Spectral arc length ( SAL ) dependency. x and y axes represent SAL values, calculated from referential motion capture system (Optotrak) and IMU based hand velocity, respectively. S A L G y r o 2 (red circles) and S A L G y r o 1 (green triangles) denote SAL values of hand velocity, determined from angular velocity for 2 IMUs per arm and 1 IMU per forearm sensor placements, respectively. S A L A c c (blue squares) presents SAL results of hand velocity, calculated by integration of forearm IMU acceleration vector. Best fitting ellipses indicate level of linearity. Pearson correlation coefficients R for the three possibilities in relation to referential motion capture system (Optotrak) are presented in top left corner.