Principal Component Analysis of Grasp Force and Pose During In-Hand Manipulation

Purpose: The manner in which healthy humans interact with objects as they move them within the hand is essential for activities of everyday life. The present study aims to identify the most salient features of the complex interactions between the fingers of the hand and the object. Specifically, the study seeks to determine whether the force applied by a healthy person follows a natural trend throughout the movement, or if it varies from individual to individual. Results will potentially inform the design of therapies, surgeries, and the design of prosthetic and orthotic devices to restore function in patients with compromised hand function. Methods: The joint angles of four healthy subjects were recorded by a magnetic motion tracking system along with the forces the hand was applying to an instrumented object. These were recorded as subjects moved the object from a pose with fingers outstretched toward the palm of the hand and back again. After the joint angles were extracted from the motion capture readings, principal component analysis was conducted on the joint angle-force space as well as the space consisting of the joint angles alone. Results: Principal component analysis of the joint angle and force data revealed that the first two principal components explained over 90% of the variance. One of these principal components was associated with the curling motion of the fingers, and the other with the squeezing of the object. The curling motion was consistent from subject to subject, but the changes in gripping force over the course of the movement varied from individual to individual. Conclusion: The study supports the notion that when conducting an in-hand manipulation of an object, the coupled motions of the fingers and the sequence of hand shapes follows a common trend across individuals. The force applied to the object, however, seems to be volitional on the part of the subject and does not seem to follow as a consequence of the hand movement.