The effect of target size and inertial load on the control of rapid aiming movements. A test of speed-sensitive and speed-insensitive strategies.

Two experiments are reported that investigated the effects of target size and inertial load on the control of rapid aiming movements. Based on kinematic profiles, movements were partitioned into their preprogrammed initial impulse- and feedback-based error correction phases. Electromyographic (EMG) rise rates were examined to investigate whether participants used a speed-sensitive or speed-insensitive control strategy. The results from both experiments showed that initial impulse velocity and EMG rise rates varied as a function of target size, i.e., a speed-sensitive strategy. This was the case whether participants were allowed to make error corrections to their movements (experiment 1) or were instructed to produce initial impulses that hit the target (experiment 2). Both experiments also showed that initial impulse velocity and endpoint variability were inversely related to inertial load. The results from experiment 2 indicated that, while the manipulation of inertial load had no effect on EMG rise rates for movements to a large target, EMG slopes were modulated between inertial load conditions when the target was small.