A TRAINING AND FEEDBACK SYSTEM FOR ARCHERS
AbstractINTRODUCTION Archery is a skill that demands utmost precision and repeatability of the motion. Minute deviations from the individual pattern are often below the JND threshold. Nevertheless they influence the score. More specifically, archers display radial or ulnar abduction of the bow-hand while aiming. This leads to changes in the energy stored in the upper and lower bow-limbs and thus to vertical deviations from the target. Also, a tremor of the drawing hand may occur. Furthermore, some archers tend to slowly rotate the bow about an axis parallel to the arrow. Since the arrow is well below the line of sight, lateral deviations occur even when the point of aim is dead on. While beginners have rather large and random inconsistencies, skilled archers have intermittent problems. The ability to control and reproduce the wrist position and the tilt of the bow is performance relevant. Small deviations may not be observable by the coach. Hence there is potential for a dedicated 'archery measurement' system that delivers fast and objective supplementary information for coaches and archers. The aim was to develop a precise and 'easy to use' tool to provide feedback. DESIGN AND FEATURES OF THE SYSTEM A HQY'T competition bow was instrumented with strain gauges above and below the bowgrip. The signals are pre-amplified and fed to an amplifier with gain and offset adjustment to accommodate bow-limbs of different stiffness. The difference between the forces F(a) and F(b) is supplied. The tilt of the bow is measured with a silicon damped inclinometer. The analog data are digitally converted using a National Instruments PC-LPM-16 board. Based on the LAB VIEW0 software an application was programmed to fulfill all user requirements regarding the data acquisition and processing as well as ease of operation and stability. In addition to the forces F1, AF2, F and the tilt angle a, two channels may be user defined to record for example the M. pectoralis EMG and an accelerometer signal to determine release and clicker times. The measuring module features programmable gain and offset, on-line help, visual data inspection as well as zoom of the display and data storage. During the data acquisition, an sound can be activated at a specified set point and in an adjustable window. This sound represents a real time feedback on whether the predefined levels are reached within the selected range. Different frequencies are used to distinguish 'force' and 'angle' information. Data can be recalled, viewed, compared and exported from a second software module. SYSTEM PERFORMANCE EVALUATION The feedback system uses up to date hardware and software technology. Implementation of bows with different strength and arrow length is possible. The software is flexible enough to accommodate future user requirements. The accuracy and reliability of the system was determined using a force vs. displacement measuring device that pulls the string to draw length without applying a torque to the handle. The system is now used in the training environment of international caliber Archers. RESULTS Archers have individual draw, anchor, aim and release patterns. Success depends on the reproducibility of the motion. Highly skilled archers have excellent inter-shot stability of the measured parameters. Deviations occur intermittently and can be recorded and analyzed with the present system. A long-term study was undertaken to establish the effect of the feedback with decreasing variability on the score for different performance levels.