Resubmitted for review to Clinical Biomechanics in August 2005 ms# cb/2005/000090




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НазваниеResubmitted for review to Clinical Biomechanics in August 2005 ms# cb/2005/000090
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Resubmitted for review to Clinical Biomechanics in August 2005

MS# CB/2005/000090


2. revision

Dynamic force-sharing in multi-digit task


C. E. Dumonta,*, M. R. Popovicb,c,e, T. Kellerb,d, R. Sheikha


aDepartment of Orthopaedic Surgery and dSpinal Cord Injury Center, University Hospital Balgrist, Forchstrasse 340, 8008 Zürich, Switzerland

bAutomatic Control Laboratory, Swiss Federal Institute of Technology Zürich, Physikstrasse 3, ETL K24, 8092 Zürich, Switzerland.

cInstitute of Biomaterials and Biomedical Engineering, University of Toronto, 4 Taddle Creek Road, Toronto, Ontario M5S 3G9, Canada

eLyndhurst Centre, Toronto Rehabilitation Institute, 520 Sutherland Drive, Toronto, Ontario M4G 3V9, Canada


*Corresponding author. Uniklinik Balgrist, Forchstrasse 340, 8008 Zürich, Switzerland.

E-mail address: charles.dumont@balgrist.ch


Abstract


Background

Dynamic hand grasping implies sophisticated motor coordination. Most knowledge on motor synergies used in grasping is deduced from experiments based on static precision grip. This experiment was aimed at better understanding the mechanisms of finger force-sharing in an active, dynamic hand task under repetitive strain conditions.

Methods

A multi-digit task consisting of holding a cylinder with the digit tips, in which the thumb and the finger opposed each other, was investigated during repetitive unidirectional wrist flexion and extension cyclic motion. Finger and thumb forces and wrist angular position were simultaneously recorded during repetitive wrist motion against 0.3 – 0.6 Nm load in 10 healthy adults.

Findings

Load torques acting during wrist movements produced in-phase increases of the thumb and the finger forces with the wrist extension and the wrist flexion, respectively. Digit forces increased proportionally to the applied load. The alternating rise of thumb and finger forces changed instantaneously at the end of the flexion and extension phases of the movement, respectively. Six subjects predominantly used the index finger, two the middle finger, one the ring finger, and the remaining one used the small finger during wrist flexion against 0.6 Nm to perform the task. Variations among individual finger forces were negatively correlated during the phase of constant rotational velocity of the wrist flexion. Repeated measures ANOVA revealed that the percentage of individual finger contribution to the total fingers’ force significantly varied during the wrist flexion (P < 0.0001) and among wrist flexion cycles (P < 0.0001) in each subject.

Interpretation

Variations in finger force-sharing among cycles were not necessitated by task dependent activities, as the task was identical. These findings indicated that motor coordination of repeated multi-finger task allowed redundant solutions in finger force-sharing. The force-sharing variation may reflect a minimal intervention principle of the central nervous system controlling only the goal-directed parameters and might help to prevent muscle fatigue in repetitive tasks through modulation of activity in multi-digit muscles.


Key-words: Wrist, motion, finger, force, motor coordination, feedback

Introduction


Although hand grasp is more often used in activity of daily living than precision grip, this second task is usual in manual works to grip and handle polyhedral (like books) or cylindrical objects. Repetitive tasks are often responsible for overuse syndromes and repetitive trauma disorders, which result in considerable burden to patients and society (Levenstein 1999).

Motor synergies required for combined multi-digit grip and wrist movements have not been previously systematically investigated (Shim et al. 2005). Two groups of muscles are used to achieve multi-digit tasks (Yu et al. 2004): 1) single-digit muscles, mainly intrinsic muscles of the hand, and 2) multi-digit muscles, mainly extrinsic muscles in the forearm. Both groups of muscles contribute to movement, force production and joint stability. Wrist movements are dependent on the activation of specific extrinsic muscles. Co-contraction of agonist and antagonist muscles of the forearm are necessary to better stabilize the wrist and the digit joints, and that way increase the strength of the grasp (Chabran et al. 2001). Therefore, grasping during wrist movements implies sophisticated motor coordination to complete the task. To accomplish reaching and grasping tasks the central nervous system (CNS) involves appropriate neuronal connections required to generate reaching and object manipulation synergies. Descending inputs from supraspinal centers in the CNS select appropriate motor synergies to execute the task, which are then reinforced by proprioceptive feedback from the moving limb and cutaneous feedback from the hand obtained once the contact is established with the object (Stein and Smith 1999; Witney et al. 2004). At the fingers and thumb levels, tactile afferents of the skin in contact with the object are providing essential information to adapt force generation to loads through continuous cutaneous input (Macefield and Johansson 1996). Anticipatory postural adjustment is a mechanism allowing the central motor controller to select postural muscle synergy in advance, based on visual estimation of the object mass and inertia, and taking benefit from the experience gained when similar tasks were performed in the past (Chabran et al. 2001; Diedrichsen et al. 2005; Nowak et al. 2002; Ohki et al. 2002).

The study of motor synergies used in hand task is matter of intense investigations (for review see Zatsiorsky and Latash 2004), but most knowledge on finger force-sharing is deduced from experiments based on static precision grip, which does not integrate the complexity of synergies involved at the wrist and digit muscles’ levels during repetitive, dynamic hand tasks.

We have investigated a multi-joint task consisting of a prismatic precision grip with the digit tips in firm contact with the object, in which the thumb and the fingers opposed each other. During the grasp repetitive cyclic wrist flexion and extension motions were performed. The present study was designed to observe individual digit forces in healthy individuals generated during the above task and how they changed during active wrist movements against variable loads. The experimental design allowed an indirect assessment of digit muscle activities, through individual digit force recordings in an identically repeated task. The experiments were aimed at better understanding the mechanisms of finger force-sharing in an active dynamic task under repetitive strain conditions.
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