection • TGF - responds to large stress, high intensity, strenuous exercise, protects against overshoot of inflammation? • TGF produced by osteoblasts with heavy exercise - released? Training increases TGF in muscle • VEGF - increases after injury and by exercise - angiogenesis • During growth IGF-1 and TGF increase in bone

21. Benefits of exercise are optimized or enhanced by: • adequate caloric intake • adequate Ca2+ in diet • adequate Vitamin D intake • adequate levels of hormones • estrogen • testosterone • growth hormone