Examining Unilateral and Bilateral Strength Training Exercises

Single-leg squats, lunges, or a traditional squat? There is an ongoing discussion about what will bring your athlete the best sport-specific result. The arguments are valid and robust for both the unilateral proponents and the bilateral enthusiasts.

The traditional squat, pegged as “the king of leg exercises,” allows the whole system to be overloaded to push adaptation, releases anabolic hormones, teaches both sides to work together, and is often used to build base strength and muscle development. Opponents would argue it is not sport-specific in many cases, causes too much spine loading, and increases the risk of injury—especially in those unfamiliar with the proper technique. Add in differences in force production (bilateral deficit or facilitation) and muscle activation (changes in co-activation), and the answers become murkier as to what is the “best.”

I’ve always followed the notion that there are very few wrong exercises, just different applications. Furthermore, there is probably no “best” option, just better solutions. Share on X

I’ve always followed the notion that there are very few wrong exercises, just different applications. Furthermore, there is probably no “best” option, just better solutions. At face value, positioning the body and executing exercises in the movement you want to improve seems to make sense, and research would also support this.^1,2 Squats are standard options for enhancing strength and power in double leg performances like rowing and skiing. Walking lunges are typical in building muscle power for running, and single-leg squats for cutting and traditional lunges for deceleration are other common exercise prescriptions for proponents of unilateral training.

But do they replicate the muscle activation patterns you expect them to mimic?

Having a curious mind, I wanted to investigate this further to get some more insight into how these exercises train (or don’t train) the involved muscles. To keep things relatively simple, I took a comparative approach. I looked at muscle activation via EMG monitoring (FREEEMG, BTS Bioengineering) and kinematic (approximate center of mass acceleration and velocity (G-Sensor, BTS Bioengineering)) measurements between the bilateral squat and various unilateral exercises.

The Results

With the results compared as a ratio of the highest EMG amplitude achieved in the given muscle during all the exercises, you can immediately see the increases in amplitude during the concentric phase, which is about the last 40% of the movement.

When looking at many squiggly lines, it can be tough to draw significant conclusions. Restating the differences as mean averages during the concentric portion allows a narrower focus—the one caveat is that the forward lunge had an overall shorter activation period than the other exercises.

Video 1. The forward lunge, showing acceleration, velocity, and muscle activation during one repetition. Red is vastus lateralis, green is gluteus maximus, purple is adductor longus, and yellow is semitendinosus in millivolts.

Shorter, higher-intensity bursts of activity achieved in the glute max, semitendinosus, and adductor longus—as seen in the forward lunge—could have a different training effect on the muscles. For simplicity’s sake, we will assume that the mean activation is a reasonable representation of muscle activity during the concentric phase.

Differences in amplitude are generally related to the tension the nervous system believes the muscle needs to generate to cause the required movement. Working backward, the amount of weight you choose and your exercise technique have an influence. I wanted to make sure the load on the lead leg was similar in all the movements, so I used a vertical force platform to measure the ground reaction force in the lowest part of the movement.

The unilateral exercises required an additional 60 pounds to counteract the unloading effect of the back leg, except for the Bulgarian squat (50 pounds). This extra weight put me in the same ballpark as the leg in the single-leg squat and presumably the bilateral squat. The added resistance for the unilateral exercises resulted in roughly an 8 to 10 rep max. Could I have gone heavier with an enthusiastic coach cheering me on? Possibly.

Another explanation for differences in amplitude between these compound movements is that coordination among muscle groups could differ in lunge variations versus squat variations. Other than the split squat and forward lunge, the exercises showed higher gluteus maximus activation, which may have assisted in decreasing the force-generating requirements of the quadriceps group. Combine this with lower co-activation in the hamstring muscles and a center of mass that shifts forward; the force required from the quads may have been less to complete the movement. Aspects of this and how it affects the bilateral deficit regarding muscle coordination are detailed in an article by Enrico Rejc et al.³

With aiming for the amount of vertical ground reaction force to be similar, one might expect comparable activation levels. However, some research has shown higher activation in unilateral over bilateral movements.⁴ Interestingly, the bilateral deficit does not occur in all athletes. Bilateral facilitation, where the force of both legs during a bilateral exercise exceeds the sum of the unilateral movements, is often seen in weightlifters, powerlifters, rowers, and downhill skiers, to name a few.⁵ Evidence reinforces this notion that you build strength in the specific way you train and move.

