Submitted on November 9, 2007
Accepted on January 26, 2008
Multi-digit control of contact forces during rotation of a hand-held object
Sara A Winges1*, Stephanie E Eonta1, John F Soechting1, and Martha Flanders2
1 Neuroscience, University of Minnesota, Minneapolis, Minnesota, United States
2 Neuroscience, University of Minnesota, Minnesota, United States
* To whom correspondence should be addressed. E-mail: swinges{at}umn.edu.
Rotation of an object held with three fingers is produced by modulation of force amplitude and direction at one or more contact points. Changes in the moment arm through which these forces act can also contribute to the modulation of the rotational moment. Therefore, force amplitude and direction as well as the center of pressure on each contact surface must be carefully coordinated to produce a rotation. Since there is not a single solution, this study sought to describe consistent strategies for simple position-to-position rotations in the pitch, roll and yaw axes. Force amplitude and direction, and center of pressure on the contact surfaces (and thus the moment arm), were measured as human subjects rotated a 420g force-transducer instrumented object, grasped with the thumb, index and ring fingers (average movement time: 500ms). Electromyographic (EMG) activity was recorded from five intrinsic and three extrinsic hand muscles and two wrist muscles. Principal components analysis of force and EMG revealed just two main temporal patterns: the main one followed rotational position and the secondary one had a time course that resembled that of rotational velocity. Although the task could have been accomplished by dynamic modulation of the activity of wrist muscles alone, these two main dynamic EMG patterns were seen in intrinsic hand muscles as well. In contrast to previous reports of shifting in time of the phasic activity bursts of various muscles, in this task, all EMG records were well described by just two temporal patterns, resembling the position and velocity traces.
