DRAG COEFFICIENT AND FRICTION COEFFICIENT IN ROLLER SKATING. AN INDIRECT DETERMINATION. SOME SUGGESTIONS ABOUT TRAINING LOADS

Abstract

INTRODUCTION: Only a few biomechanical studies of roller skating are available. There are differences between roller skating and ice skating, and consequently it is not correct to transfer a model from one of the two sports to the other. Roller skaters encounter resistance due both to air and the friction of their skate wheels. No experimental data are available regarding these values, and consequently an estimate of the power required to roller skate at different speeds is not possible. The purpose of this work is to estimate the frictional forces by an indirect method and then use their values to compute the mechanical power required to skate at various speeds. METHODS: Newton’s equation was written for a skater who is slowing down under the effect of frictional forces (air drag and wheel friction). After double integrating the differential equation, a relationship was found between t (time from the start, when the skater moved at v0) and s (covered distance). In the equation there are two unknown coefficients (air drag and wheel friction), plus some data describing the body (mass, height) and the environment (air density). A separate computation was done to estimate the area of the body, thus requiring a third coefficient (body area coefficient). Experimental data for t and s were collected for six athletes of the Italian National team in order to obtain the best-fit values for the unknown coefficients. RESULTS: The proposed equation and the experimental data were used to obtain the drag coefficient, the wheel friction coefficient and the body area coefficient under the test conditions. The residual error of the model was estimated using Ds/s, obtaining an average value of 2%. The method required only a few data, with no need for costly equipment. Even so, the results were quite good. It was also possible to recognize the differences in the friction coefficients between different wheels. CONCLUSIONS: With the model proposed, it is possible to calculate the external forces required for a given skater in a known environment to skate at various speeds. It is possible to compute the mechanical external power required to roller skate and then compare it with the power required for other sports. It is also possible to make important suggestions for coaches about the kinds of training loads to use.