Principal Investigator: David B. Burr, Ph.D.
Institution: Indiana University School of Medicine.
Title: Use of Shoe Orthotics to Reduce Bone Strain Rate
Abstract: (A) As a preliminary study to the one proposed for NOCSAE, we tested the hypothesis that tibial strain rates increase during the strenuous, fatiguing exercises that military recruits do. In a prior study an age-related relationship was found between fatigue and increased tibial strains, with older subjects having lower increases. In this study we found that the relationship between fatigue and increases in strain rate was related to the degree of fatigue reached and not to subject age. In the muscle fatigue state maximum tension strain rates may increase and may thereby contribute to the development of stress fractures, but when near total fatigue is reached the maximum tensile strain rate diminished.
(B) We tested the hypothesis that significantly higher strains and strain rates would be produced by more dynamic, impulsive types of loading, such as jumping. If so, then the reduction of impact loads by orthotics during fatiguing exercise could lower stress fracture risk. We found that peak compressive and tensile strains generated upon landing from a jump are not significantly greater than peak strains measured during vigorous uphill and downhill running. However, peak shear strains generated by landing from a jump are nearly 3x maximum shear strains measured during vigorous non-jumping activities. Peak shear strains are 3-6x higher than compressive and tensile strains. Surprisingly, strain rates created by jumping from half a meter are lower than those generated during running. Muscular and kinematic shock absorption mechanisms can explain this reduction of strain rate. Training programs that eliminate continuous high-impact activities, or teach people how to use kinematic mechanisms to reduce strain rates, are likely to result in decreased stress fracture incidence. Also, orthotic devices that can reduce high peak shear strains are likely to reduce stress fracture incidence.