Confused? Well let me explain. Given your RM for a lift with one limb (a unilateral movement), you'd expect your RM for two limbs combined (a bilateral movement), to be equal to the sum of each unilateral movement (UL), but this is not the case.
Let's go further with an example. Common sense tells us that if we can curl a 25kg dumbell with one arm, we should be able to curl a 50kg barbell with two arms, right? Sadly not. In practice the RM with the barbell will be around 15% less! Behold the bilateral deficit effect.
You can actually see this phenomena in many other exercises - press ups, chins and pull ups, squats, dips rows and presses.
In this article by Peter Vint he quotes work by Vandervoort et al. who found that,
- "...simultaneous BL leg extension strength was significantly less that the summed UL strengths under isometric and concentric conditions."
Vint addresses work by Secher, Rorsgaard, and Secher who concluded that:
- "...slow-twitch fiber activity was reduced during the simultaneous BL exercises. The results of pharmacological muscular inhibition led to a further rationalization that the UL strength dominance was attributable to a reduction in ST MU activation during the simultaneous BL exercises, despite the fact that the FT fibers were actively firing"
- "MU activation was less complete during BL versus UL leg extension",
The size of the deficit varies between individuals, whether trained or untrained, and, on efficiency of an individual's CNS.
That said, the effect is real and given how much more juice you can squeeze from unilateral movements, they may be the way to go!