This content was originally posted on athleticlab.com.
[This is a guest blog by Gaby Smith. Gaby Smith completed her MS in Exercise Science at Northeastern University and is participating in the Athletic Lab Mentorship Program. Gaby is a Certified Strength and Conditioning Specialist and holds certifications with U.S. Soccer, USAW, and USTFCCCA.]
When it comes to resistance training, you may automatically think of traditional bilateral exercises like the back squat, bench press, or deadlift which use both arms or both legs simultaneously. While these bilateral exercises have been effective in improving strength and power, there is increasing interest in the benefits of unilateral exercises – those that use only one arm or leg at a time – to improve athletic performance. Unilateral exercises are typically used as assistance exercises to bilateral exercises to add volume or variation within a strength program. Given that many sport-specific movements are unilateral in nature, many argue that unilateral resistance training provides a more specific and transferable stimulus to sport performance. Sprinting, jumping, and changing direction are all performed unilaterally, so training in a predominately unilateral manner may provide greater specificity and ultimately provide better transfer from training to competition given the similarity of the movement patterns. There have been many studies investigating the effects of both bilateral and unilateral training and determining which is best for improving speed and power.
Advantages of Bilateral Training
Improvements in both strength and power using bilateral exercises, such as the back squat, have been established and are commonly used as a primary exercise for this reason (Spiers, 2016). Training the back squat has been shown to significantly improve running velocity and jump height and may be correlated with improvements in change of direction performance (Spiers, 2016). Bilateral exercises allow for greater loading; therefore, the production of greater absolute force. This peak force production helps improve the athlete’s strength which is a key indicator of athletic performance and a main goal in most training programs. Bilateral exercises also allow for greater stability because the center of mass can be balanced between both limbs. These exercises require the coordination between both sides of the body and can be used to train both sides of the body simultaneously. Additionally, bilateral exercises may be easier to teach and perform, especially for novice athletes, and can be used as a safe option for even inexperienced athletes.
Advantages of Unilateral Training
The main argument for the use of unilateral exercises (reverse lunge, box step-up, single-arm row or press) is their greater specificity and potential transfer to sport-specific movements like running, bounding, jumping, and changing direction which are entirely or predominately unilateral. Specificity is a key principle in program design to maximize the transfer between training and competitive performance. Therefore, to most effectively improve performance, it is argued that resistance exercises must closely resemble the forces and mechanics required for the specific sport. The smaller base of support of unilateral exercises requires neuromuscular coordination and increased stability; however, this comes at the cost of external loading (Appleby, 2019). Some research also suggests that unilateral training may reduce muscular imbalances and the overall bilateral deficit (Jansson, 2013).
The Bilateral Deficit
Another theory supporting the incorporation of unilateral exercises in a training program is known as the bilateral deficit. This refers to the difference between the maximal strength of a bilateral contraction and the sum of the strength of the right and left limbs when contracting alone (Costa, 2015). This phenomenon has been observed in both dynamic and isometric contractions, in both the upper and lower limbs, large and small muscle groups, and in populations including athletes, non-athletes, elderly, and adolescents (Jansson, 2013; Costa, 2015). There are many theories regarding the cause of the increased force capacity of unilateral contractions compared to the two limbs contracting in tandem. One possible explanation as to the origin of the deficit is the difference in antagonist muscle activation between unilateral and bilateral contractions (Kuruganti, 2011). It may also be more complicated for the body to fully activate the largest and strongest motor units during bilateral contractions, as this requires coordinated neural impulses from both of the brain’s hemispheres. Therefore, it is theorized that unilateral training may be a more efficient means of training if maximal speed and strength production presents a challenge to the neuromuscular system (Jansson, 2013).
The Bottom Line
Both bilateral and unilateral exercises demonstrate improvements in their trained movement and proved to be equally effective in increasing strength (Appleby, 2019; McCurdy, 2005; Spiers, 2016). While one modality did not prove to be significantly more effective in increasing strength than the other, there was a noted reciprocal benefit: bilateral training also improved performance on unilateral tests and unilateral training also improved performance on bilateral tests (Appleby, 2019; McCurdy, 2005; Spiers, 2016). Although the exact mechanism for this finding is unclear, the improvements in strength likely have a carryover effect between tests. Even with the carryover effect, greater improvements in strength were seen on the specific test modality the athletes trained for, likely due to the mechanical specificity between the training stimulus and test (Appleby, 2019).
Because both bilateral and unilateral exercises can elicit similar relative improvements in strength, both can and should be incorporated in an athlete’s training program. Bilateral exercises will allow for the use of greater absolute loads and greater overall force production. While unilateral exercises will require lighter absolute loads than a bilateral exercise, the relative intensity may be greater than the bilateral movement and could enhance force development in a way that better corresponds to sport-specific strength gains (Jones, 2012). Another reason unilateral exercises require lighter loads is because of the increased stability required to perform the exercise. Therefore, they may be less effective for the development of maximal strength, but can be used to complement and transfer strength gains to bilateral exercises (Appleby, 2019). Also, using unilateral exercise alters the training stimulus which may also enhance recovery and reduce the risk of overuse injury while still providing a similar training result to a bilateral exercise (Jones, 2012). Practically, research suggests that both unilateral and bilateral exercises should be used as complimentary components of a training program as a means of increasing strength.
- Costa EC, Moreira A, Cavalcanti B, Krinski K, Aoki MS. 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.
- Jansson, D. (2014). Effects of Unilateral versus Bilateral Complex Training and High Intensity Interval Training on the Development of Strength, Power and Athletic Performance : An experimental study on elite male and female handball players during preseason training.
- Jones, Margaret T; Ambegaonkar, Jatin P; Nindl, Bradley C; Smith, Jeffrey A; Headley, Samuel A Effects of Unilateral and Bilateral Lower-Body Heavy Resistance Exercise on Muscle Activity and Testosterone Responses, Journal of Strength and Conditioning Research: April 2012 – Volume 26 – Issue 4 – p 1094-1100. doi: 10.1519/JSC.0b013e318248ab3b
- Kuruganti U, Murphy T, Pardy T. Bilateral deficit phenomenon and the role of antagonist muscle activity during maximal isometric knee extensions in young, athletic men. Eur J Appl Physiol. 2011;111(7):1533-1539. doi:10.1007/s00421-010-1752-8
- 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. doi:10.1519/14173.1
- Speirs, Derrick E.1,2; Bennett, Mark A.3; Finn, Charlotte V.4; Turner, Anthony P.2 Unilateral vs. Bilateral Squat Training for Strength, Sprints, and Agility in Academy Rugby Players, Journal of Strength and Conditioning Research: February 2016 – Volume 30 – Issue 2 – p 386-392