Large-scale intramuscular electrode system for chronic electromyography and functional electrical stimulation.

To understand how the central nervous system (CNS) enacts movements, it seems important to monitor the activities of the many muscles involved. Likewise, to restore complex movements to paralyzed limbs with electrical stimulation requires access to most limb muscles. Intramuscular electrodes are needed to obtain isolated recordings or stimulation of individual muscles. As such, we developed and tested the stability of large arrays of implanted intramuscular electrodes. We implanted 58 electrodes in 29 upper limb muscles in each of three macaques. Electrode connectors were protected within a skull-mounted chamber. During surgery, wires were tunneled subcutaneously to target muscles, where gold anchors were crimped onto the leads. The anchors were then deployed with an insertion device. In two monkeys, the chamber was fixed to the skull with a titanium baseplate rather than acrylic cement. In multiple sessions up to 15 wk after surgery, electromyographic (EMG) signals were recorded while monkeys made the same reaching movement. EMG signals were stable, with an average (SD) coefficient of variation across sessions of 0.24 ± 0.15. In addition, at 4, 8, and 16 wk after surgery, forces to incrementing stimulus pulses were measured for each electrode. The threshold current needed to evoke a response at 16 wk was not different from that at 4 wk. Likewise, peak force evoked by 16 mA of current at 16 wk was not different from 4 wk. The stability of this system implies it could be effectively used to monitor and stimulate large numbers of muscles needed to understand the control of natural and evoked movements. A new method was developed to enable long-lasting recording and stimulation of large numbers of muscles with intramuscular electrodes. Electromyographic signals and evoked force responses in 29 upper limb muscles remained stable over several months when tested in nonhuman primates. This system could be used effectively to monitor and stimulate numerous muscles needed to more fully understand the control of natural and evoked movements.