Interleaved multielectrode stimulation enhances fatigue resistance

Recruitment of force via independent asynchronous firing of large numbers of motor units produces the grace and endurance of physiological motion. I have attempted to reproduce this physiological recruitment strategy by activating large numbers of independent pools of motor units via intrafascicular multielectrode stimulation (IFMS) of peripheral nerve to produce fatigue-resistant forces. Demonstrating the feasibility of interleaving stimulation via multiple electrodes at low frequencies to produce fatigue-resistant yet ripple-free contractions was the starting point for this project. To this end, interleaved 15 Hz stimulation of four electrodes, producing a net stimulation frequency of 60 Hz, was compared with multielectrode 60 Hz quasi-simultaneous stimulation protocols. Across a broad range of forces (10% to 80% of maximum), responses evoked by multielectrode 15 Hz interleaved stimulation exhibited substantially less fatigue than did responses evoked by 60 Hz quasi-simultaneous stimulation, and less ripple than responses evoked by single-electrode 15 Hz stimulation. Having determined the feasibility of this strategy, the next phase of the project was to ascertain how many electrodes grant access to individual subpopulations of motoneurons innervating a given muscle. We found a mean of 17.4 +/- 4.9 (mean +/- sem) electrodes selectively excited maximal forces in medial gastrocnemius before exciting another muscle. Among electrodes demonstrating selectivity at threshold, a mean of 7.3 +/- 2.7 electrodes was shown to recruit independent populations of motor units innervating medial gastrocnemius (overlap <20%). The stimulation strategy was to use more electrodes, which grant access to independent pools of motor units, in order to lower the frequency of stimulation that can be used at each electrode. To determine the benefits of very low stimulation frequencies, fatigue trials have been run across a range (0.1 to 20 Hz) of stimulation rates. Time to fatigue exhibited a relatively early onset (<120 s) with stimulation frequencies from 20 Hz to 5 Hz, but increased monotonically with stimulation frequencies from 5 to 1 Hz. No fatigue was observed during the 12-minute stimulation trials at stimulation frequencies below 1 Hz. The reduction in stimulation history effects decreases variation from targeted forces allowing the maintenance of a fatigue-resistant contraction.