REVIEW ARTICLE P. Bawa á G. R. Chalmers á K. E. Jones á K. Sùgaard M. L. Walsh Control of the wrist joint in humans Accepted: 30 May 2000 Abstract As one considers changes in motor activity from lower mammals to higher primates, one of the major changes one observes lies in the cortical control of forelimb muscles. There has been a shift from disynaptic control of spinal motoneurons in, for example, the cat, to a greater and greater percentage of monosynaptic control of hand and forelimb motoneurons in the pri- mate. In spite of the species and evolutionary changes in the synaptic connections of the corticospinal tract, it appears that the interneurons identi®ed in the cat are retained in the monkey and human. These interneurons, under the in¯uence of descending pathways, modulate the output of motoneuron pools. Perhaps the control of these interneurons has also changed towards ®ner con- trol of movement, as has been suggested by recent studies in the monkey. Whether in cat or human, the recruitment pattern for motor units is the same; the change from disynaptic to monosynaptic connections has not changed the recruitment pattern of muscles. Dierences in the recruitment patterns of muscles may lie in the ®ner control of inputs to motoneurons in the primate. This review seeks to integrate the current knowledge of the mechanisms involved in the motor control of the wrist joint and especially in the recruit- ment patterns of the muscles. These motor control mechanisms include the biomechanics of the wrist joint, recruitment patterns of wrist muscles, interneurons and spinal cord circuits in the cervical regions mediating the output of spinal motoneurons, and the supraspinal control of these muscles. Key words Recruitment á Interneurons á Re¯exes á Corticomotoneuron á Motoneuron Introduction The human hand has received enormous attention from a wide range of specialists including behaviourists, an- thropologists, neuroscientists, surgeons ± and, of course, palm readers. The function of the dexterous hand is highly dependent on the biomechanical and activation capabilities of the forearm muscles acting at the wrist joint. There is an intricate coordination between the activation of dierent wrist and hand muscles. For ex- ample, cocontraction of wrist extensor and ¯exor muscle creates a stable basis for ®nger ¯exor and extensor muscles to produce precise ®nger movements, such as ®ne manipulation in pinch grip or pressing buttons on a computer mouse. The ®ne control of wrist and hand muscles has been shown to be re¯ected in the strong monosynaptic corticomotoneuronal connections to the hand and forearm motoneuron pools in the baboon see Fetz et al. 1999 for Clough et al. 1968) and in humans see Pierrot-Deseilligny 1996 for Palmer and Ashby 1992). This review brings together information on vari- ous peripheral and central structures and mechanisms involved in the control of wrist movements. In consid- eration of the limitations of space, many of the original articles cited are not given in the References but may be located through the indicated cross-references. Biomechanics of the wrist The contemporary work environment includes a high level of computer use that is accompanied by charac- teristic patterns of wrist and ®nger muscle activity. These occupations use movement patterns which include Eur J Appl Physiol 2000) 83: 116±127 Ó Springer-Verlag 2000 G. R. Chalmers &) Department of Physical Education, Health and Recreation, Western Washington University, Bellingham, WA 98225-9067, USA e-mail: chalmers@cc.wwu.edu Tel.: +1-360-6503113; Fax: +1-360-6507447 P. Bawa á K. E. Jones á M. L. Walsh Kinesiology, Simon Fraser University, Burnaby, BC V5A 1S6, Canada K. Sùgaard National Institute of Occupational Health, Lersù Parkalle 105, 2100 Copenhagen é, Denmark