The effect of wrist angle and finger grip on maximum trigger pull force.

A forensic investigation into a shot fired by a handgun may require analysis of the body posture of the shooter or an opinion of whether the shot was deliberate or inadvertent. Determining the amount of force which can be applied to the trigger or the direction in which the handgun was pointing could be critical to the investigation. Studies investigating the effect of arm posture on whole hand maximum grip force suggest that maximum index finger force will be highly dependent on wrist angle and finger grip. The present study was designed to quantify the effect of these factors on maximum index finger trigger pull force. We found that even under almost optimal conditions, trigger pull force is substantially reduced with wrist flexion and is significantly lower with a finger pad grip compared to a finger hook grip. Our results indicate that for wrist flexion angles greater than 60°, maximum trigger pull force dropped by 50 % for male subjects and by 38 % for female subjects compared to the neutral or extended wrist. Other studies have shown that when a firm grip cannot be established, maximum index finger force can be less than 30 % of what we measured. Thus, maximum trigger pull forces could be below 22.9 N (5.1 lb) for male subjects and 19.0 N (4.5 lb) for female subjects in certain scenarios. If a subject is unable to get a firm grip on a handgun and the arm is in an awkward posture, it is possible that the subject would not be able to exert sufficient index finger force to overcome the trigger weight of many handguns.