FIGURE 3.3 Recruitment curves for single twitch contractions at various stimulus intensity levels, measured as force normalized to the maximal twitch for 100% recruitment of the muscle. A. Comparison of recruitment of cat medial gastrocnemius by nerve cuff electrodes (open circles) and nerve hook electrodes (open diamonds) on the common sciatic nerve, as opposed to intramuscular electrodes configured as in the BION (solid circles) for acutely implanted (solid lines) and chronically implanted (dashed lines) electrodes (adapted from Singh et al., in press). B. Recruitment of cat posterior biceps muscle by a BION1 implanted acutely in a proximal or distal location in the muscle belly, with stimulus intensity (abscissa) normalized to the threshold for a just palpable twitch at each of two pulse widths (adapted from Cameron et al., 1997). The plateau effects at less than 100% recruitment represent stimulus current levels at which compliance voltage limits were reached for an earlier version of the BION1 that had 8 VDC rather than 17 VDC compliance.
30-300 us, pulse duration and current are interchangeable as a way to modulate stimulus strength. Thus the ranges of these output variables available in the BION can be used to produce very fine control of recruitment. The details of recruitment depend on the location of the device in the muscle. A stimulator located near the entry point of the nerve into the muscle is able to recruit more of the axons before they fan out in their course toward more distal parts of the muscle.14
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