ANALYSIS OF FINGER POSITION DURING TWO AND THREE-FINGERED GRASP: POSSIBLE IMPLICATIONS FOR TERMINAL DEVICE DESIGN

INTRODUCTION: Analysis of human hand kinesiology has been used to justify the design characteristics of terminal devices. The human hand is a complex mechanical structure consisting of 15 articulations controlled by over 30 muscles for the 5 fingers. It functions as a communication, sensory, grasp and manipulation structure. Separating essential hand kinesiology that is linked with each hand function might allow for the efficient development of terminal devices where there is one primary function such as grasping. It follows that to develop an effective grasping mechanism for a terminal device there should be a clear understanding of hand motions as they relate directly to the grasping function. The conceptual framework used to characterize grasping functions will most likely influence the development of terminal devices. There is a growing body of literature that is refining the way we think of grasping activities, and may impact the design of terminal devices. For example, researchers have described the simplified coordination of hand motions during grasp as “synergies” or “virtual fingers”, where there are predictable dependencies of one finger on another. Using conceptual approaches such as the “virtual fingers” concept will reduce the complexity of the description of the hand/object/task interaction.[2] Recent studies indicate that only a small part of the hand’s potential motion is used during grasp. Mason et al. [5] concluded that much of reach-to-grasp uses a base posture with small refinements in finger and thumb positions. In another study Baud-Bovy & Soechting [1] found that when two fingers oppose the thumb, there is a predictable singular task that produced a balanced lever. Kamper et al [3] studied a variety of grasping tasks using 5 everyday objects. They found that the thumb, for example, used less than 5% of the available range. If one were to extrapolate these findings, prosthetic grasping mechanisms may only need a portion of the hand’s total range of motion. Grasping patterns of the normal hand should be studied using a wide variety of objects to compare normal grasp patterns with prosthetic devices. The current study examined finger positions during twoand three-fingered grasp of a wide range of geometrical objects. The results from people with normal hands were compared to two terminal devices. METHODS: Seven subjects participated in the study (2 male, 5 female). A 6-camera video motion analysis system (Qualisys, Gothenburg, Sweden) was used to collect marker locations in a calibrated volume approximately 1 m 3 . Fourteen reflective markers were taped to the interphalangeal joints, metacarpophalangeal joints and the carpals of the subjects (Figure 1). Seated subjects were asked to grasp objects, one at a time, using either two fingers or three fingers. The 9 objects were chosen based on standard geometric shapes, sizes and varied mass distribution properties. The smallest object was a 1.9 cm diameter sphere and the largest object was a 9.5 X 12.5 X 1.7 cm rectangular prism. The objects were painted brown to avoid inferring a particular use. Data were collected at 120