Unilateral vs. Bilateral Training: Is One Better?
Written by Philip Van Dyke, PT, DPT, CSCS
Unilateral movements are incorporated in exercise programs in rehabilitation settings, strength and conditioning settings, as well as in all settings between. The use of unilateral exercises instead of, or in addition to, bilateral exercises is justified in many different ways, such as training specificity, training the smaller “stabilizer” muscles, and that fancy term called “bilateral force deficit.” Still, others contend that unilateral movements have no additional benefit when compared to bilateral movements or even produce inferior results. There are prominent strength coaches out there who are big proponents of unilateral movements, such as Mike Boyle (see one his cases for unilateral training here), and there are also those who are against their use, such as Mark Rippetoe (one of the many topics in this colorful article). The arguments made for or against either side are often anecdotal or are personal opinion; so is there any research to support either side? Or, like so many things in life, does the evidence indicate that it depends?
So, let’s look at the evidence for or against the common arguments made for the use of unilateral training.
If an athlete is strength training to improve his or her performance in competition, there is always a unilateral component in that sport to varying extents (a football lineman has far less unilateral movements in his sport than a tennis player, but certainly still requires planting and cutting, sidestepping, etc.). The same may go for an industrial worker or an elderly person needing to reach the overhead cabinet. The question is: does unilateral exercise translate to improved “performance” in unilateral tasks over bilateral exercise? In a study by Makaruk et al. (see summary below), they identified some specificity of unilateral and bilateral plyometric training after six weeks of training, but there was no difference in improvements after 12 weeks. While there were no differences in bilateral or unilateral strength or in bilateral power, McCurdy et al. (see summary below) also found that unilateral vertical jump height and relative power improved more after 8 weeks of unilateral strength and plyometric training than after 8 weeks of bilateral strength and plyometric training in untrained men and women.
Bilateral Force Deficit
The bilateral force deficit (sometimes called bilateral deficit) is an observed phenomenon that the sum of maximal strength of both the right and left limbs is greater than the simultaneous use of both limbs in a bilateral movement. This bilateral force deficit is usually attributed to neural inhibition when bilateral limbs are contracted. Often, research studies use a leg extension machine or some other machine in which the movement should be exactly the same with the exception of whether one or two limbs are used. This would seem like the most controlled setup to determine a valid measure of the difference between bilateral and unilateral strength. Unfortunately, a recent study by Simoneau-Buessinger et al. brought this, and the commonly accepted theory of neural inhibition, into question. When they allowed for postural adjustments to occur in a more open configuration of their mechanical dynamometer, there was no longer a bilateral force deficit in plantar flexor strength. In the locked configuration, they reported that postural changes factored into the increased force despite no difference in EMG activity of the agonist muscles. Similarly, a study by Buckthorpe et al. found that there was no difference in EMG activity of the agonists or antagonists of a knee extension activity. They also found that a bilateral force deficit only occurred during “explosive isometric contractions” and not during maximal voluntary force contractions. There is a large body of evidence to support the existence of a bilateral force deficit, but the theory that it is caused by neural inhibition during bilateral contractions has recently been brought into question.
Training the “Stabilizer” Muscles
This is less of an argument used in literature and research, and more of an explanation commonly heard given by personal trainers and, very likely, physical therapists when asked by clients/patients. While the individuals giving “stabilizer muscles” as an explanation for unilateral training probably did not have the specific evidence in mind, there may be something to this argument. Makaruk et al. suggested that improvements in stabilizing muscles during unilateral jumping tasks to improve the body’s alignment over the legs, which has been shown to attenuate ground reaction force absorption on landing, are a possible explanation for early improvements observed in peak power and jumping ability of unilateral and alternating tasks. Additionally McCurdy et al. found that gluteus medius and hamstring EMG activity was higher during a rear elevated split squat (called a modified single-leg squat in the study) than during a back squat in female athletes. The gluteus medius is often considered an important muscle in stabilizing the pelvis during unilateral or alternating activities ranging from walking to high level sports activities.
Effects of Unilateral vs. Bilateral Training
No matter what the argument is for the inclusion of unilateral training, we need to know how it actually affects performance measures when compared to bilateral training. The evidence for or against the use of unilateral training is not overwhelming, but it seems to be effective for achieving particular goals. Below are a few studies that compared unilateral and bilateral training in various scenarios. Some of the summaries are a bit dense with results, but I will attempt to synthesize some takeaway points in the following section.
- Speirs et al. found that five weeks of unilateral strength training (with a rear elevated split squat) and five weeks of bilateral strength training (with a back squat) both improved 1-RM strength of both exercises, as well as 40-m sprint speed and change of direction speed in trained rugby players.
- Makaruk et al. compared the effects of 12 weeks of unilateral plyometric training with bilateral plyometric training in active, but untrained, female college students on peak power and jumping ability in unilateral, alternating, and bilateral tasks. Total gains were not significantly different between the two experimental groups (both improved compared to control group). However, the unilateral training group had greater improvements at 6 weeks, whereas the bilateral training group was the only group to not significantly decrease in performance after 4 weeks of detraining.
