Comparable Safety Margins of the Ulnar Fingers When the Thumb Remains on an Unsteady Slider

Among various daily life activities, grasping an object is a common task performed by all healthy individuals. For a stable grasp of an object, fingertip force distribution plays a major role. In the current study, an instrumented handle was designed on which five-fingertip force/torque measuring sensors were mounted. A vertical railing was provided on the left surface of the handle, and a slider platform with a force sensor to measure the forces from the thumb was placed over the railing. The present study focuses to investigate how the fingertip forces varied to keep the handle in equilibrium when the slider platform was made unsteady. The experiment involved two conditions. In one condition, the platform was fixed, and in the other condition, the platform was mobile. The participants had to lift the handle and match the flat mark on the platform to the mark on the handle during “fixed” and “free” condition. As a consequence of reduction in the thumb load force during the “free” condition, grip forces of ring and little fingers increased together, and they were statistically equivalent. We hypothesized that the load force exerted by the ring finger would be larger than the small finger, and the “safety margin” of the little finger would be greater than ring. However, both the tangential forces and safety margin of the ulnar fingers were found to be “comparable” during the free condition, while they were significantly different during the fixed condition.