Electrical phrenic nerve stimulation (EPNS) applied at end-expiration during exclusive nasal breathing can be used to characterize upper airway (UA) dynamics during wakefulness by dissociating phasic activation of UA and respiratory muscles. The UA level responsible for the EPNS-induced increase in UA resistance is unknown. The influence of the twitches expiratory timing (200 ms and 2 s) on UA resistance was studied in 9 normal awake subjects by looking at instantaneous flow, esophageal and pharyngeal pressures, and genioglossal EMG activity during EPNS at baseline and at - 10 cm H₂O. The majority of twitches had a flow-limited pattern. Twitches realized at 200 ms and 2 s did not differ in their V_Imax, but esophageal pressure measured at V_Imax was significantly less negative with late twitches (- 6.6 ± 2.7 and - 5.0 ± 3.0 cm H₂O respectively, p = 0.04). Pharyngeal resistance was higher when twitches were realized at 2 s than at 200 ms (6.4 ± 2.4 and 2.7 ± 1.1 cm H₂O.L^-1.s respectively). EMG activity significant rose at peak esophageal pressure with a greater increase for late twitches. We conclude that twitch-induced UA collapse predominantly occurs at the pharyngeal level, and that UA stability assessed by EPNS depends on the expiratory time at which twitches are performed.