A STUDY OF THE VARIABILITY IN ENERGY CALCULATION RELATED TO THE VISIBILITY OF THE MARKERS, AFTER A MANUAL DIGITIZING PROCESS

Abstract

INTRODUCTION - CAVANAGH (1990) described a variation from 170 to 1700 W in power output for the same movement (running at 3.6 m.s-I) calculated by six different authors. He pointed to four factors as being responsible for the variation.We have concentrated our efforts in the numerical methods, studying differences generated by certain variables that in general take part in all the energy calculation methods, but that are not necessarily always the same. The aim of this paper is to analyse the different components of the mechanical energy (potential, kinetic and rotational) of the segments considering the differences between the right side (visible) and left side (hidden). We will consider as variables that interfere in this variation: different movemei conditions - treadmill and overground. A stride from one male subject was filmed with two video-cameras (Sony- 50Hz) while walking at 1.5 mls on the treadmill and overground. Each trial was repeated for at least 3 times on overground and on treadmill it was filmed for at least 30 seconds. From the kinematics we performed a 3D analysis after a manual digitizing process.The analysis was based on a 13 segment model, positions of segmental centers of gravity, segmental weights, and moments of inertia were estimated on the basis of tables devised by DEMPSTER(1955) as revised by WINTER (1979). The components of mechanical energy were calculated at each instant of time, using basically the equations described by ZATSIORSKY (1 987). RESULTS - It was not possible to identify differences between the average energy curves of the visible and hidden side considering total kinetic and rotational energies of the segments both for walking on treadmill as for walking overground. Differences were easily to be noted in the total potential energy. After we have divided the whole side into two parts: upper and lower extremity curve, we observed that for the potential energy the greatest lateral difference lies in the lower extremity curves. Observing the potential energy curves from the three segments of the lower extremity; we note that the curve that shows a great lateral difference is the thigh's curve with the curves for the leg and foot showing no lateral differences. A problem that was observed for almost all segments in the three energies was the greater difference between the right and left side of the body in the energy curves on treadmill in relation to overground. We analysed this difference by comparing the average curves for the sum of the potential, kinetic and rotational energies for the upper extremity and lower extremity in walking on treadmill and overgound. The curves for both conditions have similar patterns, but the curves for left side in the treadmill condition seem to be a little downwards dislocated . CONCLUSIONS - The discrepancies in the energy calculation between the right and left side of the body are considerable, especially in relation to the potential energy of the thigh. We consider this is a parameter to be controlled when it is of interest to digitize the body landmarks for the whole body, having also an influence in the calculation of the total energy of the body. We also noted that the variability between the energy curves of the sides appears to be different for overground and treadmill.