Training with unilateral movements appears to carry over by enhancing the bilateral deficit, which seems to improve unilateral power production and change of direction ability but not necessarily linear speed.⁶ One caution is that it does not seem to relate to the total number of unilateral exercises performed in the training period.⁷ The adage here would imply quality over quantity, so choose your unilateral exercises carefully and don’t necessarily think the more unilateral exercises, the better.

The more critical issue here is that each athlete can have different intra- and inter-muscle coordination patterns based on training history and genetic makeup. Share on X

The more critical issue here is that each athlete can have different intra- and inter-muscle coordination patterns based on training history and genetic makeup. In the past, I have been surprised at how similar coordination patterns can be in the same individual across a wide variety of exercises despite the exercise having different movement patterns.

Overall, quad muscle activity was similar to slightly lower between the squat and the unilateral exercise. Also, the single-leg squat resulted in higher vastus medialis activity at the beginning of the concentric movement during the propulsion stage. The unilateral training resulted in lower rectus femoris activation, possibly because of the more significant forward lean.

On average, biceps femoris activation was lower, along with slightly lower muscle activity in semitendinosus. The hip stabilizers (glute max, glute med, and hip adductors) tended to be greater in unilateral exercises (particularly glute med) than in the bilateral squat. We often see this pattern in the literature, most likely due to unilateral movements requiring more significant hip and knee stabilization.^8,9

This last point got me wondering if that activation was enough to cause an actual training effect. Although the difference in muscle activation may have statistical significance, does it have physiological significance? In other words, were those muscles working as hard as a typical “work set” of 8 to 10 reps that we might prescribe for regular strength or muscle development? Can we expect to switch from squats to a single-leg squat and still adequately strengthen the hip stabilizers enough to withstand the high loads experienced during competition?

Can we expect to switch from squats to a single-leg squat and still adequately strengthen the hip stabilizers enough to withstand the high loads experienced during competition? Share on X

This last notion keeps me up at night, so to put my suspicions to rest and for better sleep for all coaches, I put it to the test.

Are the Muscles Getting the Right Amount of Stimulation?

If we assume that a set of 8–10 reps to failure is enough to cause strength gains, and the resultant muscle activation is a decent gauge of that intensity (generally accepted, but not without debate^10,11), my next step was to compare unilateral isolation-type exercises on the examined muscles once again to the bilateral squat at the same intensity.

The following charts show muscle activation in the selected exercises compared to traditional squats. Also, as we mentioned previously, keep in mind that the forward lunge had a shorter concentric propulsion phase with briefer muscle activity. This would cause the mean values for the forward lunge to be lower because of similar periods but shorter bursts of muscle activity.

When looking at the difference between directly training the abductors (glute med), there is no contest comparing cable adduction to the traditional squat. Previously, we saw that the single-leg squat had approximately 200% greater activity than squats during the lifting portion and was most likely acting in a limited range as a stabilizer. Focusing on training the glute med with cable abduction had pushed activation to well over 350%, which is nearly double how it behaved in the single-leg movement. Granted, in this case, it was more of a dynamic motion across a more extensive range that can give greater values, but also may be more representative of how it behaves during lateral movements.

There was less of a difference in glute max activation among all the exercises where it played an active role in the movement. In the forward lunge, although the glute max had the most significant peak EMG amplitude from the line graphs, it had the lowest MEAN activation (remember the length of the concentric period as it drove the body back to the starting position). Single-leg and Bulgarian squats, along with reverse lunges (and most likely forward lunges), were similar to the two types of kickbacks, probably because they were already reasonably active in the squat. Perhaps statistically significant, but was it physiologically important?

