Purpose: Activity of the motor cortex in one hemisphere reduces the maximum motor outflow of homologous parts of the opposite hemisphere, causing a reduction in the maximum force a muscle can exert when the contralateral homologous muscle is activated concurrently. The purpose of this study was to establish whether this bilateral deficit is large enough to explain limitations in performance in bilateral exertions. Methods: Voluntary force production and neural drive during unilateral and bilateral exertions were compared in three experiments, consisting of unilateral maximum contractions, synchronous bilateral contractions, and asynchronous bilateral contractions of finger flexors and knee extensors. Results: Maximum voluntary force was overall about 7% lower in bilateral knee extension as compared with unilateral knee extension (P < 0.001). In finger flexion, a bilateral voluntary force deficit of as much as 20% was found (P = 0.001). Corresponding deficits in agonist EMG activity were also significant and on average found to be of similar size, though the magnitude of the bilateral deficit in EMG was not consistently related to the magnitude of the bilateral force deficit. In knee extension, a deficit in voluntary activation of 4% (P = 0.003) was demonstrated by means of superimposed tetanic stimulation. The magnitude of this deficit was correlated to the magnitude of the voluntary force deficit (r = 0.80, P = 0.002). The maximum rate of force development in bilateral knee extensions was 13% lower than in a unilateral knee extension (P = 0.002). Conclusion: These results suggest that deficits in bilateral force production are large enough to constitute an important performance-limiting factor. Furthermore, the data suggest that a reduced neural drive underlies this bilateral deficit.