What the heck is the bilateral deficit? Watch the video.

There may be some skepticism or confusion regarding the bilateral deficit. People may wonder why they cannot 1-arm bench press more than half of their max bilateral bench. Not all movements work very well for demonstrating the bilateral deficit, because unilateral movements often use different positions than their bilateral counterparts and are generally less stable.

For example, you may want to compare your back squat and your 1-leg squat. But 1-leg squatting uses major forward translation of the knee as well as back rounding. It is mostly just a knee/quad exercise, which should make it weaker compared to the back squat, which involves the hips/glutes much more. And of course 1-leg squatting is limited by stability as well. That being said, a good 1-leg squatter may still demonstrate more strength per leg on one leg compared to two if body weight is factored in. For example, there was a time when I weighed 205 lbs and squatted 345, so I lifted 550 lbs total. At the same time, I could also do a deep 1-leg squat with 100 lbs, lifting 305 lbs total on one leg. To be more accurate, in a squat the weight of the legs is not really being lifted. The legs are doing the lifting. If we subtract 40 lbs per lifting leg, I was able to squat 470 lbs on two legs and 265 on one. So I demonstrated more strength per leg in a unilateral squat, even though knee dominance and instability should make it weaker. On a similar note, without any practice I have been able to walk up to 250 lbs on a barbell and pick it up on one leg. I have never been able to deadlift 500 lbs. This is possible because of the bilateral deficit.

Other exercises offer simpler demonstration of this strength deficit. Compare unilateral and bilateral strength in simple dumbbell exercises like I did in the video with bicep curls. Find a pair of objects that are hard to grip. Note how difficult it is to grip one in each hand compared to just one at a time. Strength training machines work well for this too. Compare unilateral and bilateral leg press, quad extensions, or hamstring curls. You should find the bilateral deficit quite evident.

As I mentioned in the video, it is easy to take this information and jump to the conclusion that unilateral strength training is superior to bilateral. If you can produce more strength in one limb at a time, then training that way will make you stronger, right? Mike Boyle (the wonderful, famous, genius, top notch performance coach, Mike Boyle) fell into this trap years ago when he proclaimed “the death of squatting.” He found that his athletes could lift much more weight per leg in rear foot elevated split squats than in regular back squats. His explanation for this was that squatting is not a lower body exercise but is limited by lower back strength. Apparently Mike Boyle does not know anything about proper squatting. The actual explanations are (1) contribution from the back leg and (2) the bilateral deficit. But regardless of the explanation, the strength deficit is still there. And replacing bilateral strength exercises with unilateral is an intriguing idea.

Unfortunately, focusing on unilateral strength training does not work very well in practice. Unilateral strength does not carry over very well to bilateral strength, and more importantly it does not carry over very well to athleticism. You could spend a couple months increasing your weight on lunges and then find that you have not improved in any other way. On the other hand, if you improve your squat strength, you will probably find that your deadlift went up, lunges and pistol squats are easier, you are jumping higher and quicker, you can accelerate faster, you can throw a medicine ball higher, so on and so forth. Squats make you a generally stronger, more powerful person. Lunges make you better at lunges. So, Mike Boyle, let’s get something straight. Bilateral back squats are the best strength exercise known to man, and there is no replacing them.

The rear foot elevated split squat, not a replacement for bilateral squats.

