Human Grip Responses to Perturbations of Objects During Precision Grip

Grasp stability of a precision grip requires fine control of three-dimensional fingertip forces. This chapter begins with a review of the literature on how precision grip forces are affected by intrinsic object properties, anticipation, load direction, and sensory feedback. Previous studies have established that reactive, initial increases in grip forces (pulse-like ‘‘catch-up responses’’ in grip force rates) are elicited by unexpected translational perturbations and that response latency and strength scale with the direction of linear slip relative to the hand as well as gravity. To determine if catch-up responses are elicited by unexpected rotational perturbations and are strength-, axis-, and/or directiondependent, we imposed step torque loads about each of two axes which were defined relative to the hand: the distal-proximal axis away from and towards the palm, and the grip axis which connects the two fingertips. First dorsal interosseous activity, marking the start of the catch-up response, began 71–89 ms after the onset of perturbation. Onset latency, shape, and duration (217–231 ms) of the catch-up response were not affected by axis, direction, or magnitude of the rotational perturbation, while strength scaled with axis of rotation and slip conditions. Rotations about the grip axis induced rotational slip at the fingerpads and elicited stronger catch-up responses than rotations about the distal-proximal axis. The chapter concludes with a discussion of this study that, to our knowledge, is the first to investigate grip responses to unexpected torque loads and to show characteristic, yet axis-dependent, catch-up responses for conditions other than pure linear slip.