Trajectory dynamics and endpoint accuracy in targeted ballistic contractions.

Although effort alters the neural commands driving ballistic contractions, its impact on endpoint accuracy remains poorly understood. Here, we examined how effort-related changes in trajectory fluctuations and inconsistency (trajectory dynamics) and muscle activation affect the accuracy of ballistic goal-directed contractions. Eighteen healthy young adults (26.4 ± 6.4 years; 9 women) performed ballistic isometric index finger abductions for 40 trials at each of the seven randomly assigned force levels (2, 5, 15, 30, 50, 70, 85% of maximum voluntary contraction; MVC). We quantified endpoint accuracy (bias error, absolute error), force trajectory dynamics, and first dorsal interosseous (FDI) muscle activity across different frequency bands. Bias error showed effort-dependent reversal of accuracy, with overshooting at low force levels, maximum accuracy at 30% MVC, and undershooting at high effort levels. Absolute force error decreased with effort, reflecting a 2/3rd power-law trend, indicating an improvement in accuracy with increasing force level regardless of the direction. Trajectory fluctuations and muscle activity changed with effort level, whereas an increase in 8-13 Hz electromyography (EMG) power predicted reductions in bias error from 2-30% MVC. These findings demonstrate an effort-dependent effect on endpoint accuracy and identify trajectory dynamics and muscle activation characteristics that explain endpoint accuracy changes in targeted ballistic contractions.