The aim of this human study was to investigate the effect of experimentally-induced muscle pain on the modifications of motor unit discharge rate during sustained, constant force contractions. Intramuscular and multi-channel surface electromyographic (EMG) signals were collected from the right and left tibialis anterior muscle of 11 volunteers. The subjects performed two 4-min long isometric contractions at 25% of the maximal dorsi-flexion torque, separated by 20-min rest. Before beginning the second contraction, hypertonic (painful; right leg) or isotonic (non-painful; left leg) saline was injected into the tibialis anterior. Pain intensity scores did not change significantly in the first 150 s of the painful contraction. Exerted torque and its coefficient of variation were the same for the painful and non-painful contractions. Motor unit discharge rate was higher in the beginning of the non-painful contraction than the painful contraction on the right side (mean ± SE, 11.3 ± 0.2 pps vs. 10.6 ± 0.2 pps; P<0.01) while it was the same for the two contractions on the left side (11.6 ± 0.2 pps vs. 11.5 ± 0.2 pps). The decrease in discharge rate in 4 min was smaller for the painful (0.4 ± 0.1 pps) than for the control contractions (1.3 ± 0.1 pps). Initial value and decrease in motor unit conduction velocity were not different in the four contractions (right leg, 4.0 ± 0.1 m/s with decrease of 0.6 ± 0.1 m/s in 4 min; left leg, 4.1 ± 0.1 m/s with 0.7 ± 0.1 m/s decrease). In conclusion, stimulation of nociceptive afferents by injection of hypertonic saline did not alter motor unit conduction velocity but reduced the initial motor unit discharge rates and the difference between initial and final discharge rates during sustained contraction.