Inter-finger Connection Matrices

Fingers of the hand are interdependent: when a person moves one finger or produces a force with a fingertip, other fingers of the hand also move or show force production. Hence, no direct correspondence exists between the neural commands to individual fingers and finger forces. The relations among fingers are described with inter-finger connection matrices, IFM. The IFMs depend on the number of fingers involved in the task. This presentation addresses three aspects of the IFMs: (1) computation of the IFMs, (2) role of finger interdependence during manipulation of hand-held objects and (3) inter-individual differences in the IFMs. When a person moves one finger or produces a force with a fingertip, other fingers of the hand also move or show force production (Schieber 1991, 1995; Kilbreath and Gandevia 1994; Hager-Ross and Schieber 2000; Schieber et al. 2001. This phenomenon has been termed enslaving (Zatsiorsky et al. 1998, 2000). The finger interdependence is due to three sources/mechanisms: (1) peripheral connections, both tendinous (Leijnse 1997) and intermuscular myofascial (Huijing 1999a, 1999b), (2) multi-digit motor units in the extrinsic flexor and extensor muscles (Kilbreath and Gandevia 1994), and (3) central neural connections (Schieber and Hibbard 1993). Due to the enslaving, there is no direct correspondence between neural commands to individual fingers and finger forces. The relations among fingers can be described with inter-finger connection matrices, IFM (Zatsiorsky et al. 1998; Li et al. 2002). The IFMs depend on the number of fingers involved in the task. The reason behind this dependence is a so called force deficit: a maximal force exerted by a finger in a multi-finger task is smaller than a maximal force produced by this finger in a single-finger test. The deficit increases with the number of fingers involved in the task (Li et al. 1998a, 1998b). Existence of the force deficit makes determination of the IFMs in static tasks nontrivial: recording of finger forces while the subject tries to press with only one finger does not account for the force deficit and, hence, is not sufficient to determine an IFM. This presentation addresses three aspects ⋆ This study was partly supported by NIH grants AR 048563, NS-35032 and AG-18751. The support from the Whittaker Foundation to Dr. Z.M. Li is also acknowledged. of the IFMs: (1) computation of the IFMs, (2) role of finger interdependence during manipulation of hand-held objects and (3) inter-individual differences in the IFMs. 1 Computation of the IFMs So far, two techniques have been used to compute the IFMs: (a) neural networking and (b) algebraic approximation.