Tremor Amplitude is Greater in the Mediolateral Direction than Vertical During Handgun Aiming Tasks.

Abstract

PURPOSE: In aiming at a target, humans produce small involuntary fluctuations in the aiming limb which may hamper performance. While often studied in the vertical axis, these fluctuations occur in all axes. The current study compared vertical and mediolateral tremor amplitude during different handgun aiming postures. METHODS: Thirty volunteers stood 21 feet from a target and aimed a weighted mock handgun for 10 sec with an accelerometer affixed to each wrist and the gun barrel. Participants performed five trials per condition. Amplitude (RMS) of the acceleration signals were computed. Two hand grip (bilateral, unilateral), two arm positions (bent, straight), and two segments (forearm, gun) were analyzed in conjunction with the two directions (VT, ML). RESULTS: Across all conditions tremor amplitude in the mediolateral direction was significantly greater than the vertical direction (VT: .017+.006g vs. ML: 0.021+.005g, p<.001). Additionally significant interaction effects were seen for direction by hand grip (p<.001) as well as direction by arm position (p=.002). For hand grip, vertical tremor was significantly lower than mediolateral and bilateral tremor was significantly lower than unilateral. For arm position, vertical tremor was significantly lower than mediolateral and bent arm tremor was significantly lower than straight arm. There were no significant interaction effects for direction by segment (p=.173). CONCLUSION: Across numerous hand gun aiming postures mediolateral tremor amplitude was consistently higher than the vertical tremor amplitude. This may be due to the neurological and mechanical factors of the limb, the limbs resistance to gravity, or the effort of control. Further research is needed to examine the interaction of mediolateral and vertical tremor.