Lee Cockrell- Creating Magic
Any time we bring long held beliefs into question there is bound to be controversy. However, imagine that I was going to show you a new spin on lower body strength training that would allow you to train with heavier weights and yet was far safer and potentially more effective than what you currently do? I think many intelligent coaches would at least initially say “show me”.
Unfortunately, as I began to describe how to lift heavier loads more safely, our sense of what is conventional and acceptable would take over. As I began to describe unilateral exercise as a method to use greater, not lesser loads the automatic reaction is “but I'm using less weight”.
In truth, you might be using less weight than you would use in the comparable bilateral lift but you would still be using more weight with the targeted muscles. By working only one side at a time you use what appear to be lighter loads. However, this is primarily a problem of math and secondarily a problem of perception.
As an example, a 1 Leg Straight Leg Deadlift using 135 lbs supplies 135 lbs to the involved leg and a load of 135 lbs on the spine. Using 225 lbs in a conventional two leg Romanian Deadlift ( yes, I hate the term but many find it recognizable) places 225 lbs of load to the spinal column but supplies only 112 lbs of load to the posterior chain of each leg ( if we assume equal contribution from each leg for discussion purposes).
In this example the targeted muscles are the glutes and hamstrings and, the unilateral exercise involves a higher load for the target muscles (glutes and hamstrings). The added bonus is a lower load to an area of injury concern ( lumbar spine). In other words, the unilateral exercise provides more posterior chain load with less low back stress. Can that be a bad thing? The rational answer is no, however our reaction is often more emotional than rational.
The next complaint or rationalization revolves around the thought that “the exercise doesn't look like what I'm used to “. Unilateral exercises are viewed as weird, different, dare I say functional? Unfortunately, I would again have to say that is not a good reason not to lift heavier loads, is it?
Another frequent complaint about unilateral exercise is that these new training ideas require too much balance. This is the most interesting complaint from the free weight community. If I brought up machine based training, comments like “free weights blow away machines”, “machines don't allow you to balance and stabilize the weight” etc. etc. would immediately come to mind. In fact, for years we have been told that free weights are superior to machines because you are required to balance the load and are incorporating important, underworked stabilizers?
However when the thought process moves one step further and we ask someone to perform a unilateral exercise the argument is often reversed. The unilateral opponent takes the position that unilateral exercises are not good because they are too unstable and require too much balance? Funny? In one breath we glorify free weights because of the balance and stability requirements and in the next breath denigrate unilateral training because of the balance and stability needed? To me this sounds like more of the Simon Senek lament:
“Why can't we do what we have always done?”
Is it really just the change that we dislike? Do we cling to ideas that are outdated even in the face of solid evidence?
Henry Ford had a great quote that parallels my position on unilateral training.
“If I'd listened to my customers I would have invented a faster horse”
If I'd listened to everyone else, we would still be doing back squats and proclaiming them as the King of All Exercises.
Unilateral training is not accepted primarily because it is different and secondarily because it is unconventional. Fortunately (for me) or unfortunately ( for most everyone else) the evidence is becoming more and more clear that unilateral training allows for training with higher loads on the targeted muscles.
This brings us to the critical concept of bilateral deficit.
“The bilateral limb deficit (BLD) phenomenon is the difference in maximal or near maximal force generating capacity of muscles when they are contracted alone or in combination with the contralateral muscles. A deficit occurs when the summed unilateral force is greater than the bilateral force. The BLD has been observed by a number of researchers in both upper and lower limbs, in isometric and in dynamic contractions. The underlying cause of the deficit remains unknown. One possible explanation is that the deficit occurs due to differences in antagonist muscle coactivation between unilateral and bilateral contractions.”
 Kuranganti et.al
Another potential explanation is much simpler. The brain does not like bilateral exercise. Numerous studies have shown that opposite hemispheres of the brain control movement ( in another words the right side of the brain controls the left side of the body). This is the natural way that the body works. Attempts at simultaneous bilateral contraction is in effect neurologically confusing. In other words the body wants to work one side at a time and, does it more efficiently.
The simplest illustration of bilateral deficit can be seen in a hand grip dynamometer test. The sum of right plus left is greater then the combined score of the two working together. This phenomenon is also seen in leg extension, the one leg straight leg deadlift ( illustrated previously) and is also seen in power in the vertical jump/ hop ( Boyle unpublished data).
The most interesting and useful examples from a strength training perspective can be seen in three exercises. The bilateral deficit is clearly seen in the Olympic lifts ( and this drives purists crazy) and in both the Rear Foot Elevated Split Squat and the previously discussed One Leg Straight Leg Deadlift. The deficit is so evident in the posterior chain that we have seen greater One Leg Straight Leg Deadlifts ( when right and left are combined) than in bilateral conventional Deadlifts. ( see Max Shank and Meghan Duggan video below)
Interestingly enough a 1965 study by Bill Kroll showed that bilateral deficit may be absent in certain lifts ( most notably in our experience bench press) but, present in others. Another study showed bilateral deficit to be greater in some lifts than others. 
The bench press example may be explained by the fact that training can in fact eliminate the bilateral deficit. In the bench press the extensive emphasis in many programs may overcome what is a natural tendency in the body.
In any case, bilateral deficit is real and has been identified and studied since the 1960's. However in our “why can't we do what we have always done” world of strength and conditioning we have continually rejected ideas that move us out of our comfort zone. It's funny that coaches like Anatoli Bondarchuck and Franz Bosch laugh at our attempts to copy powerlifters, Olympic lifters and bodybuilders in our quest to create great athletes. Great European coaches like Bondarchuk and Bosch have emphasized unilateral training for years to improve elite athletes in Europe but, in the US were are so stuck in the old paradigms that we continue to fight change in the face of science.
If you truly want to improve your athletes it may be necessary to take a second look at Cockrell's words and ask yourself “what if the way we have always done it was wrong”.
European Journal of Applied Physiol. 2011 Jul;111(7):1533-9. doi: 10.1007/s00421-010-1752-8. Epub 2010 Dec 3.
Bilateral deficit phenomenon and the role of antagonist muscle activity during maximal isometric knee extensions in young, athletic men.
2- American Journal of Physical medicine 1965 ( 44, 218-223)
Central Facilitation in Bilateral versus Unilateral Isometric Conractions
1- Kroll, W
European Journal of Applied Physiology 1987 , 56, 201-205
Strength- Velocity Realationship and Fatiguability of Unilateral vs Bilateral Arm Extension