Bilateral Deficit, or Why You Should Favor Unilateral Exercises

Here’s a quick hypothetical scenario for you.  Suppose you go to the gym and decide to test your strength on the leg curl machine.  Since it’s hard to test your one-rep max for isolation exercises, you decide to go for your five-rep max.

After a few test sets, you determine that you can do six reps at 80 pounds.  Cool.  But then you get an idea- what about unilateral leg curls?  If your leg curl is 80 pounds for six reps, your unilateral leg curl is probably around 40 pounds for six reps.  I mean duh, right?  Still, it couldn’t hurt to test it. 

But what happens next blows your mind.  Because your max unilateral leg curl isn’t 40 pounds.  It’s not even 50 pounds.  It’s 55 pounds, for seven reps.

Naturally, you assume that one leg must just be way stronger than the other.  So you test the other leg- and find that one can do 55 pounds for six reps.  Weird.  So you try the regular, bilateral leg curl again.  Can you do 110 pounds?  Nope, still only 80.  What the hell is going on here?

What you’ve just experienced is called bilateral deficit.  Simply put, you’re almost always stronger at unilateral movements than their bilateral equivalents.  And you can experience it for yourself by trying the exact experiment I just described- leg curls and leg extensions work best for this. 

Why does this happen?  It seems to be central nervous system mediated- your brain can send a stronger signal to the muscles when it’s able to focus on one side of the body at a time.

Most gym exercises are bilateral- think of the bench press, the squat, the deadlift.  Now, you want your body to be mostly symmetrical- not perfectly symmetrical, but mostly- but does this mean you need to train it with symmetrical movements?  No, not really. 

Most movements we perform naturally are unilateral.  Walking alternates left and right.  When we jump, we usually have one foot forward. 

We usually throw stuff one-handed.  Even if we throw two-handed- chest-passing a basketball, for instance- we step into the throw, making the movement asymmetrical. 

This is what our bodies have evolved for.  All other things being equal, unilateral movements are going to be superior to bilateral ones.

How to Use Bilateral Deficit in Your Training

The “all other things being equal” is important here.  Unilateral exercises are only going to be flat-out superior if they’re otherwise identical to their bilateral equivalents. 

In practice, this means many unilateral exercises aren’t really better, because they bring you off balance so much they can only be performed with excessively low weights.  You wouldn’t want to do a unilateral lateral raise, for instance. 

So back to the leg curl and leg extension examples- they’re ideal because you’re seated, can hold onto handles on the machine, and so you don’t need to worry about stabilizing your body.  Exercises where you’re standing up and need to keep yourself stable are hit or miss- they work best as unilateral movements when the weight is held near the centerline of the body- think of how the weight is pressed straight up in a shoulder press- so that you aren’t dragged off-balance.

Unilateral movements also combine well with cluster sets, as you can work one limb while the other rests.  Do a few reps with one limb, then a few with the other, and go back and forth 3-4 times until both limbs are fatigued- that’s one cluster set. 

Here are some of my favorite unilateral exercises:

Unilateral lying leg curl cluster sets (lying is better than seated for the leg curl)

Unilateral leg extension cluster sets

One-armed dumbbell sumo press or Arnold press

Bulgarian split squats from a deficit (both feet elevated so your knee can dip below your feet)

Dumbbell lunges, ideally performed from a deficit (inferior to the split squat but requires less equipment- more practical in some cases)

Unilateral standing machine calf raise (if you have handles to hold for balance)

Unilateral seated calf raise

One-armed dumbbell rows

Bayesian curl

One-armed Zottman curl

One-armed cable or dumbbell triceps extension

One-armed dumbbell Romanian deadlift (Not one-legged though, unless your main goal is balance rather than strength or hypertrophy)

Dumbbell swing

Side planks

Bicycle crunches

Unilateral movements allow you to lift harder and get stronger.  Incorporate them into your workouts as much as you can, just so long as they don’t make balance- rather than muscular fatigue- the limiting factor in how heavy or how hard you can train.