The aim of the present study was to determine whether the difference in maximal voluntary contraction (MVC) forces between unilateral (UL) and bilateral (BL) contractions could be due to a difference in the timing of MVC peak force production between legs during BL MVC, or due to the maximal voluntary activation level (VAL) in each leg between UL and BL MVC. Thirteen active men (28 +/- A 6 years) volunteered to participate in an experimental session requiring the performance of MVC with right, left and both knee extensor muscles. An ergometer equipped with two strain gauges was used to assess the timing of MVC peak force production of each leg during BL MVC. Transcutaneous electrical stimulation was used to investigate VAL and electromyography (EMG) activity. The present results showed that the sum of the right and left UL MVC was higher compared to BL MVC force, i.e., a BL deficit of force. The timing of MVC peak force production of each leg during BL MVC was not different, but BL MVC force was lower than the sum of BL MVC(R) and BL MVC(L) force (i.e., maximal force production of each leg during BL MVC). No changes of EMG activity, M-wave amplitude, VAL, and peak doublet between UL and BL contractions were observed. The difference found in the production of force during BL MVC cannot be explained by the timing of force production of each leg.