- McCurdy et al. compared 8 weeks of progressive unilateral and bilateral lower body strength and plyometric training (only during weeks 3-8) in untrained college-aged men and women. Both types of training produced equal improvements in bilateral and unilateral lower extremity strength and bilateral jump height and power. However, unilateral vertical jump height and relative power improved significantly more in the unilateral training group than in the bilateral training group. The only significant difference between men and women was that men improved significantly more in unilateral strength (assessed with rear elevated split squat). The effect size for this difference was small, so it warranted further investigation.
- In a study by Costa et al., despite the presence of a bilateral force deficit in 1-RM of bilateral compared to unilateral leg extensions, the total volume of load lifted (sets x repetitions x load), RPE, and blood lactate levels, were similar between groups of trained adult males. Even though a bilateral force deficit was observed, individuals in the unilateral training group were unable to complete as many total repetitions when working to exertion on three sets. Thus, the total volume of load lifted, along with RPE and blood lactate levels, were similar between groups.
- In a longer-term study, Janzen et al. examined the effect of 26 weeks of unilateral and bilateral full body resistance training with machine exercises in post-menopausal women. At baseline, a bilateral force deficit was present for the leg press and lat pull-down exercises, but not for the leg extension exercise. The bilateral force deficit for leg press and knee extension significantly decreased following bilateral training, and the between-groups difference in change in bilateral force deficit was significant for all exercises. Both unilateral and bilateral training groups improved in strength and in lean tissue mass with no significant between-group differences.
So, should we train exclusively bilaterally or unilaterally? Are there certain cases in which one method is better? An important thing to note is that all of the studies mentioned above, along with every other study that I saw while researching this topic, compared unilateral training to bilateral training in healthy individuals ranging from untrained to highly trained. So, it is difficult to make any conclusions about how to apply this information to any rehabilitation population. A second important point is that there are clear cases in physical therapy in which unilateral training is warranted (e.g., a post-CVA patient with hemiplegia). Additional factors such as tissue healing stage, neurological involvement, and presence of pain with certain movements play into the prescription of exercise in a rehabilitation setting.
What we can take away is that it appears to not matter in most cases whether unilateral or bilateral training is utilized across most populations. Strength gains were not significantly different after training with unilateral or bilateral approaches. Plyometric training may see quicker improvements with unilateral training, but there may longer lasting improvements with bilateral training. Additionally, bilateral training may lead to a decrease in bilateral force deficit. There does not appear to be a strong element of training specificity when it comes to unilateral training. The one consistent result between studies was that both unilateral and bilateral resistance training brought about improvements in strength across many different populations with varying levels of training.
What are your reasons for or against utilizing unilateral training? If you have additional insights, please feel free to leave a comment.
- Aune TK, Aune MA, Ettema G, Vereijken B. Comparison of bilateral force deficit in proximal and distal joints in upper extremities. Hum Mov Sci. 2013;32(3):436-44.
- Buckthorpe MW, Pain MT, Folland JP. Bilateral deficit in explosive force production is not caused by changes in agonist neural drive. PLoS ONE. 2013;8(3):e57549.
- Costa E, Moreira A, Cavalcanti B, Krinski K, Aoki M. Effect of unilateral and bilateral resistance exercise on maximal voluntary strength, total volume of load lifted, and perceptual and metabolic responses. Biol Sport. 2015;32(1):35-40.
- Hay D, De Souza VA, Fukashiro S. Human bilateral deficit during a dynamic multi-joint leg press movement. Hum Mov Sci. 2006;25(2):181-91.
- Janzen CL, Chilibeck PD, Davison KS. The effect of unilateral and bilateral strength training on the bilateral deficit and lean tissue mass in post-menopausal women. Eur J Appl Physiol. 2006;97(3):253-60.
- Mccurdy KW, Langford GA, Doscher MW, Wiley LP, Mallard KG. The effects of short-term unilateral and bilateral lower-body resistance training on measures of strength and power. J Strength Cond Res. 2005;19(1):9-15.
- McCurdy K, O'kelley E, Kutz M, Langford G, Ernest J, Torres M. Comparison of lower extremity EMG between the 2-leg squat and modified single-leg squat in female athletes. J Sport Rehabil. 2010;19(1):57-70.
- Makaruk H, Winchester JB, Sadowski J, Czaplicki A, Sacewicz T. Effects of unilateral and bilateral plyometric training on power and jumping ability in women. J Strength Cond Res. 2011;25(12):3311-8.
- Simoneau-Buessinger E, Leteneur S, Toumi A, et al. Bilateral Strength Deficit Is Not Neural in Origin; Rather Due to Dynamometer Mechanical Configuration. PLoS ONE. 2015;10(12):e0145077.
- Speirs DE, Bennett MA, Finn CV, Turner AP. Unilateral vs. Bilateral Squat Training for Strength, Sprints, and Agility in Academy Rugby Players. J Strength Cond Res. 2016;30(2):386-92.