Hip adduction resulted in a much higher increase in the activation of the adductor longus (over four times greater mean activation) compared to the squat and more than any of the unilateral exercises.

It is intriguing and perhaps meaningful that the lunges resulted in 50% less activation in the rectus femoris for the lead leg. Share on X

Squats were still the leader in quad activation for all the relevant exercises. The exceptions were for vastus medialis in the single-leg squat and rectus femoris in the leg extension. It is intriguing and perhaps meaningful that the lunges resulted in 50% less activation in the rectus femoris for the lead leg. If this is physiologically relevant, it may be good to include exercises that challenge this muscle specifically; however, it is also possible that the rear supporting leg (which was not assessed) may have had more activation in the rectus femoris than the front leg. Whether this level of lower activation in the front leg would increase the risk of injury or impede hip flexion would be an excellent question to ask.

The other finding of interest was the relatively low activation level of the hamstrings during the compound movements versus the single-leg curl exercise and kickback exercises. The lower level of the semitendinosus could be of significant interest to those looking to avoid ACL injury. Although research appears to be sparse, at least one study I came across suggests that the medial hamstrings (of which the semitendinosus is one) could play a role in stabilizing the knee.¹²

Muscles Creating Movement

With a better understanding of how the muscles are activating to produce force, we can pull it together to see how the body accelerates through unilateral and bilateral exercises.

In the propulsion stage of any movement, you will see increasing force typically at the lowest center of mass, both at the end of the eccentric phase and at the beginning of the concentric phase. We hope to mimic this in specific strength training to match the movement’s knee, hip, and ankle angles. However, force production is also the coordination of agonists working together while antagonists relax. Since most unilateral leg exercises focus on vertical force, lateral force production suffers, resulting in less influence on change of direction (COD).¹³

Side lunges and other resisted lateral movements combined with gameplay-specific drills would help address this aspect for improving COD. The forward lunge showed the highest side-to-side and frontward changes in acceleration and velocity from our exercises, with the reverse lunge coming in a distant second.

Trying to get game-specific velocities in the gym can be problematic, if not impossible, for many sports. Therefore, focusing on the rate of force development can be a wise choice. Currently, there is a need for more specific research to confirm the benefits of choosing unilateral or bilateral exercises and the effect on speed and RFD. Still, the safe assumption is it develops from both approaches,¹⁴ mainly when the focus is on the intention to move quickly.¹⁵

Generally, bilateral movements can generate higher velocities through a similar range of motion, partly because they require less balance. Speeds were not vastly different in our case, as there was no intention to move as quickly as possible. Additionally, during many lower-body unilateral strength movements, the center of mass often shifts forward, closer to the knee, as the athlete leans forward, reducing the torque on the knee.

This motion minimizes the force the quadriceps muscles need to generate, and we can see the lower activation levels that reinforce this idea. The upside is that our unilateral movements are improving proprioception and getting closer to the requirements of the sport. The downside is that they could be unloading the quadriceps in both the eccentric and concentric ranges of motion, which is the opposite of the demands of producing higher forces during the propulsion stage.

Applying Unilateral and Bilateral Training for Sport

If your goal is to transfer appropriate force production during unilateral-type sports (running, throwing, field events, etc.), you should incorporate unilateral strength movements during the appropriate phase closer to the season. Unilateral exercises give the most significant crossover benefit during the first 6-8 weeks, with diminishing returns for more extended periods. Therefore, using these during the final prep phases would be wise, as long-term use probably does not improve the result.¹⁶

Unless your athletes spend a significant amount of time moving up and down, you would want to choose strength exercises, plyometrics, and drills that emphasize horizontal movement for training change of direction.¹⁷ Although the ability to move quickly to decelerate and change direction should improve with unilateral training, linear speed may not.¹⁸ To be able to outrun or catch your opponents, stick with the basics—good old actual sprint training to pull it all together.¹⁹

When assessing bilateral strength levels, differences of around 10% between left and right limbs are considered normal. With differences that are greater than 15%, the concern for injury arises, as well as how it may ultimately affect performance. More specifically, the power production for the side that is less than the other may impair performance on that weaker side, though only to a certain point. Once critical power has reached a certain power threshold, it does not seem to be as big of a factor for performance.²⁰ If there’s too great of an imbalance in strength and power below threshold levels (greater than 15%), you may get to the point where one side may overpower the other.

