Initial doublet discharges are important for quickly producing force due to the catch-like property of muscles, resulting in a non-linear summation of motor unit twitch forces (Burke et al., 1976). The doublet literature regarding healthy humans has focused primarily on repetitive doublets close to the minimal rhythmic firing rate (Bawa and Calancie, 1983) and ballistic contractions with high force levels (Desmedt and Godaux, 1977). However, whether a fast onset of synaptic input at lower force levels is sufficient to produce initial doublets is unclear. We performed a human experiment comprising a slow ramp followed by superimposed fast sinusoidal movements (5-15% MVC-range) at low force levels using the tibialis anterior while recording multichannel surface electromyography. Initial doublets were produced in some higher-threshold units that were phasically active during the superimposed sinusoidal contractions (mean inter-spike interval 9.9 ms). Motor units that were tonically active throughout the contraction did not exhibit doublets. In parallel intracellular recordings of high-threshold sacral motoneurons in adult mice, initial doublets (8-10 ms) were also produced in response to large rectangular current pulses (>3 nA), which allowed the afterdepolarization (AD) in the first action potential to reach the firing threshold. Once the afterhyperpolarization (AHP) was activated after the doublet spike, the size of subsequent afterdepolarizations was reduced, and no further doublets were produced. The outward SK currents activated during the AHP likely counteract the inward voltage-activated Ca+2 currents mediating the AD to restrict doublet discharges to the first action potential, thereby producing a firing pattern that maximizes force production.