Workspace Shape and Characteristics for Human Two- and Three-Fingered Precision Manipulation

Goal: To study precision manipulation, which involves repositioning an object in the fingertips and is used in everyday tasks such as writing and key insertion, and also for domain-specific tasks such as small scalpel cuts, using tweezers, and hand soldering. Methods: In this study, the range of positions (workspace) through which 19 participants manipulated a 3.3-4.1 cm-diameter object are measured with a magnetic tracker. Each participant performed two conditions: a two-finger thumb-index finger condition and a three-finger thumb-index-middle finger condition. Results: The observed workspaces, normalized to a 17.5 cm hand length, are small compared to free-finger trajectories; for the two-finger trials, 68% of points are within 1.05 cm of the centroid and 95% are within 2.31 cm, while the three-finger case shows a narrower distribution, with 68% of points within 0.94 cm of the centroid and 95% of points within 2.19 cm. The longest axis is a long thin arc in the proximal-palmar plane. Analysis of fingertip workspaces shows that the index fingertip workspace volume is the most linear predictor of object workspace (R² = 0.98). Conclusion: Precision manipulation workspace size and shape is shown, along with how the fingers are used during the manipulation. Significance: The results have many applications, including normative data for rehabilitation, guidelines for ergonomic device design, and benchmarking prosthetic and robotic hands.

I. Introduction

Considering the immense scope and broad reaches of the topic, there has been relatively little work done related to understanding normal human hand use, and especially quantifying function. More prior research has studied human grasping [1]–[4] and finger force modulation [5], while understanding the human hand's kinematic capabilities during dexterous, within-hand manipulation [6] has been largely ignored. Some studies have analyzed the kinematic capabilities of individual fingers in healthy participants [7], [8], as well as comparing the finger capabilities of participants with impaired hand functionality [9] –[11]. However, only a few studies (see Section II) directly consider the precision manipulation task of multiple fingertips interacting with an object, as in the present work.