Directional coordination of thumb and finger forces during precision pinch

Abstract

The human opposable thumb enables the hand to perform dexterous manipulation of objects, which requires well-coordinated digit force vectors. This study investigated the directional coordination of force vectors generated by the thumb and index finger during precision pinch. Fourteen right-handed, healthy subjects were instructed to exert pinch force on an externally stabilized apparatus with the pulps of the thumb and index finger. Subjects applied forces to follow a force-ramp profile that linearly increased from 0 to 12 N and then decreased to 0 N, at a rate of ± 3 N/s. Directional relationships between the thumb and index finger force vectors were quantified using the coordination angle (CA) between the force vectors. Individual force vectors were further analyzed according to their projection angles (PAs) with respect to the pinch surface planes and the shear angles (SAs) within those planes. Results demonstrated that fingertip force directions were dependent on pinch force magnitude, especially at forces below 2 N. Hysteresis was observed in the force-CA relationship for increasing and decreasing forces and fitted with exponential models. The fitted asymptotic values were 156.0 ± 6.6° and 150.8 ± 9.3° for increasing and decreasing force ramps, respectively. The PA of the thumb force vector deviated further from the direction perpendicular to the pinching surface planes than that of the index finger. The SA showed that the index finger force vector deviated in the ulnar-proximal direction, whereas the thumb switched its force between the ulnar-proximal and radial-proximal directions. The findings shed light on the effects of anatomical composition, biomechanical function, and neuromuscular control in coordinating digit forces during precision pinch, and provided insight into the magnitude-dependent force directional control which potentially affects a range of dexterous manipulations.

Figure 1. Experimental setup for the pinch task.

(A) A subject interfacing with the pinch apparatus while viewing the monitor for force feedback. (i) The horizontal line in the middle of the tank serves as the target line; (ii) the vertical bar indicates the real-time pinch force of the subject; and (iii) the pinch apparatus in the experiment. (B) Close-up of the typical pinching posture assumed while the digits are interfacing with the apparatus.

Figure 2. Definition of coordinate system and angular parameters.

Figure 3. Force vector components of the thumb and index finger from one representative trial.

(A) The F_x, F_y and F_z components of the thumb force vector; (B) The F_x, F_y and F_z components of the index finger force vector.

Figure 4. Coordination angles of the two force vectors at different pinch force magnitudes during increasing and decreasing force conditions.

The solid lines represent the average of CA across all trials. The dotted/dashed lines represent the plus/minus standard deviations of CA for all trials.

Figure 5. Force projection angles for the thumb and index finger.

Each data point represents the average of PA for all trials within the specified 1-N force interval.

Figure 6. Projection angle for the thumb and index finger.

Each data point represents the average of SA for all the trials within the specified 1-N force interval.

Figure 7. Shear force tuning curves for the thumb and index finger.