(a) Schematic illustration of an aluminium handle (black circle with a large hollow inside) and six-component sensors (white rectangles) at digit contacts and (b) detailed schematic illustration of the little finger producing a force at a contact. O_G = origin of the global reference system (GRS) of coordinates; X = x-axis in GRS; Y = y-axis in GRS (z-axis is not shown in the figures, but its positive direction follows the right-handed coordinate system and is from the paper to the reader); O_L = origin of local reference system (LRS) of coordinates of the little finger sensor; x_l = x-axis in LRS of little finger sensor; y_l = y-axis in LRS of the little finger sensor; = moment about z-axis in LRS of little finger sensor (z-axis in LRS for each sensor is parallel to Z-axes in GRS); F^l = little finger force; = little finger normal force; = little finger tangential force; = required normal force to avoid slipping of little finger with and friction coefficient (μ = 1.5) of the contact surface; = safety margin of normal force( ); = position of LRS origin in GRS; = position of little finger centre of pressure in GRS; = angular position of in GRS; α^l = angle between and F ^l; = angle between and ; = safety margin of angle ( ) for little finger force. O_L was fixed to the centre of the contact surface of the sensor and a cap (shown grey) was fixed on the sensor surface. The distance between the apex of the cap and O_L was ~0.81 mm.

Individual digit normal forces at the time of maximum resultant moment about anterior – posterior (AP) and medio-lateral (ML) axes in opening (OP) and closing (CL) directions. T, I, M, R and L, respectively, represent thumb, index, middle, ring, and little fingers. (a) Mean (± standard error) normal forces across subjects; (b) sharing of total normal force by individual digits (mean ± standard error).

(a) Internal force and (b) resultant force at the maximum moment production in opening (OP) and closing (CL) tasks about the anterior – posterior (AP) and medio-lateral (ML) axes. Mean ± standard errors across subjects are reported for each task condition.

Schematic top view of a subject posture during maximum moment production about anterior – posterior (AP) or medio-lateral (ML) axes in opening (OP) and closing (CL) directions.

Moments of individual digit tangential forces at the time of maximum resultant moment about anterior – posterior (AP) and medio-lateral (ML) axes in opening (OP) and closing (CL) directions. T, I, M, R and L, respectively, represent thumb, index, middle, ring, and little fingers. (a) Mean (± standard error) moments across subjects; (b) sharing of total moment by individual digits (mean ± standard error).

Safety margins of individual finger normal forces and force angles about the anterior – posterior (AP) and medio-lateral (ML) axes in opening (OP) and closing (CL) tasks. The normal and tangential forces are shown as horizontal and vertical arrows, respectively. The mean values and standard errors of the safety margin of normal force and force angle across all subjects are shown next to each force vector diagram for each digit in each task condition. The mean values and standard errors of the safety margin of normal force are respectively shown as the length and thickness of a solid arrow while the mean values and standard errors of safety margins of force angle are respectively shown as the angles and their thickness.