Unilateral Training for Strength, Hypertrophy, and Performance

According to research, how does unilateral training compare to bilateral training, and what is the best program for your client’s needs?

Most exercises can be categorized as unilateral or bilateral. A unilateral exercise mainly or completely involves one limb (e.g., single-leg squat, Bulgarian split squat, and single-leg jump). In contrast, a bilateral exercise is done evenly and simultaneously by both limbs (e.g., back squat, deadlift, and countermovement jump). 

Clients are as diverse as the world itself, with many goals and needs. Personal trainers’ role is to program their clients’ workouts, whether they are athletes or older adults. Deciding whether to program a unilateral or bilateral exercise to meet a client’s goal is a daily thought process for personal trainers. 

Much research has been conducted, some conflicting, on the effects of unilateral versus bilateral training for training parameters such as strength, sprinting speed, jumping, change of direction ability, and balance. In this article, we’ll review the research on unilateral training and help personal trainers with the following learning outcomes:

1. How does unilateral training compare to bilateral training in jumping, sprinting, and changing direction ability? 
2. How does unilateral training compare to bilateral training in terms of strength?
3. How to apply the principle of specificity to unilateral training.
4. The unique advantages of unilateral training.

Unilateral vs Bilateral on Jumping, Sprinting, and Change of Direction Ability

Jumping, sprinting, and change of direction ability are some essential sports performance metrics. It is important to note that the studies used for this section only examined lower-body exercise, so their findings may not necessarily apply to the upper body when investigating these performance metrics. 

Jumping

According to a 2023 meta-analysis (a study that analyzes many studies), unilateral training had a more significant effect on unilateral jumping ability, and bilateral training had a more substantial impact on bilateral jumping ability (Wenfeng Zhang). These results align with the principle of specificity, which states that to improve a task, you should train in a way that most resembles that task. 

A 2022 meta-analysis found similar results, citing a larger improvement in unilateral jump performance from unilateral resistance training. In contrast, larger improvements in bilateral jump performance were seen with bilateral resistance training (Kai-Fang Liao). Personal trainers should match bilateral performance goals with bilateral resistance training and unilateral goals with unilateral training, respectively. 

Sprinting

Zhang et al.’s meta-analysis on jumping also examined sprinting similarly. It showed that unilateral training was more likely to improve the athletes’ straight-line sprinting ability. This could be due to sprinting’s unilateral nature, which more closely matches the biomechanical characteristics of unilateral training. The main factor affecting sprint ability is the pedal extension speed of the lower limbs. 

Through nerve stimulation, unilateral training promotes strength growth in small and deep muscle groups. It helps improve the nervous system’s ability to control and coordinate muscle strength during running. The author recommends plyometric training to exploit the stretch-shortening cycle (Wenfeng Zhang) fully.

On the contrary, another meta-analysis found that bilateral and unilateral resistance training are equally effective in enhancing sprinting. This is also contrary to popular opinion, supported by the concept of training specificity (Moran). How could two meta-analyses published within a couple of years come to different conclusions on the topic? 

One possible explanation is that the Zhang et al. study only included studies involving athletes, whereas the Moran et al. study only included studies with healthy participants. Both forms of resistance training are equally helpful in increasing sprint speed in healthy individuals because they are not at as high a training status and receive greater adaptations to the training, regardless of whether it is unilateral.

Change of Direction Ability

The ability to change direction is likely to improve more with unilateral training since it requires a single leg to be involved as a support foot for propulsion after a change of direction (Kai-Fang Liao). However, another study found no significant effect of unilateral and bilateral training on the indicators of the athletes’ ability to change direction (Wenfeng Zhang). The different populations tested in these studies may again explain these contradicting results, with higher-trained individuals requiring more specific training.

Unilateral vs Bilateral on Strength, Hypertrophy, and Balance

It is important to note that the studies used for this section only examined lower-body exercise, so their findings may not necessarily apply to the upper body when investigating these performance metrics. 

Strength

Many studies that investigated the effect of unilateral versus bilateral resistance training on strength had results aligned with the principle of specificity (Botton, Wenfeng Zhang, Kai-Fang Liao, Stern). Specifically, these studies showed that unilateral training improved the maximal force of the athletes’ unilateral limbs, and bilateral training improved the maximal force of bilateral limbs. 

It is important to note that not all unilateral exercises are created equal since some use a support limb while others solely use one limb. For example, the Bulgarian split squat was found to place nearly 15% of the load on the rear support leg (Helme).

Some studies found that strength increases were not significantly different between unilateral and bilateral resistance training (Appleby; Zhao). The results show that practical increases in leg strength can be achieved using bilateral or unilateral resistance training. This indicates that transferring strength between exercises of similar joint movements and muscles is possible. Personal trainers may incorporate unilateral strength training, where the prescription of bilateral training may be inhibited, and benefits may still be seen.

Hypertrophy

Research shows muscle thickness increased similarly in both training groups (Botton). This makes sense as we know that the main drivers for hypertrophy aren’t necessarily a bilateral vs. unilateral discussion but more so about creating maximum time under tension, full range of motion, and attaining enough weekly volume. 

