A MECHANICAL MODEL FOR MEASURING IN THREE DIMENSIONS THE SMALL AMPLITUDE COUPLED MOTION THAT CHARACTERIZES MOTOR PATTERNS IN SHOOTING ACTIVITIES

Abstract

INTRODUCTION: During the aiming in air rifle shooting from a standing position, the shooter, in order to align his eye with the rifle and the target, adopts a posture characterized by a pronounced extension with simultaneous lateral bend and slight twist of the trunk (coupled motion) with respect to the pelvic girdle. Shooters try to maintain this posture, passing mechanical loads across the passive structures and elements of the locomotor system. However, the viscoelastic properties of muscles disturb the shooter - rifle system introducing small amplitude oscillations that tend to deflect the aiming line from the target. The purpose of this study is to develop and validate a mechanical model for measuring with precision these small amplitude rotational movements of the shooter’s trunk and the rifle. METHODS: The spatial coordinates of nine markers that define three segments on the shooter-rifle system are obtained using sonic digitizing techniques (Gianikellis et al., 1994). Position-time data are smoothed using quintic splines, and orthogonal coordinate frames are assigned to the three segments, namely upper trunk, pelvic girdle and rifle. Then the orientation of each segment with respect to the global system of reference is expressed by the Cardan angles, where the rotation matrix is parameterized in terms of three independent angles, resulting from an ordered sequence of rotations with respect to the three axes of the global system of reference. RESULTS: By means of this method it is possible, for first time, to measure the small rotations that take place in such particular motor activity. In the table the average values of the range of angular movement of the three segments are represented for the concrete case of 60 trials that we studied. The model was validated by simulation procedures, knowing the precision in the coordinates data. So the random errors (p<.01) included in the values of the Cardan angles and the relative error with respect to the average values of the range of the real rotational movement were calculated (in brackets). [table] CONCLUSIONS: This representation of coupled motion is expected to improve the knowledge about the motor patterns of sport shooting activities, given that the rotations are smaller than 10º, angles are anatomically interpreted and it is possible to avoid mathematical singularities (gimbal lock).