Bilateral deficit (BLD) is a phenomenon that leads to decrease of motor output from one limb in motor task that is performed bilaterally when compared with motor output from the same limb in motor task that is performed unilaterally. There is no clearly defined mechanism that lies behind this phenomenon. Previous research suggested two mechanisms in vertical jumping (VJ); neural inhibition in bilateral contractions and change of force-velocity relationship in leg extensor muscles. The purpose of this research was to determine whether the reason of BLD in force during VJ is neural inhibition in bilateral contractions or a change in force-velocity relationship in leg extensor muscles. Results are indicating occurrence of BLD in mechanical output of relative average force and power, but also in the relative work. In the performance od VJ with subject’s own body mass BLD in relative average force was 37%, while the size of BLD in power was 10%. BLD in work was 31%. Main findings show significant impact of unloading on the size of BLD in relative average force. There was also a significant influence of unloading on mechanical output of force, power and work. Results of EMG activity showed simultaneous decreas of muscle activity and mechanical force output. While bilateral EMG deficit was found in the most of muscles, there was also a bilateral EMG facilitation found in some muscles. This research has indicated neuro-mechanical source of bilateral force deficit phenomenon in ballistic motor task like VJ, and it has improved our understanding of human neuromuscular function. Regarding the results of this research, there has been found a significant influence of the force-velocity properties of the muscles on the occurrence of BLD phenomenon in mechanical force output. It has been also confirmed that in the natural tasks like VJ a significant mechanism behind aforementioned phenomenon is change in forcevelocity relationship between unilateral and bilateral performance of VJ. This conclusion does not discard influence of neural factors in the occurrence of investigated phenomenon. Bilateral EMG deficit and in some cases, bilateral EMG facilitation in VJ performance clearly shows the effect of neural factors on BLD in mechanical output of force. Altogether, the given results show two segmental BLD and divided role of biomechanical and neural factors behind the occurrence of bilateral force deficit.