Prior to the collection of torque data on the planter flexion axis, the 0˚ contact in the self-sizing spiral cuff electrode had been identified as located close to the common peroneal fascicle in the sciatic nerve trunk. Through stimulation of the distal branch of the common peroneal nerve the maximum torque produced by activating all motor neurons of the common peroneal nerve branch was identified.
Using the monopolar cuff configuration, with stimuli applied to the 0˚ contact, in the cuff, the evoked torque as a function of current was plotted, shown in magenta colored symbols. The slope of this plot can be calculated between the 10% and 90% torque values and is defined as the gain, in this case measured to be -0.4 N-cm/µA. I’m illustrating increased recruitment using time for increasing current spread through the common peroneal fascicle.
Next, a similar plot was constructed using 0˚ contact in the tripolar configuration, blue symbols. Calculating the recruitment gain we find a lower gain, -0.2 N-cm/µA. This means that with the tripolar configuration there is a more gradual recruitment characteristic, illustrated by the slower spread on current in the common peroneal fascicle.
There are a couple of points I want you to take note of. First, the maximum torque generated when stimuli were applied to the 0˚ contact in both monopolar and tripolar configurations was the same as the maximum torque generated by stimuli applied to the isolated common peroneal nerve branch. A change in the direction of the torque as the current was increased indicates the stimuli was spreading the other fascicles in the sciatic nerve trunk. The greater the current magnitude, required to spread the stimulus in to an adjacent fascicle with the tripolar configuration suggests greater control of current spread, isolating the activation to a single fascicle is cleaner. These observations are consistent with the modeling results by Chintalacharuvu.
Tarler, M.D. and J.T. Mortimer, “Comparison of Joint Torque Evoked With Monopolar and Tripolar-Cuff Electrodes, IEEE Trans. on Rehabilitation Engineering, Vol 11, pp227-235. 2003.