The Effects of Stimulation Strategy on Joint Movement Elicited by Intraspinal Microstimulation

The goal of this study was to characterize the effects of stimulation parameters and multielectrode stimulation on selectivity, range of motion, recruitment characteristics, and fatigue during intraspinal microstimulation (ISMS). A custom-made multielectrode array was implanted into the activation pool of the rat dorsiflexor muscle where the stimulation produced the highest movement range on the ankle joint and the least effect on the other joints. The results show that the selectivity could be significantly enhanced using multielectrode stimulation strategy. Moreover, the fatigue was significantly reduced using multielectrode synchronous stimulation with respect to single-electrode stimulation. For a given charge, stimulation with higher current amplitude and shorter pulse duration produced greater range of motion than that with lower amplitude and longer pulse duration. However, the stimulation with shorter duration caused greater fatigue than that with longer. In addition, there was a significant difference in time constant of spinal response obtained with different pulse amplitudes during pulse width (PW) modulation. The time constant decreased with increasing pulse amplitude. However, there was no significant effect of pulse duration on time constant during pulse amplitude (PA) modulation. The results suggest that the motor neurons (MNs) within the spinal cord can be recruited according to size principle by appropriate selection of stimulation parameters. Based on these results an efficient stimulation strategy can be designed for control of movement performance (i.e., speed of movement, fatigue, range of motion, and selectivity) during ISMS.