Balance and Muscle Imbalances

One meta-analysis showed that unilateral and bilateral training did not significantly affect the athletes’ balance ability (Wenfeng Zhang). This result makes sense because balance improvement is directly correlated with strength, which resistance training increases. 

However, there is a clear winner when offsetting muscle imbalances or asymmetry between limbs. In one study on basketball players, only unilateral training reduced between-limbs asymmetry and achieved more significant enhancements in actions that mainly required applying force unilaterally in basketball players. (Gonzalo-Skok). 

Conclusion

Both unilateral and bilateral resistance training are advantageous for many performance goals. If the trainee or athlete is a beginner, both forms of exercise may produce similar performance benefits. However, more experienced athletes need specific training to match their performance goals, some of which may require unilateral training. 

Following the principle of specificity may be the best tool when deciding to program a unilateral exercise. For example, unilateral resistance exercises may be better than bilateral exercises if an athlete plays a sport with many single-limb movements. Specifically, bilateral exercises should be chosen to enhance bilateral strength and performance, and unilateral exercises should be selected to improve unilateral strength and performance.

Whereas most performance metrics showed no significant differences between training styles, sprinting and direction change improve most with unilateral training. Furthermore, unilateral training was superior in improving asymmetries and muscle imbalances. 

Unilateral training will likely benefit any program, especially when specificity is considered. Personal trainers can also incorporate unilateral strength training where the prescription of bilateral training may be inhibited. Lastly, further research is needed on upper-body unilateral training, as all research to date that this article is aware of is for the lower body.

References

1. Appleby BB, Cormack SJ, Newton RU. Specificity and Transfer of Lower-Body Strength: Influence of Bilateral or Unilateral Lower-Body Resistance Training. J Strength Cond Res. 2019 Feb;33(2):318-326. doi: 10.1519/JSC.0000000000002923. PMID: 30688873.
2. Botton CE, Radaelli R, Wilhelm EN, Rech A, Brown LE, Pinto RS. Neuromuscular Adaptations to Unilateral vs. Bilateral Strength Training in Women. J Strength Cond Res. 2016 Jul;30(7):1924-32. doi: 10.1519/JSC.0000000000001125. PMID: 26348920.
3. Gonzalo-Skok O, Tous-Fajardo J, Suarez-Arrones L, Arjol-Serrano JL, Casajús JA, Mendez-Villanueva A. Single-Leg Power Output and Between-Limbs Imbalances in Team-Sport Players: Unilateral Versus Bilateral Combined Resistance Training. Int J Sports Physiol Perform. 2017 Jan;12(1):106-114. doi: 10.1123/ijspp.2015-0743. Epub 2016 Aug 24. PMID: 27140680.
4. Helme M, Emmonds S, Low C. Is the Rear Foot Elevated Split Squat Unilateral? An Investigation Into the Kinetic and Kinematic Demands. J Strength Cond Res. 2022 Jul 1;36(7):1781-1787. doi: 10.1519/JSC.0000000000003727. Epub 2020 Aug 12. PMID: 32796420.
5. Liao KF, Nassis GP, Bishop C, Yang W, Bian C, Li YM. Effects of unilateral vs. bilateral resistance training interventions on measures of strength, jump, linear and change of direction speed: a systematic review and meta-analysis. Biol Sport. 2022 Sep;39(3):485-497. doi: 10.5114/biolsport.2022.107024. Epub 2021 Jul 3. PMID: 35959319; PMCID: PMC9331349.
6. Moran J, Ramirez-Campillo R, Liew B, Chaabene H, Behm DG, García-Hermoso A, Izquierdo M, Granacher U. Effects of Bilateral and Unilateral Resistance Training on Horizontally Orientated Movement Performance: A Systematic Review and Meta-analysis. Sports Med. 2021 Feb;51(2):225-242. doi: 10.1007/s40279-020-01367-9. PMID: 33104995.
7. Stern D, Gonzalo-Skok O, Loturco I, Turner A, Bishop C. A Comparison of Bilateral vs. Unilateral-Biased Strength and Power Training Interventions on Measures of Physical Performance in Elite Youth Soccer Players. J Strength Cond Res. 2020 Aug;34(8):2105-2111. doi: 10.1519/JSC.0000000000003659. PMID: 32541618.
8. Zhang W, Chen X, Xu K, Xie H, Li D, Ding S, Sun J. Effect of unilateral training and bilateral training on physical performance: A meta-analysis. Front Physiol. 2023 Apr 13;14:1128250. doi: 10.3389/fphys.2023.1128250. PMID: 37123275; PMCID: PMC10133687.
9. Zhao X, Turner AP, Sproule J, Phillips SM. The Effect of Unilateral and Bilateral Leg Press Training on Lower Body Strength and Power and Athletic Performance in Adolescent Rugby Players. J Hum Kinet. 2023 Jan 20;86:235-246. doi: 10.5114/jhk/159626. PMID: 37181263; PMCID: PMC10170531.