Bilateral Deficit Explained

On the advantages of unilateral and iso-lateral exercises

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Here’s something I want you to try next time you go to the gym.

First thing, head over to the leg extension machine and find your five-rep max– the heaviest weight you can perform five repetitions with. Try a weight, if you can do six reps with it, stop, wait a minute or two, then try the next heaviest weight, and so on until you find one you can’t do at least five reps with.

Once you’ve found your five-rep max, wait three more minutes, then set the weight to half as heavy as your five-rep max with two legs was– or if you can’t precisely halve that weight, then the next highest weight– and see how many reps you can do.

Since you’re lifting half as much weight with only one leg, you’d think it would be five reps again. In all likelihood however, you’ll be able to do more– perhaps seven or eight reps, maybe even nine or ten.

Now raise the weight, wait a minute, and then do it again with the other weight. You’ll very likely be able to do at least five reps, perhaps even six or seven.

Ultimately, your results may look something like this:

• Two legs, 100 lbs, 5 reps

• One leg, 50 lbs, 7 reps

• One leg (the other one this time), 60 lbs, 5 reps

What’s going on here? Why does using only one leg allow you to produce more force with a given amount of muscle?

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.  

The reason this happens is because strength isn’t just a function of the physical properties of your muscles. Some of it has to do with the nervous system. Some of that nervous system component in turn relates to the nerves in your muscles, but there is also a central nervous system subcomponent.

In other words, your brain factors into it— and 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.

But let’s back this up a moment and clarify our terms. Exercises can be bilateral, unilateral, or iso-lateral.

Bilateral exercises work both sides of the body symmetrically, like a squat or bench press.

Unilateral exercises work only one arm or one leg, like a one-armed dumbbell press, or one-legged leg extension or leg curl.

Iso-lateral movements involve both limbs– most commonly the arms, but occasionally the legs– at the same time, but each one moves a separate weight. Some machines are iso-lateral because each arm moves a separate weight stack. Two-armed dumbbell exercises are inherently iso-lateral.

There are also a couple of groups of exercises that straddle these categories.

Many cable exercises have separate handles for each hand, and allow each hand to move freely and independently, but they both share one weight stack. This lies somewhere between bilateral and iso-lateral.

Then we have asymmetrical exercises like the lunge or Bulgarian split squat. These work both sides of the body in different ways, so they lie somewhere between iso-lateral and unilateral.

Natural movements are rarely bilateral like most gym exercises; instead, they’re usually asymmetrical, or occasionally iso-lateral or 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 and asymmetrical movements are going to be superior to iso-lateral movements, which in turn are 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.  

Of course all other things are not equal– sometimes bilateral movements are preferable because they let you maintain your balance with a heavy weight. You couldn’t squat or deadlift heavy weights if you didn’t keep your body symmetrical, for instance.

Or as another example, a unilateral lateral raise would be likely to pull you off balance. Unilateral dumbbell shoulder presses are possible only because you can lean to one side to balance yourself.

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.

Another technique that can be used with unilateral movements is the alternating unilateral cluster set. Perform 3-5 reps with one limb, wait 5 seconds, perform 3-5 reps with the other limb, rinse and repeat until you near muscle failure. You’ll want to alternate which limb you start with to maintain muscle balance.

A few examples of the different types of movements:

Barbell bench press (bilateral), dumbbell bench press (iso-lateral), one-armed dumbbell bench press (unilateral). Note that the unilateral version works best with lighter weights for balance’s sake.

Barbell shoulder press (bilateral), Arnold press (iso-lateral), one-armed dumbbell shoulder press (unilateral). In this case, for the one-armed shoulder press, you’ll lean to the side that isn’t holding the dumbbell in order to keep the weight near your center line.

Leg curl (bilateral), one-legged leg curl (unilateral).

Leg extension (bilateral), one-legged leg curl (unilateral).

Barbell squat (bilateral), Bulgarian split squat (asymmetrical), pistol squat (unilateral).

Chin-up (bilateral), one-armed chin-up (unilateral). Unless you’re extremely strong you’ll need to use an assist machine for this, and ideally you should use one that allows you to grasp the bar near your body’s centerline.

Unilateral and iso-lateral movements allow you to exert more force while performing a more natural movement than their bilateral equivalents. Try incorporating a little asymmetry into your workouts.