Motor responses to FES electrodes in a growing limb

A canine model was used to investigate the chronic performance of surgically implanted electrodes in a growing limb. Seven skeletally immature dogs were implanted with epimysial and intramuscular electrodes. Recruitment characteristics for each electrode, including the activation threshold, spillover muscle threshold, peak force, and force at spillover were recorded with tendon force transducers placed on the tendon of the target muscle and the first spillover muscle. Recruitment properties for the electrodes were measured at the time of implant and after growth was complete. Appropriate electrode positions were also remapped after maturity was reached. Data for 23 electrodes are reported. Average longitudinal growth of the ulna was 5.6 cm (39% increase). The measurements of threshold for target and spillover muscle activation show that appropriate muscle recruitment order was retained in 22 of 23 (96%) of the electrodes. The target muscle thresholds remained stable between implant and explant with variation on the order of that reported for these electrodes placed in mature animals. Overall, a multivariate analysis of variance for repeated measures indicates electrodes performed equally well at explant as they did at implant as measured by threshold and usable range of target muscle before spillover. New electrodes placed in the adult animal did not perform better than the chronically placed electrodes. The recruitment properties of 22 or 23 electrodes did not vary significantly with growth. This is encouraging evidence that a stable motor response can be retained with implantable intramuscular and epimysial electrodes in the presence of growth.