When it comes to resistance training, most movements fall into two categories—unilateral exercises or bilateral exercises.
Unilateral exercises are performed with one arm or a leg at a time. Bilateral exercises are performed with both arms or both legs at a time. While unilateral training is not a new concept, its popularity in the strength and conditioning field has exploded in recent years. There are many positives to the unilateral training method, but does this mean that bilateral training (which are performed with two arms or legs at a time) should be regarded as inferior?
Let's take a closer look at these two types of training to help you figure out how they should fit into your routine.
As mentioned previously, unilateral exercises are performed with one arm or one leg at a time. Let's break down some of the key advantages with this type of training.
Advantage 1: Sports are Played on One Leg
This may sound a little odd, but generally speaking, any sport that involves running will require most actions to be performed off one leg. Whether you're sprinting, throwing, cutting or jumping, it's rare for both of your feet to be on the ground simultaneously during these movements. If we are attempting to make our training specific to athletics, then it makes sense to only train one leg at a time, because that more accurately reflects what the athlete will be asked to execute on the field.
Advantage 2: Training Balance and Core Stability
During lower-body unilateral movements, the individual must produce forceful contractions while standing on one leg. This requires greater proprioception and core stability than bilateral movements. The same goes for upper-body unilateral movements, as the core must work to prevent the torso from unnecessarily rotating during unilateral pushing or pulling exercises. A 2012 study in the European Journal of Applied Physiology found that unilateral exercises more effectively activated superficial core musculature than bilateral exercises.
Advantage 3: Each Side Must Do an Equal Amount of Work
Athletes are typically riddled with muscular imbalances. This is due to the nature of sports. Every athlete has a dominant arm or a dominant leg. A more well-rounded athlete has a smaller gap between the skills and strength of their limbs, but there is almost always a deficit. When an athlete takes those deficits into the weight room to perform bilateral exercises, they will many times rely on the stronger side to perform more work. If you're performing a heavy Back Squat, for example, it's only natural that your stronger leg will contribute more significantly to the exercise than your weaker limb. But unilateral training avoids this issue by forcing you to train both sides in isolation. When you train unilaterally, you cannot rely on your stronger side to make up for a lack of strength in your weaker side. This ultimately helps decrease muscular imbalances throughout the body.
Advantage 4: Cross-Education Effect and Rehab
Via a phenomenon known as cross-education, unilateral exercises also strengthen the "unused" side of the body. Training one limb can cause strength gains in the contralateral (opposite) untrained limb. Essentially, unilateral exercises can increase strength in a limb without directly training it. A recent meta-analysis in the European Journal of Applied Physiology found that cross-education in unilateral exercise produces, on average, an 11.9% increase in contralateral strength. This fact could be especially useful for preventing atrophy and maintaining strength in an injured limb, which could potentially limit the amount of imbalance between the two limbs when the injury heals.
Advantage 5: You Generally Won't Be Limited By Your Core or Grip
Mike Boyle talks about this concept in his book New Functional Training for Sports. Unilateral exercises are generally only limited by the limb doing the work. For example, in a Split Squat, the legs are going to fatigue or give out before the low back. But in a bilateral Back Squat, the weakest link could very well be the low back. Unilateral exercises can eliminate limiting factors, allowing you to better focus on building strength in the limb.
Advantage 6: Very Tall Athletes Tend to Handle Them Better
If you've ever trained a basketball center, you know these athletes often do not have the ideal biomechanics for performing a Back Squat. On the other hand, I have had 6-foot-10 athletes perform Split Squats and Split Squat variations with perfect form. This is because there isn't as much ankle and thoracic spine mobility required during the single-leg versions, making them more friendly to taller athletes.
A Key Misconception
Some people will argue that there is greater force produced in unilateral exercises (when the force of each limb is summed) in comparison to bilateral exercises. This is known as the bilateral deficit. There is evidence to show this deficit does occur, but a 2006 study published in The Journal of Applied Physiology found that the difference in force production is due to the force-velocity curve. The greater the force output, the lower the velocity, and the greater the velocity, the lower the overall force output. Under the same load, bilateral exercises are able to be performed at a greater velocity than unilateral exercises. This means that greater force is produced unilaterally, because it is performed at a slower velocity. Click here to see a video outlining a unilateral exercise progression.
As mentioned previously, bilateral exercises are performed with both arms or both legs at a time. Let's break down some of the key advantages with this type of training.
Advantage 1: You Can Lift More Weight
Unilateral exercises can not be loaded as heavily as their bilateral counterparts. When you combine the load used on both sides, there may be a similar amount of load, but it's still not quite the same. For example, say an athlete has a max Rear-Foot-Elevated Split Squat of 225 pounds on each leg. They also have a max Barbell Back Squat of 450 pounds. The amount of total load is the same at 450 pounds. However, the core needs to support 450 pounds during the Back Squat but just 225 pounds during the Rear-Foot-Elevated Split Squat. This difference is crucial for sports like football where the athlete will face significantly greater forces on the field of play.
Advantage 2: Reciprocal Inhibition
Reciprocal inhibition is the relaxation of muscles on one side of a joint so that prime movers on the other side can perform an action with greater force. This is crucial for smooth athletic movement and is one of the central nervous system adaptations that allows for increases in strength. In his book Supertraining, Yuri Verkhoshansky explains that unilateral training causes co-contraction. This is when muscles on both sides of the muscle contract simultaneously. Due to the instability of unilateral exercises, both the agonist (prime mover) and antagonist muscles naturally contract to increase stability. Training bilaterally allows the antagonist muscles to relax more rapidly and to a greater extent.
Advantage 3: Training Both Sides Simultaneously
As stated above, the cross-education effect allows for some strength adaptations to occur in the untrained limb during unilateral exercise. While this is a nice benefit, it's not the same effect you'll get from training both limbs simultaneously. When training heavy on a unilateral exercise, the athlete will perform a set that fatigues not only one leg, but also their central nervous system. The athlete then has to perform an equal number of reps on the other side while in a fatigued state. The effort on the other side will not be able to be the same as the first side. There are ways to program around this, but bilateral exercises significantly increase the efficiency of the workout.
Advantage 4: Limited by Core
Although unilateral exercises not being limited by core strength or stability was listed as an advantage above, this fact is not without a drawback. The advantage of this limitation is you are unable to train the legs beyond what the core can support. As long as the athlete does not throw on a weight belt for their Squats, the limitation of bilateral exercises can force the athlete to increase their core strength at the same rate as their legs. For more on this topic, read about how ergogenic aids effect strength training for athletes.
To wrap things up, there's no clear cut answer. I'm not going to tell you to only perform unilateral exercises, nor will I tell you to perform only bilateral exercises. In a well-designed program, both bilateral and unilateral exercises should be utilized. The ratio of bilateral to unilateral depends on the individual's sport and size. With my athletes, it's almost always a 50-50 split between the two. If they are very tall (above 6-foot-4), than I tend to utilize more unilateral movements. If their sport is played primarily on two legs (golf, volleyball, wrestling, etc.), than I use a greater proportion of bilateral movements. These two styles of exercise complement each other well for athletes in any sport, so I would recommend finding ways to get both into your programs.
Photo Credit: Zocha_K/iStock, bernardbodo/iStock
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