Exercise selection is one of the most important variables in regard to creating adequate training stimulus and adaptations necessary to get stronger, faster, more skilled, and so forth. Some of the earliest branches of the tree of exercise selection involve choosing between unilateral or bilateral exercise. Bilateral exercises refer to those that require using both homologous limbs (both legs or both arms) at the same time, such as the squat, bench, deadlift, snatch, and clean and jerk. Unilateral exercises are ones that involve contraction of only one limb at a time, such as the Bulgarian split squat, single arm kettlebell press, single arm dumbbell bench press, and single leg Romanian deadlift.
 
Classically, bilateral training has been deemed superior to unilateral training given its ability for greater force production, which leads to heavier loads and increased training stimulus. However, more than 50 years ago, two researchers described a phenomenon termed the Bilateral Force Deficit (BLFD), bringing into question the line of classical thinking. The BLFD describes the finding that the total cumulative force of two unilateral contractions is greater than the force produced by one bilateral contraction. In other words, the maximal force produced by a single limb decreases when both limbs contract simultaneously. This phenomenon has led some people to believe that unilateral training may, instead, be the superior training method.
 
The BLFD has been replicated in multiple studies and is of no significant difference in others. To complicate matters more, the presence of bilateral facilitation (bilateral force is greater than the sum of two unilateral forces) is a common occurrence in these types of studies. The BLFD seems to be most prevalent when untrained individuals perform multi-joint exercises. The mechanism of what actually produces this discrepancy in force production is not entirely understood and is likely multifactorial. A few leading hypotheses include decreased neural drive in bilateral exercises and the thought that increased activation of stabilizer muscles during unilateral exercises can produce transferable forces, such as torque.  
 
As mentioned above, data suggests that the BLFD is most prevalent in untrained individuals. On the contrary, trained individuals tend to have a smaller BLFD, no BLFD, or exhibit bilateral facilitation. In fact, the magnitude of the BLFD reliably decreases (or reverses) as both training experience and time spent training predominantly bilateral exercises increase. The opposite is also true; BLFDs are larger in less experienced athletes and those who spend more time performing unilateral exercises.
 
At this time, it is actually unclear whether these deficits are important to athletic performance or not. A recent study performed by Bracic, et al. correlated an increased BLFD with slower starting times in sprinters, suggesting that BLFD may be important in athletic performance. However, at this time there is not sufficient evidence to suggest one way or the other.
 
While the importance of the BLFD is not obvious at this time, it does bring to light two important points. First, BLFD is an interesting finding that has been gaining popularity in the exercise science and physiology literature, and it may provide useful insights to guide training in the future. Second, it evokes a useful discussion about bilateral versus unilateral training.
 
If I convinced you to believe in the BLFD (which you should, because it’s real), then there is a clear argument for unilateral exercises: more force production leading to increased training stimulus. In addition, Kurt Mullican points out some other important factors in regard to unilateral training:

1. Allows for similar utilization of the target muscle group with alterations in secondary muscle group involvement.  For example, both the back squat and split squat target the quads, but the split squat results in more adductor and abdominal muscle stimulation.
2. Similar to number one, balance and rotation/anti-rotation are more actively tested and stimulated.
3. Allows for overload of target muscles while decreasing total load required. This is extremely useful in injury and rehab settings. Following the example cited in the first point, think about substituting a split squat for a back squat in an athlete who is dealing with a nagging neck injury that worsening with spinal loading.

 
While specificity in training is clearly important (ex: to snatch heavier weight, it is important to perform the full snatch), specificity, especially in regards to unilateral versus bilateral training, may not be as important in terms of general strength development as we once thought.
 
McCurdy and colleagues found that unilateral and bilateral training were equally effective for improving both unilateral and bilateral leg strength and power in untrained subjects. However, the limitations of this study include the subjects (untrained men and women) and the time course (the study only lasted eight weeks). Untrained and trained subjects often respond differently to a given stimulus, and muscular adaptions often change over a prolonged time course.
 
Luckily, in 2016, Speirs, et al. compared lower-body strength and 40-meter sprint times in rugby players (trained athletes) after training their lower body with exclusive unilateral (rear elevated split squat) or bilateral (traditional back squat) methods. Interestingly, there were no significant differences in improvements in strength or speed, leading the authors to conclude, in agreement with the McCurdy study, that unilateral and bilateral training may be equally efficacious in improving these variables.
 
Additionally, Makaruk and colleagues found that unilateral and bilateral training effects may have temporal differences. They found that unilateral training produced its greatest strength gains early in the training phase, quickly plateaued, and was followed by a quick regression in performance after a period of detraining. On the contrary, bilateral training appeared to show greatest strength gains midway through the training cycle, with a continued increase throughout the training cycle, and no regression after a period of detraining.
 
A study conducted by Mike Lauder and Jason Lake examined the differences between a traditional barbell power snatch and a unilateral single-arm dumbbell power snatch. It found significant biomechanical differences in the concentric muscular contraction of the load (vertical displacement of the bar or dumbbell) and differences in landing mechanics. The researchers concluded that these differences suggest that unilateral movements provide a novel stimulus to athletes and should be considered useful additions to most training programs.
 
Lastly, David Behm, et al. conducted an electromyographic analysis of trunk muscle activation with unilateral and bilateral exercises. They found that unilateral exercises are more effective at stimulating trunk muscles and likely result in increased trunk strength when compared to bilateral exercises.
 
These studies are broken down into practical points and summarized below. 
 
To summarize:

• The bilateral force deficit is an interesting phenomenon that suggests that, in certain populations, combined unilateral force production is greater than that produced by the same bilateral movement.
• The actual importance of the bilateral force deficit, in terms of physical performance, is still being studied, so altering exercise selection based on how it may affect this variable is unwarranted.
• Unilateral training appears to be most beneficial early on in a training cycle and when working around injuries. As such, the bulk of unilateral training could be incorporated early in a training block and phased out for more specific bilateral exercises in athletes that perform bilateral movements in competition.
• Unilateral exercises create a novel stimulus, mostly by changing muscular requirements needed for balance and rotation/anti-rotation forces.
• Unilateral exercises are an effective way to improve trunk/core strength—and are more effective than bilateral exercises,
• Unilateral training may be just as effective as bilateral training for increasing general strength and power. However, it’s crucial to remember the importance of specificity when training for a goal or competition that will test a specific movement pattern.
• Unilateral exercises can be incorporated into any training program in a number of ways:
  • Warm up: Choose three exercises and perform three sets as a super set of 12-15 reps each. Don’t even think about coming close to failure. These should be a light, easy, fun way to actively warm up for the main segment of the workout.
  • Working sets: Choose five exercises and perform five sets of five to eight reps. Pick a weight that encourages approaching failure on the last one or two reps of sets four and five.
  • Accessory work: Choose three exercises and perform three sets of eight to 10 reps. Pick a weight that encourages approaching failure on the last set.

A few useful unilateral exercises:

• Single arm kettlebell clean and press
• Single arm kettlebell snatch
• Landmine single-leg Romanian deadlift
• Landmine single-arm shoulder press
• Rear leg elevated single leg split squats
• Reverse lunge with single arm kettlebell overhead press