The purpose of this study was to examine whether motor units, grouped by speed of contraction in the human first dorsal interosseous muscle, differed in their sensitivity to ischemia and motoneuron afterhyperpolarization (AHP) time-course. Motor units were recorded while subjects held an abduction force for approximately 10 min. Subsequently, subjects abducted for 4-5 min under ischemic conditions. Motor unit twitches derived using spike-triggered averaging were allocated into "fast" or "slow" contracting groups based on twitch time to peak (TTP) force. Motor units in the "slow" group had a greater sensitivity to ischemia than the "fast" group. When upper and lower quartiles of TTP were compared, motor units with slow TTP had long AHP time-constants (as estimated by an interspike interval histogram transform). Thus, motor units grouped by speed of contraction differed in both their sensitivity to ischemia and motoneuron AHP time-course. This provides preliminary evidence that the estimated AHP time-constant may be used to deduce motor-unit type in humans.