Sensorimotor integration at spinal level as a basis for muscle coordination during voluntary movement in humans

Spinal reflexes have traditionally been treated as separate from voluntary movements. However, animal experiments since the 1950s and human experiments since the 1970s have documented that sensory activities in afferents from muscles, skin, and joints are integrated with descending motor commands at the level of common spinal interneurons. Two different roles of this sensorimotor integration at the spinal level may be discerned. First, sensory feedback evoked by the active muscles may help to drive the motoneurons. Second, external stimuli, such as sudden perturbations of a limb, may give rise to "error signals," which are integrated into the ongoing motor activity and form the basis of corrective responses. When interpreting experimental data, it is important to consider these two different roles. Application of external stimuli may provide little information about how the spinal cord integrates sensory feedback evoked as part of ongoing movements. The complexity of the spinal machinery that is activated by external stimuli also makes the interpretation of data obtained from experiments dealing with artificial external stimuli, such as electrical stimuli, difficult. Nevertheless, such experiments have provided and will continue to provide very valuable information about how the brain and spinal cord ensure coordination of muscle activity during voluntary movement. So far, spinal control mechanisms have only been investigated to a limited extent in relation to sports and occupational activities. Provided that researchers consider the methodological problems of the techniques and that they seek independent validation of the findings, this may be a very fruitful research field in the future.