Reversing the recruitment order with electrode array stimulation

Electrical stimulation of peripheral nerve activates large-diameter fibers before small ones, a recruitment order opposite to the physiological one. Methods proposed to solve this problem require a long-duration stimulus pulse which could lead to electrode corrosion and nerve damage. This study presents a stimulation technique capable of activating small fibers before large ones by manipulating the extracellular voltage using an array of electrodes placed along the nerve and the technique is independent of stimulus pulsewidth. The electrode array was tested in two cats on lateral gastrocnemius (LG) nerve which innervated both LG and soleus muscles. The LG has fast-twitch fibers and is innervated by large axons, whereas the soleus has slow-twitch fibers and is innervated by smaller axons. A stimulus electrode was placed around LG nerve. Tendons of LG and soleus muscles were separated and attached to two independent force transducers. The recruitment curves generated by tripolar and array electrodes were compared. Tripolar stimulation recruited LG before soleus muscles as expected, whereas the electrode array fully activated soleus while activating only 50% of LG muscles. These results show that the electrode array is capable of reversing the recruitment order by manipulating the extracellular voltage along the nerve.