Effect of training on voluntary activation of human fusimotor neurons.

The study was designed to determine if human subjects could develop a strategy that would allow them to activate muscle spindle afferents selectively, without contraction or stretch of the receptor-bearing muscle. Recordings were made from 19 identified muscle spindle afferents using insulated tungsten microelectrodes inserted into motor fascicles innervating ankle dorsiflexors, ankle plantar-flexors, and intrinsic muscles of the hand. The discharge of nine of the spindle endings accelerated in voluntary contractions at low levels of effort (less than 10% of maximum force). The remaining 10 endings had relatively high thresholds for activation in voluntary contractions. Despite periods of relative freedom to move and prolonged feedback of the spindle discharge and relevant electromyographic signals, subjects did not develop a strategy with which they could activate any of the afferents selectively. The findings suggest that fusimotor neurons in awake human subjects cannot be activated voluntarily without also activating low-threshold alpha-motoneurons. This is in contrast to reports of selective activity in muscle spindle afferents in freely moving cats. There are two possible explanations: 1) the motor tasks studied in man and cat are not equivalent, or 2) there is a species difference in the control and excitability of fusimotor neurons.