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Isolated activation of the tongue protrusor muscles is the current technique for treating obstructive sleep apnea (OSA) via functional electrical stimulation. Recent studies, however, have shown marked improvements in upper airway (UAW) patency by co-activating the tongue protrudor and retractor muscles. As such, selective stimulation of the hypoglossal (XII) nerve with a single implantable device presents an attractive alternative for treating OSA. In order to demonstrate the feasibility of such a device, the maximum achievable stimulation selectivity of the Flat Interface Nerve Electrode (FINE) was investigated. The beagle XII nerve was stimulated with an implanted FINE, while the corresponding neural and muscular responses were recorded. The overall performance of the FINE, as defined by the selectivity index (SI), showed a high degree of selectivity at both the fascicular and muscular levels: 0.91 ± 0.05 (n = 5) and 0.85 ± 0.03 (n = 4), respectively. The results clearly demonstrate the feasibility of the FINE for selective stimulation of the XII nerve and UAW muscles.