Arm posture and hand mechanical impedance in the control of a power drill

This research investigated the control of a hand-held power drill by human operators. A better understanding of human performance in this context may be helpful for the automation of industrial processes, or to prevent occupational disorders such as Carpal Tunnel Syndrome or Hand-Arm Vibration Syndrome. We chose to investigate the control of a drill by analyzing the upper limb posture and hand stiffness of drill operators.

We found that posture was repeatable within a subject, that it was insensitive to both the level of axial force exerted and the drill length. In addition, posture was similar while drilling or pushing on the drill when turned off. These results suggest that upper limb kinematics is an important factor. However, neither maximization of hand manipulability nor isotropy characteristics of a 2D two link model could explain the posture selected by subjects. Instead, minimization of joint torques appeared plausible. Right handed subjects selected a posture that minimized shoulder torque, in contrast to left handed subjects who selected a posture that minimized elbow torque.

We have developed a model to demonstrate how hand stiffness can be used to compensate for the static instability induced by the axial force exerted on a drill handle. As hand stiffness could not be measured directly while subjects were drilling, it was measured while they were asked to push on a drill handle mounted on a pivoting stick. Results showed that subjects did more than simply compensate for the static instability; hand stiffness either remained constant or increased with force. We found no evidence that one would voluntarily modulate his/her hand stiffness to fulfill some criteria. Stiffness increase may just be a well-tuned side effect of muscle activation. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)