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 (F_CV), 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 (rEMG_H) of the vastus lateralis muscle. The cross-correlation between F and rEMG_H was also calculated. F_CV was significantly higher in FE₁₀₀ than in FE₀ and FE₅₀ (p < 0.01). The peak power values of F and rEMG_H was significantly higher in FE₁₀₀ than in FE₀ (p < 0.05). Cross-correlation between F and rEMG_H 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.