In addition, it appears unilateral training has its most significant effect on younger and more inexperienced athletes. As an athlete improves skill and execution with increased experience, their ability to transfer existing strength and power to performance improves, and unilateral exercise may have a less significant role in improving stabilization.¹⁴ Spending more time on bilateral movements to increase overall strength appears to make sense for elite athletes. Of course, someone who has been competing for years may also have to balance their training to decrease the risk or irritation of injuries, so there is still a role in unilateral training from this perspective.

Takeaways

The biggest takeaway from what we have learned is that you should be cautious in prescribing unilateral exercises to adequately train the quads and hip stabilizers. Strength routines should include direct work for the hip stabilizers and the hamstrings, and coaches should not rely on unilateral exercises to achieve this. It does not appear, at least in this case, that these muscles would get enough stimuli to get proportionally stronger or hypertrophy at fast enough rates with the exercises we examined. An analogy would be doing biceps curls and expecting your shoulders, rotator cuffs, upper back, and chest to respond. Probably not that effective.

Strength routines should include direct work for the hip stabilizers and the hamstrings, and coaches should not rely on unilateral exercises to achieve this. Share on X

It’s important to remember that strength changes in individual muscles can also affect muscle coordination patterns, affecting force production. Coordination patterns can adapt, either positively or negatively, based on the force production capabilities of the muscles. However, our case study was just one individual, and we would expect that not everyone will have this specific result.

As with almost everything in life, moderation and variety keep the needle moving forward, so training your athlete’s newfound strength requires specific drills and lots of actual game play to complete the transfer. Starting with building overall strength and muscle development during the off-season with bilateral movements and focused work for the hip stabilizers establishes the foundation for more specific work with unilateral strength movements. Unilateral training also appears to be most relevant for those in the early stages of their athletic careers. Incorporating movement-specific drills and real-life performances should help complete the transfer as the season progresses.

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References

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3. Rejc, E., Lazzer, S., Antonutto, G., Isola, M., and di Prampero, P.E. “Bilateral Deficit and EMG Activity During Explosive Lower Limb Contractions Against Different Overloads.” European Journal of Applied Physiology. 2009,108(1),157–165.

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12. Toor, A.S., Limpisvasti, O., Ihn, H.E., McGarry, M.H., Banffy, M., and Lee, T.Q. “The Significant Effect of the Medial Hamstrings on Dynamic Knee Stability.” Knee Surgery, Sports Traumatology, Arthroscopy. 2018;27(8):2608–2616.

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16. Nicholson, G. and Masini, D. “Bilateral Deficit: Relationships with Training History and Functional Performance.” Kinesiology. 2021;53(1):86–94.

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18. Bishop, C., Berney, J., Lake, J., et al. “Bilateral Deficit During Jumping Tasks: Relationship With Speed and Change of Direction Speed Performance.” Journal of Strength and Conditioning Research. 2019;35(7):1833–1840.

19. Lockie, R.G., Murphy, A.J., Schultz, A.B., Knight, T.J., and Janse de Jonge, X.A. “The Effects of Different Speed Training Protocols on Sprint Acceleration Kinematics and Muscle Strength and Power in Field Sport Athletes.” Journal of Strength and Conditioning Research. 2012;26(6):1539–1550.

20. Hoffman, J.R., Ratamess, N.A., Klatt, M., Faigenbaum, A.D., and Kang, J. “Do Bilateral Power Deficits Influence Direction-Specific Movement Patterns?” Research in Sports Medicine. 2007;15(2):125–132.