Effect of intertrial delay on whole nerve cuff recordings of muscle afferents in rabbits.

Objective. The effect of the stationary period on the muscle afferent responsiveness to passive stretch was studied to determine if muscle afferent activity might be suitable as feedback in motor-neural prostheses control. Methods. Muscle afferent activity was recorded using nerve cuff electrodes applied individually to the tibial and peroneal components of the sciatic nerve in five rabbit preparations. The ankle joint was rotated using a servomotor. The delay between trapezoidal movements was varied (excursion = 30°, velocity = 20°/s) and the duration was increased stepwise in successive trials(2 s, 5 s, 10 s, 20 s, 30 s, and 180 s). Results. An increase in amplitude of the afferent activity with increasing intertrial delay was observed. The afferent responses following a 2-s delay were subtracted from the responses following each of the remaining intervals (5 s-2 s, 10 s-2 s, 20 s-2 s, 30 s-2 s and 180 s-2 s) and were then compared. During the ramp phase a statistically significant difference was found in 85% and 76% of the cases for the tibial and peroneal nerve responses. Significant differences were below 21% for both nerves during four other phases examined. Conclusions. It is suggested that the enhancement in afferent responses caused by the increased intertrial delay may be explained by the change in the mechanical properties of the intrafusal and extrafusal muscle fibers. The consequences of this, when applying muscle afferent activity in motor-neuro prostheses, are discussed.