Adjustments to local friction in multifinger prehension

Abstract

The authors studied the effects of surface friction at the digit-object interface on digit forces and moments when 12 participants statically held an object in a 5-digit grasp. The authors changed low-friction contact (LFC) with rayon and high-friction contact (HFC) with sandpaper independently for each digit in all 32 possible combinations. Normal forces of the thumb and virtual finger (VF), an imagined finger with a mechanical effect equal to that of the 4 fingers, increased with the thumb at LFC or with an increase in the number of fingers at LFC. When the thumb was at LFC, the thumb tangential force decreased. The VF tangential force decreased when the number of fingers at LFC increased. The interaction of the local responses to friction and the synergic responses necessary to maintain the equilibrium explain the coordination of individual digit forces.

FIGURE 1

Schematics of the instrumented handle (A) and the computation of the moment arms of the normal forces (B). In B, w is the handle width (w = 76 mm); y_i is the vertical coordinate of the index finger force application with respect to the sensor center; d_i is a projected vertical distance between the sensor centers of the index finger and thumb; and Y_i is the vertical coordinate of the point of the index finger force application. Note that the lowercase y_i designates the coordinate with respect to a sensor center, whereas the uppercase Y designates a coordinate in the handle-fixed reference system. F. = finger.

FIGURE 2

Normal forces of the thumb: group averages and standard errors (bars). HFC = high-friction contact. LFC = low-friction contact. When one finger was at LFC, Combinations 1, 2, 3, and 4 corresponded to LHHH, HLHH, HHLH, and HHHL, respectively (the four-letter sequences here indicate high [H] and low [L] friction at the index, middle, ring, and little fingers, respectively). When two fingers were at LFC, Combinations 1, 2, 3, 4, 5, and 6 corresponded to LLHH, HLLH, HHLL, LHLH, HLHL, and LHHL, respectively. When three fingers were at LFC, combinations 1, 2, 3, and 4 corresponded to LLLH, HLLL, LLHL, and LHLL, respectively.

FIGURE 3

Normal forces of the middle finger: group averages and standard errors (bars). In the legends in this and the following figures, the first letter indicates surface friction of the finger of interest (in this figure, the middle finger), and the second letter indicates surface friction of the thumb. The solid line corresponds to the finger at high-friction contact (HFC), and the dotted line corresponds to the finger at low-friction contact (LFC). The squares correspond to the thumb at HFC, and the circles to the thumb at LFC.

FIGURE 4

Percentage contribution to the virtual finger (VF) normal force of (A) the index finger and (B) the ring finger: group averages and standard errors (bars). The symbols are similar to those used in Figure 3.

FIGURE 5

Tangential forces of (A) the thumb and (B) the virtual finger: group averages and standard errors (bars). The solid line represents the thumb at high friction contact (HFC), and the dotted line represents the thumb at low friction contact (LFC). When one finger was at LFC, Combinations 1, 2, 3, and 4 corresponded to LHHH, HLHH, HHLH, and HHHL, respectively (the four-letter sequences here indicate friction at the index, middle, ring, and little fingers, respectively). When two fingers were at LFC, Combinations 1, 2, 3, 4, 5, and 6 corresponded to LLHH, HLLH, HHLL, LHLH, HLHL, and LHHL, respectively. When three fingers were at LFC, Combinations 1, 2, 3, and 4 corresponded to LLLH, HLLL, LLHL, and LHLL, respectively.

FIGURE 6

Tangential forces of the little finger: group averages and standard errors (bars). The symbols are similar to those used in Figure 3.

FIGURE 7

Safety margins for (A) the thumb and (B) the virtual finger: group averages and standard errors (bars). The solid line represents the thumb at high-friction contact (HFC), and the dotted line represents the thumb at low-friction contact (LFC).

FIGURE 8

Safety margins for the ring finger: group averages and standard errors (bars). The symbols are similar to those used in Figure 3.

FIGURE 9

Moment of tangential forces, M^t (A), and moment of normal forces, Mⁿ (B), as the function of slipperiness: group averages and standard errors (bars). Solid lines represent the thumb at high-friction contact (HFC); dotted lines represent the thumb at low-friction contact (LFC).

FIGURE 10

Schematic graph showing the effects of increasing slipperiness on biomechanics of prehension. When the slipperiness increased, the virtual finger (VF) tangential force F^t decreased and the thumb F^t increased by equal amount. The effects of those changes on the friction-induced moments (M^t), however, depended on the thumb friction. (A) The thumb at high-friction contact (HFC). At zero slipperiness (all fingers are at the HFC), the thumb and VF F^t were equal and the friction-induced M^t was zero. With the increased slipperiness, the difference between the thumb and VF F^t—as well as the friction-induced M^t—emerged and built up. The friction-induced M^t in the clockwise direction was generated. (B) The thumb at low-friction contact (LFC). At zero slipperiness (all fingers were at the HFC), the thumb and VF F^t were not equal and the friction-induced M^t was in the counterclockwise direction. With the increased slipperiness, the difference between the thumb and VF F^t, as well the friction-induced M^t, decreased. Fⁿ is the normal force. See Figure 2 for explanation of conditions.