INTRODUCTION: The combined force production of the lower limbs during synchronous bilateral contractions (BL) has been reported to be lower than the summed force production of each limb when performed unilaterally (UL), and is known as the bilateral deficit (BLD)[1,2]. Although, the BLD during maximal voluntary contractions (MVCs) has received considerable attention there has been little investigation of explosive contractions (EXP). Further, the investigation of BLD may have been confounded by contrasting UL performance with either: (i) single limb performance in a BL situation; or (ii) combined BL performance, that may under represent the performance of one limb in a bilateral situation (i.e. the best effort of both legs when measured together may not represent the best effort of either leg in the bilateral situation). METHOD: Thirteen healthy untrained males (24 ± 4 yrs) performed a series of isometric MVCs (3 × 3-s duration, 30-s rest; 2-min between conditions) and EXP (10 × 1-s duration, 20-s rest; 2-min between conditions) in 3 conditions: UL, with each leg, and BL. Contraction order was randomised (UL dominant or non-dominant, BL, remaining UL limb). Additionally, UL and BL twitch and octet (8 pulses at 300 Hz; evokes the muscles maximal rate of force development [RFD]) contractions were electrically evoked via supramaximal stimulation of the femoral nerve. EMG was assessed from three superficial agonists. Two separate strain gauges were used to measure maximum voluntary force (MVF) during the MVCs and explosive force (EF) at 50, 100 and 150 ms during EXP. RFD and EMG were measured over consecutive time windows (0-50, 50-100 and 100-150 ms) from their respective onsets. Peak force (PF) and peak RFD (pRFD) were reported from the evoked contractions. Performance during UL contractions was compared to: single limb performance measured during the BL contractions (BLUL); and combined BL performance (BLBL). RESULTS: There was no BLD for MVF (UL, 1438 ± 202; BLUL, 1452 ± 212; BLBL, 1444 ± 217 N, ANOVA, P=0.551). BLUL and BLBL had similar EF at 50 ms to UL, but at 100 ms BLUL (-8.9%, P=0.036) and BLBL (-11.5%, P=0.003) had lower EF than UL. EF at 150 ms was lower for BLBL only (-9.6%, P=0.029). RFD 50-100ms was lower for both BLUL (-13.3%, P=0.01) and BLBL (-15.6%, P=0.001) compared to UL. There were no differences in EMG between UL and either BL measurement for either type of contraction (P≥0.107). There was a BLD in twitch PF for both BLUL (-8.9%, P<0.001) and BLBL (-10.2%, P<0.001), but not twitch pRFD, or octet PF or pRFD. CONCLUSION: There was a BLD during explosive but not maximal isometric contractions, and this occurred due to lower RFD 50-100 ms. There was no difference in muscle activation, but twitch PF was lower during BL contractions, which is a novel finding and may help explain the BLD in explosive force production.