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 (body) weight above the joint that is being used to lift a weight.
So lets compare a theoretical bilateral (two-legged) back squat and compare it to its unilateral cousin, the single leg squat. For the sake of argument, lets use the example of an individual perhaps your everyday endurance athlete, uhhh, Dave, that weights 75kg.
Lets say Dave can do a nice set of 8x single leg squats while holding just 10kg of added weight. Nice one Dave.
Not quite the same as a single leg squat, but a rear foot elevated split squat (RFESS) or a Bulgarian split squat can be a great way to make the best of the bilateral deficit. This is not Dave.
The super-incumbent load of this athlete is 65kg. That is, if we chopped his leg off just below the knee, his bloodied stump of a 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 and we have 75kg of weight being lifted on each rep, on that leg. So for his set of 8 reps - he’s lifted 600kg on that one leg. WAY. TO. GO. DAVE.
Now, lets say look at Dave’s back squat. 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 10kg?
Because he’s on two legs, his super-incumbent load is now only 55kg as we have 2x10kg metaphorically bloodied stumps not being lifted and supporting him (75-(2x10kg stumps) = 55kg). Remember, we still need to achieve 600kg in just 8 reps. But this is also PER leg. So assuming both legs do the same amount of work, we need to hit 1200kg across 8 reps. He’s still lifting his super-incumbent load of 55kg for 8 reps, so thats 440kg done and dusted, but we need to find the remaining 760kg (1200 - 440 = 760kg) in 8 reps. 760 divided by 8 = 95kg.
Don't fall foul of the bilateral deficit. Sometimes there is an easier way. This is also not Dave.
Now for an endurance athlete like Dave - this is a no brainer. It will be much faster, safer and easier to get him to work up to a set of 8 single leg squats with 10kg than 95kg for 8 back squat. For example I don’t know many endurance that at around 75kg bodyweight can squat their bodyweight well for 1 rep, much less 1.26x body weight for a set of 8.This would put their 1RM at nearly 120kg or 1.6x their bodyweight. To be fair, I don’t actually know many endurance athletes that can single leg squat for 1 rep, but with a bit of coaching and practice, this is still a far more sensible and achievable prospect (especially in the short term) than getting a 75kg athlete to 95kg for 8 reps. For a start, it requires much less equipment, no gym membership and can be more easily practiced at home.
While there are other benefits to a single leg work, especially as an endurance athlete, I’ll leave it there for now. Now you know about the bilateral deficit, you can use it to your advantage. If you want to find out more or have any questions, please get in touch with me either via twitter
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