A kinematic model of the human hand to evaluate its prehensile capabilities.

Abstract

A kinematic model has been developed for prediction of joint angles during power grasp of objects that can be described as ellipsoids or elliptical cylinders. The kinematic skeleton of the hand is characterized by ideal joints and simple segments. The model is based on an algorithm that determines contact between two ellipsoids, which are used to approximate the surface geometry of the h and segments. The model predicts the hand posture for power grasp of ellipsoidal objects by "wrapping" the fingers around the object. Graphics procedures are included for visual display. Linear correlations using h and length and diameter as independent variables explained between 41 and 94% of the variation in joint angles measured on circular cylinders. Increasing cylinder diameter decreased flexion and increasing h and length increased flexion for both measured and predicted joint angles. On average, the model overpredicted measured joint flexion by 2.8 $\\pm$ 12.2 degrees or 5.3%. Good agreement was found for the MCP and PIP joints, but results for DIP were variable because of its dependence on the predictions for the proximal joints. Models for estimating the lengths of the kinematic segments using external h and length as the independent variable account for between 49 and 99% of the variability in segment length. Models for estimating the X- and Z-locations of the finger MCP and thumb CMC joints using h and length and breadth, respectively, as the independent variable account for between 82 and 96% and between 30 and 74%, respectively, of the variability in these locations. These models have st and ard errors below 2 mm. The breadth and depth of the h and segments were measured using calipers, to describe their three-dimensional shape. Linear models using h and breadth as the independent variable explained from 12 to 47% of the variation in segment breadths and from 6 to 74% in segment depths. Ellipsoids do not approximate the three-dimensional geometry of the h and segments exactly. Segment shape is better described as a truncated cone or an elliptical cylinder with a slight taper toward the distal end. Differences between the ellipsoidal approximations and the breadth and depth measurements were largest near the joints.