Bilateral Deficit and Transfer and the Case for Unilateral Strength Training
For the longest time, resistance training has been all about generating equal force with two legs or arms at the same time, with both sides of the body bearing equal load. This is the case in the three powerlifting movements – bench press, squat, and deadlift – the Olympic lifts, and most general strength programs. But a physiological limitation, an unusual neuromuscular phenomenon, and the demands of sports performance demonstrate the usefulness of using unilateral variations alongside bilateral exercises. Let’s take a closer look.
The Bilateral Deficit: 2 + 2 = 3
As any old-school coach will tell you, there are a lot of benefits to training bilaterally. “Bilateral exercises allow for greater loading; therefore, the production of greater absolute force,” Gaby Smith wrote in an article for Athletic Lab. “This peak force production helps improve the athlete’s strength, which is a key indicator of athletic performance and a main goal in most training programs. Bilateral exercises also allow for greater stability because the center of mass can be balanced between both limbs.”
That being said, there are some drawbacks to only training in this way. In sports and even daily activities, fewer movements are executed bilaterally than unilaterally. Perhaps this goes some way to explaining a counterintuitive and self-limiting effect that researchers have observed when evaluating single- and double-limb exercise performance. According to a paper published in The Journal of Strength & Conditioning Research, “The bilateral deficit (BLD) phenomenon refers to the observation that force production during a maximal bilateral contraction tends to be lower than the sum of total force produced by the left and right limbs combined.”
Although bilateral deficit was first observed in lower body movements, it has since been seen in upper body exercises as well and across a wide spectrum of force and speed, from static holds to dynamic plyometrics. It’s an example of two plus two equaling three instead of four. If you can do a single-leg split squat with 100 pounds on both legs independently, you’d assume that you could do a bilateral squat of 200 pounds. But if there’s a bilateral deficit, perhaps you can only manage 170 or 180. In other words, the whole is less than the sum of its parts.
While there are multiple possible explanations for bilateral deficit, Brazilian researchers suggested that “they seem to be related to neural mechanisms, such as a blockage of the brain hemispheres, that may occur during bilateral contractions thus reducing the activation of motor units and therefore the strength produced.” It could be that the brain sends a signal to one limb it leads to maximal contraction, whereas when it’s split between both sides of the body, there’s some data loss like two IT systems that aren’t well integrated.
Looking at bilateral deficit through a coach’s eye, I see it as a missed opportunity to train to your full potential. If your goal is to maximize single-sided and whole-body strength, it also suggests that just training heavy in bilateral movements can lead to diminishing returns. For example, barbell back squats would develop greater strength to a point but could eventually be limited by factors like bilateral deficit. Whereas if you added in single-leg split squats, you could increase both unilateral and bilateral strength, as well as improving balance, core stability, and coordination.
Topping Up Every Capacity Bucket
Bilateral deficit indicates the need to look at training outputs with greater nuance and to consider both sides of the body independently as well as together. This is where unilateral training can come into play. Two of my coaching mentors, James Chassin and Devan McConnell, taught me that you need to find out in which areas an athlete has little to no load exposure and at least fill those buckets a little. While they both utilize bilateral training effectively, this often means incorporating more unilateral training.
If someone is new to strength work, they’ve likely done very little single-sided training. In which case, they’ll make the kind of rapid and significant beginner’s gains that those with a more advanced training age envy when they start doing at least some unilateral exercises. Simply building up volume will be enough to improve motor skill and lead to neuromuscular adaptation. They will learn how to recruit more muscle fibers, do so quickly, and engage them in a synergistic sequence by getting more reps. Unilateral exercises might be secondary to begin with, as an athlete with a low training age will still need to work on their bilateral movement.
An intermediate athlete might have done some unilateral training, but it may well have only been accessory work with “finisher” exercises or in an unstructured way. If they can demonstrate competence in unilateral movements, we can make a single leg or arm exercise the primary focus of a training block and utilize multiple variations and tempos.
This often leads to gains in single- and double-sided exercises. As a 2019 study put it, “strength improvements from a unilateral exercise can improve strength in a bilateral movement. An advantage of unilateral exercises may be in the development of coordination and stabilizer musculature that may not be sufficiently stimulated in stable, bilateral movements.” So instead of two plus two equaling three, we could eventually get to four plus four totaling eight.
An advanced athlete will probably have been exposed to more unilateral training and so might have diminishing returns from doing the basics. In which case, we could experiment with the French Contrast Method to pair a unilateral strength exercise with a ballistic one like a single-leg bound. Somone with a higher training age can also benefit from making a unilateral movement the focal point of an APRE workout and a key part of triphasic periodization. In addition, we can challenge them by having the athlete explore the sagittal, frontal, and transverse planes so they get more comfortable moving unilaterally in all directions.
