Bilateral deficit - what is it and why do I need to know about it?

So first of all, what is the bilateral deficit? This is basically the phenomenon whereby the sum of forces produced unilaterally (on a single leg) can exceed the amount of force you can produce bilaterally (on two legs).

Another term that is important to understand in relation to this is ‘super-incumbent load’. This is the bodyweight above the joint that is being used to produce force.

So lets compare a theoretical bilateral back squat and compare it to its unilateral cousin, the single leg squat. For the sake of argument, let’s use the example of an everyday endurance athlete, called Dave, who weights 75kg.

Let’s say Dave can do a nice set of 8x single leg squats while holding just 10kg of added weight. With a bit of training, this is not very hard to achieve.

The super-incumbent load of this athlete is 65kg. Again for the sake simplicity, his shin, ankle and foot conveniently weights exactly 10kg. We don’t want to count the weight of his shin, ankle and foot because he’s not lifting it, so 75kg-10kg = 65kg. Add that 65kg to his 10kg dumbbell he’s holding and we have 75kg of weight being lifted on each rep, on that single leg. So for his set of 8 reps - he’s lifted 600kg on that one leg.

Now, let’s look at Dave’s back squat. To compare like for like, the key question is:

How much weight do we have to slap on his back to get through the same amount of work as his 1 set of 8 rep single leg squats with a 10kg dumbell?

Due to the bilateral deficit. The answer may surprise you!

Because he’s on two legs, his super-incumbent load is now only 55kg. We don’t count both feet, ankles and shins now, as they’re not being lifted and are just supporting him (75-(2x10kg) = 55kg). Remember, we still need to achieve 600kg in just 8 reps, PER leg. So assuming both legs do the same amount of work, we need to hit 1200kg across 8 reps.

In a back squat, he’s still lifting his super-incumbent load of 55kg for 8 reps, so thats 440kg accounted for, but we need to find the remaining 760kg (1200 - 440 = 760kg) in 8 reps. 760/8 = 95kg.

Now for an endurance athlete like Dave - this seems like a no brainer. It will be much faster, safer and easier to get him to work up to a set of 8x10kg single leg squats than 8x95kg back squat. Single leg variations also make a lot of sense for group and youth settings. They, require far less equipment, setup, risk, and result in a very time effective way to train your athletes.

While there are other benefits to a single leg work, I’ll leave this here for now. So now you know about the bilateral deficit, use it in your training!