Fingertip Moisture Is Optimally Modulated During Object Manipulation

1 Rehabilitation and Physical Medicine Unit and 2 Laboratory of Neurophysiology, Université catholique de Louvain, Brussels; and 3 Center for Systems Engineering and Applied Mechanics, Université catholique de Louvain, Louvain-la-Neuve, Belgium Submitted 6 October 2009; accepted in final form 9 November 2009 ABSTRACT Coordination between the normal force exerted by fingers on a held object and the tangential constraints at the fingertips helps to successfully manipulate objects. It is well established that the minimal grip force required to prevent an object from slipping strongly depends on the frictional properties at the finger–object interface. Moreover, interindividual variation in the modulation of grip force suggests that the moisture level of the skin could influence grip force strategy. In the present study we asked subjects to perform a horizontal point-to-point task holding an object with a precision grip. The object was equipped with a moisture sensor. We found large inter- and intraindividual moisture level variations. There was a strong correlation between grip force exerted and moisture level at the fingertips. Indeed, the grip force was minimal when the fingertip moisture was optimal with respect to friction. Furthermore, fingertip moisture tended toward this optimal level at which grip force is minimal. In conclusion, we showed a modulation of the grip force with moisture level and hypothesized novel mechanisms of moisture regulation that tend to stabilize the moisture level toward the value that minimizes grip force. Address for reprint requests and other correspondence: J.-L. Thonnard, Unité de réadaptation et de médecine physique, Tour Pasteur, 53 avenue Mounier (5375), 1200 Brussels, Belgium (E-mail: Jean-Louis.Thonnard{at}uclouvain.be ).