The importance of thermo-mechanical properties in the selection of athletic shoe cushioning foams

Abstract

VALIDATION OF A MATHEMATICAL MODEL OF THE EFFECT OF SOLE ELASTICITY DISTRIBUTiON ON PRONATION Perry Lit and Zvi LadinS iDepartment of Mechanical Engineering, University of California, Berkeley, CA 94720 SBiomedical Engineering Department and NemoMuscular Research Center, Boston University, Boston, MA 02215 Excessive pronation has been cited as a major cause for many musculo-skeletal injuries associated with running. An accurate mathematical model is a useful tool in the design of midsoles in running shoes to prevent excessive pronation. A validation experiment is presented to verify a mathematical model which predicts the effect of the coronal plane elastic distribution in the midsole on pronation and the center of pressure location. The model is based on a rigid body model of the sole obtained via an energy minimization approach presented earlier. Experimental results show that the predictions of both the pronation angles and the center of pressure locations are accurate when the elastic distribution was systematically varied. The model is believed to have the potential of being a useful quantitative tool for engineers and shoe designers. THE IMPORTANCE OF THERMO-MECHANICAL PROPERTIES IN THE SELECTION OF ATHLEI’K SHOE CUSHIONING FOAMS Jeff Poliner, Spencer White, and Mark Fenton Reebok International, Ltd. Material tests have demonstrated that the foams used for cushioning in athletic footwear exhibit a large amount of hysteresis energy loss during impact. Some of this energy is converted to heat, which can modify the stiffness characteristics of the foam. This study was designed to measure the temperature increase in running shoes, to examine its effect on material stiffness during the course of a single exercise bout, and to compare materials with high and low hysteresis. Six subjts ran in shoes constructed with midsoles composed of either EVA or Hytrel foam (I-IF) of similar stiffness, which exhibited approximately half the hysteresis energy loss of the EVA. This teat resulted in an average increase in temperature of 11.7T in EVA and 90°C in HF shoes over the course of a 7.3 km run. testing found a similar rise in temperature, and a significant drop in stiffness (EVA 36%, Mechanical impact HF 11%) over 4ooO cycles. This test also demonstrated that EVA was significantly more sensitive to changes in temperature than HF. These results demonstrate that during a single bout of exercise there is a significant increase in temperature, and a concomitant decrease in stiffness, of foam materials. The increase in temperature appears to be related to mechanical energy losses during impact, since a material with lower hysteresis (Hytrel foam) demonstrated less temperature increase than one with higher hysteresis (EVA). I-IF also demonstrated less sensitivity to changes in temperature. These results indicate that them-to-mechanical behavior should be considered when choosing cushioning materials for use in athletic footwear. SESSION t&CERVICAL SPINE Chairperson: Gary Y amaguchi VERIFICATION OF ANALYTICAL MODELS APPLICABLE TO MOTION ANALYSIS IN THE CERVICAL SPINE. Peter Rootmon, Serge A. Gracovetsky, Gerard J Gouw, Nicholas Newman. Diagnospine Research Inc, Concordia University Departments of Electrical and Mechanical Engineering, Department of Orthopaedic Surgery, Hotel Dieu Hospital, Montreal,Quebec. In recent years, numerous in-vivo and in-vitro studies have been undertaken to quantify the mobility of cervical motion segments. As a result, much of the information required to verify comprehensive analytical models of the cervical spine is now available. A new opto-electronic device has been developed which may be used to help in such verifications. This is a high resolution system which tracks light emitting diodes affixed to the subjects’ skin. Our purpose is to consider some of the existing models and to see how they may be correlated with motion studies. We begin by examining the role of analytical models in studying the entire spine. Then we look at experimental methods employed in modelling of the spine, as well as some specific examples of analytical and physical models which are applicable specifically to the cervical region. We also consider the current state of modelling for the spinal cord. The physical models available do not yet appear to simulate head/neck motion adequately. Hence, entirely accurate predictive injury modelling is not currently possible. Nevertheless, normal and pathological motion can be approximated. Hence motion analysis through tracking of skin markers is expected to be useful in the verification of analytical models. Ti!I 25~6-6 663