NONINVASIVE MEASUREMENT OF THE INPUT-OUTPUT PROPERTIES OF PERIPHERAL-NERVE STIMULATING ELECTRODES

A non-invasive method was developed to determine the input-output (I/O ) properties of peripheral nerve stimulating electrodes. An apparatus was fabricated to measure the 3-dimensional (3-D) isometric torque gen erated at the cat ankle joint by electrical activation of the sciatic nerve. The performance of the apparatus was quantified, and the utilit y of the method was demonstrated by measuring the recruitment properti es of multiple contact nerve cuff electrodes. Torque-twitch waveforms, recruitment curves of peak torque as a function of stimulus current a mplitude, and 2-D joint torque vectors were used to analyze the recrui tment properties of the cuff. The peak of the twitch torque was an acc urate measure of excitation even for muscles having fibers with varyin g speeds of contraction. The evoked twitch waveforms and torque vector s generated by selective stimulation of individual nerve branches with a hook electrode were compared to those produced by stimulation of th e nerve trunk with the cuff electrode. These data allowed determinatio n of the regions of the nerve trunk that were activated by different e lectrode geometries and stimulus parameters. The positional stability of electrode recruitment properties could be quantified by measuring I /O characteristics at different limb positions. The methods described are useful for characterization of neural stimulating electrodes and f or studies of motor system physiology.