Figure 2

Marker placement, hand local reference system and finger joint angles. Markers position. Mi: head of the metacarpal bone of finger i (i = 1-5); Pi: head of proximal phalanx of finger i (i = 1-5); Di: head of distal phalanx of the thumb (i = 1) and head of middle phalanx of long fingers (i = 2-5); SU: styloid process of ulna; SR: styloid process of radius. Local reference system XYZ. The origin is in correspondence of the marker M2. Vectors (M2-M5) and (M2 - SR) define the metacarpal plane of the hand (grey triangle). Z-axis is normal to the metacarpal plane pointing palmarly, Y-axis has the direction of vector (M2 - SR) pointing distally, while X-axis is calculated as the cross-product of Y and Z-axis, pointing radially. Joint angles in transverse plane YZ (a) and in sagittal plane XY (b) of the hand. MCPJ _i : metacarpophalangeal joint flexion angle of finger i (i = 1-5); IPJ _i : proximal interphalangeal joint flexion angle of finger i (i = 1-5); TAB: thumb abduction angle. MCPJ _i (i = 2-5) is defined as the angle between Y-axis and the projection of the vector (Pi - Mi) on the YZ plane; IPJ _i (i = 2-5) is the angle between the projections of vectors (Di-Pi) and (Pi-Mi) on the YZ plane. TAB is the angle between the vector (P1 - M1) and the XY plane. MCPJ ₁ is the angle between X-axis and the projection of vector (P1 - M1) on the XY plane. IPJ ₁ is the angle between vectors (D1 - P1) and (P1 - M1).