Why is it that unilateral strength does not seem to translate well into improved all around strength and athleticism? I cannot offer a definitive explanation, but it does make some sense if we consider the mechanism behind the bilateral deficit. In sports training, one of the biggest questions is “What carries over to what?” In particular, what strength training movements carry over to what explosive movements? Decades of experimentation have taught us that simply working the relevant muscles is not enough. To improve athleticism, there must be neurological carryover from the strength training to the explosive movements. This requires some level of similarity in the coordination. For example, triple extension provides the power for sprinting and jumping. Since squats, deadlifts, cleans, and snatches all train triple extension strength, they are effective exercises for improving sprint and jump performance. But getting in a machine and training knee extension in isolation is not nearly as effective even though it trains the quad muscles in intense fashion. Now consider the aforementioned neurological characteristic of unilateral strength training, both sides of the brain contributing to force production in one limb. This is the source of unilateral strength. During a bilateral movement, you cannot tap into this source of strength. This may explain why improved unilateral strength tends to not translate into improved bilateral strength. A disconnect exists due to the difference in coordination. What about unilateral explosive movements? In this case, I suspect the disconnect is the result of the difference in time frame for producing force. When the body performs a movement, it does not start with instant maximal force production. Rather there is a process of figuring out the demands of the movement and then a buildup of neural drive. During unilateral strength training, it is safe to say that full bilateral brain contribution to one limb is not instantaneous but rather builds up over a short period of time. Maybe that time is only half a second, but in the realm of 1-leg explosive movements half a second is a really long time. Single leg ground contact times are generally well under half a second, and the better the athlete, the shorter the times get. It may be that the ground contact time during a running vertical jump for example is not long enough to allow an athlete to access much of the bilateral brain contribution that produces increased unilateral force production. This reduces the translation of force production from strength training to explosive movements. Of course there is always some disconnect between strength and athleticism. But unilateral explosive movements are particularly quick, and unilateral strength training is particularly slow, which makes the disconnect greater.

Does this mean unilateral strength training is worthless? Certainly not. Consider the following…

  • Basic movement competency. Lunges and step ups are movements that athletes (and people in general) should be able to do. They are useful for developing good general mechanics and range of motion as well as exposing flaws in those areas. They provide a simple context for training these qualities. That being said, I think you would be hard pressed to find someone who squats beautifully but cannot do a good lunge, so perhaps just focusing on squats would suffice.
  • Structural strength. Because of the bilateral deficit, unilateral strength training can potentially feature higher muscle tension than bilateral training. So even if it has no neurological benefit, unilateral training can at least develop structural strength. And something like a lunge can easily be manipulated to target a particular muscle. For example, I know that a big reverse lunge will tear up my glutes in a way that squats cannot. Or I can easily change a lunge to focus on the quads if I want.  However, if you are already playing a sport and doing bilateral lifting, you may not need any more structural breakdown from additional exercises. On the other hand, if you are unable to do bilateral lifting, unilateral is certainly better than nothing.
  • Variety of movement. Exclusively performing a limited set of movements is a good way to over-develop some muscles and under-develop others. It’s also a good way to get bored with training. Unilateral exercises give you more options for movement variety. Of course, if you play a multi-directional sport, you probably already perform a wide variety of movements and can probably just focus on fundamentals in the weight room.
  • Simplicity for beginners. When new lifters are working on squat and deadlift technique, they may not be able to get much of a workout from those movements. Unilateral strength training provides a simple, safe way for beginners to try hard and get a good amount of work done. And at the beginner level, any means of producing a lot of muscle tension should be effective, even if it is not ideal neurologically. For example, I know of a few young athletes who improved athleticism by training pistol squat strength.

Here is some advice regarding unilateral strength training protocol. It’s important to understand that unilateral exercises do not do much for your maximum strength. There is no reason to go heavy on lunges. All the potential benefits of unilateral exercises come from repetition. These exercises should be done with lighter weight for at least 6 reps per side in a set. And there should be no struggling to finish. Knock out some smooth, quality reps and be done. You do not have to prove how strong you are in every exercise. In fact, on split-leg exercises (lunges, split squats) using heavy weight is dangerous for the adductor muscles. Mike Boyle, after he declared the death of squatting, had his athletes pursuing max strength with heavy rear foot elevated split squats. Go figure, some of his guys sustained adductor injuries. At my university, the track team has one week every year of heavy split squats. They also have one week every year of multiple adductor injuries. Years ago I did a set of lunges with 265 lbs. The following day I had some minor groin pain. It lasted for a long time and was bad enough to stop me from squatting at times. To this day, if I do lunges with more than 135 lbs, that old pain will come back again. Do not do heavy split-leg training!

Lifting for athleticism should be centered around bilateral strength development. Unilateral exercises do not need to be universally prescribed. For example, step ups are not an exercise that everyone must do. However, each athlete has his/her own needs. Unilateral exercises can be quite useful for addressing particular needs. Think of them as tools in your bag that you bring out for specific purposes.