Wednesday, 24 June 2009

Bilateral Deficit (How to Work 13% Harder)

Apart from being a bit of a conversation killer at dinner parties, the bilateral deficit is the phenomenon of a reduction in performance during synchronous bilateral movements when compared to the sum of identical unilateral movements.

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"
This finding was endorsed by Vandervoort et al. (in the same article), who found that:
  • "MU activation was less complete during BL versus UL leg extension",
So what causes this phenomena? There seems to be a bit of disagreement around the underlying cause and little agreement other than muscle unit activation might be 'less complete', activation speed and/or possibly some local neural control mechanism.

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!


Jonathan said...

I imagine this as lying somewhere along the continuum between isolation and whole body movements. Rather than it being a matter of one method being superior, it's another tool in the belt to help achieve one's goals (whether that's maximizing motor unit recruitment, hormonal response, neural adaptation, etc.).

Methuselah said...

pistols rule!

Asclepius said...

Jonathan, you are absolutely on the money about this being "another tool in the belt to help achieve one's goals".

I don't think there is an optimal set/rep scheme from a paleo perspsective. For me the goal is simply to really mix it up and ensure there is variety and intensity.

One thing to note with the bilateral deficit effect is that UL means more work done. To get stronger, you need to lift heavier. But apparently there is evidence amongst highly trained Olympic lifters of a 'negative' or inverse BD.

Asclepius said...

Methuselah - pistols were a revelation to me. You NEVER forget your first pistol nor HSPU. I guess it will be the same for my first one arm pull up!

Methuselah said...

one arm pull up - now that will be impressive.

Asclepius said...

OAPUs are a looooong term goal. A one arm chin-up is not too far away.

As with many body weight exercises, I reckon you could probably get AOPUs sooner by training more often and with more focus on the move - but this then entertains the possibility of injury.

It is a tough balancing act!