Learning curves. Tracking error along the ten training trials for the EMG-, force- and joystick-based control interfaces. An exponential function was fitted to the mean tracking error of each training trial. The first training trial was used as a reference to identify significant reductions of tracking error. The green markers indicate significant reduction of tracking error (p <0.05) relative to the first trial. The red markers indicate non-significant reduction of tracking error (p >0.05) relative to the first training trial. The vertical lines indicate the trial in which the performance plateau was identified. The error bars indicate ± 1 SD. Stars indicate statistically significant differences. (*) indicates p <0.05.