The aim of the present study was to determine the effects of expiratory effort on the magnitude and low-frequency components of force variability during submaximal isometric knee extension (IKE). Eight subjects performed 5-s IKE of the dominant leg at the intensity of 10% of maximal voluntary contraction (10% MVC). During the IKE, the subjects performed one of three forced expirations (FEs) of different effort levels. The three effort levels of FE were 0% effort (normal breathing, FE0), 50% effort (FE50) and 100% effort (FE100). In the expiratory phase of each IKE, the amplitude of the force variability was quantified by the coefficient of variation (FCV), and the low-frequency components of the force variability were evaluated by calculating the peak power values of the filtered force signal (F) and rectified EMG signal with high-pass filtering (rEMGH) of the vastus lateralis muscle. The cross-correlation between F and rEMGH was also calculated. FCV was significantly higher in FE100 than in FE0 and FE50 (p < 0.01). The peak power values of F and rEMGH was significantly higher in FE100 than in FE0 (p < 0.05). Cross-correlation between F and rEMGH with the latter preceding the former was observed in each FE condition. These results suggest that FE with maximal effort (FE100) increased not only the absolute amount of force variation but also low-frequency oscillatory components of force and EMG and that low-frequency oscillations of motor unit firing may be responsible for the increase in force variation induced by FE.