Fig. 5

Graphical explanation of the performance metrics. a Illustration of the raw movement trajectory (green) and the distances used to calculate the percentage of task completion (PTC) the tracing error (TE). b Illustration of the spectral arc length (SPARC) derived from the Fourier magnitude spectrum of the speed profile. The green and red curves illustrate the speed profiles in time (left) and frequency (right) domain of two movements with different duration. The SPARC measure calculates the spectral arc length with a magnitude threshold (\(\overline {V}\)) that selects a cut-off frequency (ω _c ) for each power spectrum signal, making the smoothness measure independent of the movement duration