The motor unit twitch torque is modified by sustained contraction but the association to changes in muscle fiber electrophysiological properties is not fully known. Thus, twitch torque, muscle fiber conduction velocity, and action potential properties of single motor units were assessed in 11 subjects following an isometric sub-maximal contraction of the tibialis anterior muscle until endurance. The volunteers activated a target motor unit at the minimum discharge rate in eight 3-min long contractions, three before and five after an isometric contraction at 40% of the maximal torque, sustained until endurance. Multi-channel surface EMG signals and joint torque were averaged with the target motor unit potential as trigger. Discharge rate (mean ± SE, 6.6 ± 0.2 pulses per second) and inter-pulse interval variability (33.3 ± 7.0 %) were not different in the eight contractions. Peak twitch torque and recruitment threshold increased significantly (93 ± 29 % and 12 ± 5 %, P < 0.05) in the contraction immediately after the endurance task with respect to the pre-endurance values (0.94 ± 0.26 mNm and 3.7 ± 0.5 % of the maximal torque) while time-to-peak of the twitch torque did not change (74.4 ± 10.1 ms). Muscle fiber conduction velocity decreased and action potential duration increased in the contraction after the endurance (6.3 ± 1.8 % and 9.8 ± 1.8 %, respectively, P < 0.05; pre-endurance values, 3.9 ± 0.2 m/s and 14.9 ± 0.9 ms) while the surface potential peak-to-peak amplitude did not change (27.1 ± 3.1 µV). There was no significant correlation between the relative changes in muscle fiber conduction velocity or surface potential duration and in peak twitch torque (R² = 0.04 and 0.10, respectively). In conclusion, modifications in peak twitch torque of low-threshold motor units with sustained contraction are mainly determined by mechanisms not related to changes in action potential shape and in its propagation velocity.