Reckoning with Rotation and Sport-Specific Demands
Another use case for unilateral training is evening out the demands on rotational athletes whose sport requires them to be dominant on one side, such as baseball pitchers, tennis players, and golfers. To balance all the unidirectional twisting, I’ll count how many rotations they do during practice and competition and then have them do the same number on the other side. For example, if they rotated left 100 times, I’d have them do 100 med ball throws against a wall to their right. Not only will this eliminate an imbalance on the underutilized side by making it active, but it will also increase the output on the other due to the cross-education effect that we’ll explore later.
In a study published in Biology of Sport, a research team acknowledged the positive impact unilateral training can have on strength and power development. Then they noted that “many key sport-specific skills involved with the basic lower-body movements (e.g. running, changing direction, jumping, kicking) are executed completely or predominantly unilaterally. Under the specificity of training adaptation and to maximize the transfer of training, it could be argued that unilateral exercises similar to sport-specific skills, may be the best choice to improve athletic performance and prioritized as a key exercise in such training programs.”
When I have limited time with an athlete, then I try to train them in a way that both prepares them to withstand the loads they’ll be exposed to in their sport and mirrors how they move when performing. Another reason for doing so is that it facilitates a holistic approach that ensures I’m not overtraining someone. When taking into account other stressors and trying to avoid overtaxing an athlete, it’s important to get the biggest bang for our buck. This necessitates prescribing a minimum effective dose that includes or even prioritizes unilateral training.
The Rehab Benefits of Bilateral Transfer
In addition to the many performance advantages it delivers, unilateral training can also offer significant benefits from an injury rehab standpoint. If someone faces a long layoff, the strength and muscle that might have taken years to build up can rapidly diminish. A Danish research team found that inactivity in younger men led to almost twice as much loss in muscle mass – 485 compared to 250 grams – as older participants. The younger group that was observed lost a third of their leg muscle strength, while the other group lost a fourth. “A young man who is immobilized for two weeks loses muscular strength in his leg equivalent to aging by 40 or 50 years,” said study co-author Andreas Vigelsoe.
His fellow researcher Martin Gram noted that the loss of muscle mass and strength might be even more significant because they had less of each to begin with. He stated that it could take three times as long to gain back muscle size and return to previous levels of strength and overall fitness after two weeks, even if incorporating resistance training with cycling or similar aerobic conditioning. I saw this firsthand when coaching college athletes at the University of Hawaii, the University of Massachusetts Lowell, and West Point. Physical therapists would tell my colleagues and I that an injured athlete couldn’t train at all. Rather than taking this at face value, I started asking questions that would hopefully lead to this person at least being able to do unilateral exercises.
If they forbade all lower body work, I’d challenge this assumption and want to know if the individual might actually still be able to perform squats, deadlifts, or other exercises. Then I’d explore what ranges of hip and knee flexion might make variations of these possible. Did they actually have partial range of motion that we could utilize? If one leg was off limits because of a more serious injury, could we train the other one instead?
Accelerating Return to Play
That last query might have raised an eyebrow, but as a Movement as Medicine blog post stated, “it is important as hell to train the injured limb to whatever degree you can. You should also train the uninjured limb as hard as possible.” There is plentiful research to suggest that when one arm or leg is immobile or restricted, it can gain strength from the other being exercised unilaterally. In 1898, psychologist Edward Wheeler Scripture had one woman train her dominant arm for a muscle power task while another did likewise for a fine motor skill. The participants not only improved results in the trained limb, but also in the untrained one. Scripture called this effect “cross-education” and attributed it to “indirect practice,” according to an overview of this phenomenon.
Since then, there have been many studies showing evidence of cross-education, which is also known as bilateral transfer or the contralateral training effect. A meta-analysis published in the Journal of Applied Physiology found that in 16 prior studies, “the size of the contralateral strength training effect is ∼8% of initial strength or about half the increase in strength of the trained side.” This kind of unliteral work doesn’t impose mechanical load on the passive side, leading researchers to believe that the impact is more neuromuscular.
Going back to my college coaching example, we accelerated the return-to-play process when we could responsibly train an injured limb to some degree and the other fully, but also when single-sided work was the only option. Unilateral training gave athletes the mental benefits of being with their teammates and attacking a challenge while also improving circulation, recapturing fitness, and activating their kinetic chain.
It's arguable that incorporating unilateral training into a balanced program can also be a preventative measure. Coach Alex Stott wrote that it offers “the ability to help address and counter any strength imbalances that could be present, reducing therefore the potential injury risk for athletes too.” It is a smart way to train the musculature around each joint from every angle, and can also help reduce the incidence of overuse issues.
Mike Boyle, a pioneer of unilateral training, has noted how single-leg split squats, RDLs, and other single-limb exercises also reduce strain on the low back, a common injury site. The improvements he has seen in his athletes’ output from unilateral training show how it can be equally effective in improving performance, increasing durability, and addressing any injuries that still occur.
