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A computer model of the human precision grip

The term precision grip refers to the fine human grip between the pulps of the index finger and the thumb. It is used for the manipulation of small and discrete objects, e.g., lifting an object from a table. The precision grip is composed of two orthogonal force components: the grip force (squeeze) in the normal direction, and the load force (lift) in the tangential/vertical direction. Complex sensory and cognitive mechanisms form internal neural representations of the objects physical properties (e.g., its weight, size, and surface friction), which are used to adjust the force output both in feedback and feedforward modes [1]. The circuits controlling the precision grip involve several cortical areas, and is conveyed through the descending cortico-motoneuronal system. Here, a computer simulation model of the human precision grip is presented. The hand is represented by a non-linear model of the lift force, and a linear grip force generator. The lift force and the vertical movement are based on a lumped parameter agonist-antagonist biomechanical Hill-model [2], where the parameters are taken from the litterature. The grip force generator, however, generates isometric forces only, i.e., no grip movements are involved. In humans, the precision grip is a rather complex multijoint function known to involve approximately 15 muscles. The parameters for the grip force generator are therefore extracted from two specially designed grip force experiments, as described below (see System identification). System identification