The Bilateral Deficit: Why Two Limbs Lift Less Than One

Grip a barbell with both hands and pull as hard as you can, then grip a dumbbell with one hand and pull just as hard. Add the two single-arm numbers together and, on paper, that sum should predict what both arms can do at once. It rarely does. Across decades of force-plate, dynamometer, and EMG research, two limbs working together consistently produce less force than the arithmetic sum of what each limb produces alone. Exercise scientists call it the bilateral deficit, and it sits quietly underneath almost every barbell lift, jump, and sprint a lifter performs.

It is not a training flaw or a sign of poor coordination. It appears to be a built-in feature of how the nervous system drives muscle when both sides of the body act at once, and its size depends heavily on what is being measured, how fast, and in whom. For anyone using surface EMG to interpret bilateral lifts, understanding when the deficit shows up, and when it quietly does not, changes how a lot of activation data should be read.

What the Bilateral Deficit Actually Is

The bilateral deficit is defined as a reduction in total force, power, or EMG amplitude during a simultaneous two-limb contraction relative to the sum of the same limbs contracting one at a time.¹,² It was first documented in handgrip dynamometry in the 1960s and has since been replicated across knee extension, elbow flexion, plantar flexion, jumping, and multi-joint leg press tasks.²,³ A comprehensive review by Škarabot and colleagues puts the typical magnitude at roughly 2 to 20 percent, with isometric tasks generally showing smaller deficits than fast, ballistic ones.²

The opposite pattern, a bilateral facilitation where two limbs together produce more than the unilateral sum, does occasionally appear in the literature, particularly in trained or sport-specific populations.² This inconsistency is part of what makes the phenomenon interesting rather than a fixed rule: the deficit is real and reproducible on average, but its presence and size in any one person, on any one task, is not guaranteed.

The Neural Story, and Its Limits

The leading explanation is neural rather than mechanical. When both limbs contract together, central drive to the working muscles appears to be reduced compared with a unilateral effort of the same intended intensity, likely through interhemispheric and transcallosal inhibition between the two motor cortices.²,⁴ Reduced voluntary activation and altered corticospinal excitability during bilateral contractions support this account, and cortical involvement has been observed directly using movement-related cortical potentials during bilateral handgrip.⁵,⁶

But the evidence is not tidy. A frequently cited study by Buckthorpe and colleagues found that a clear bilateral deficit in explosive force during knee extension was not accompanied by a corresponding drop in agonist EMG amplitude, arguing against agonist neural drive as the whole explanation and pointing instead toward antagonist co-activation or intermuscular coordination changes.⁷ Girompaire and colleagues later confirmed reduced voluntary activation during bilateral knee extension using cortical stimulation techniques, while other work has struggled to detect the deficit at all under certain isometric conditions.⁸ The honest summary is that the bilateral deficit is a robust behavioral finding with a plausible, partially supported neural mechanism, not a fully solved puzzle. Reading rectus femoris or vastus lateralis EMG amplitude alone during a bilateral lift will not always reveal it.

It Depends on What You Measure

The size of the deficit is highly sensitive to the outcome variable. Carr and colleagues found a clear bilateral deficit in maximal handgrip strength but no equivalent deficit in the rate of force development during the same contractions, with EMG amplitude unaffected in either condition.⁹ This distinction matters for anyone tracking early-phase force output rather than peak force, since the two do not necessarily move together.

Contraction velocity matters just as much. Padulo and colleagues showed that the bilateral deficit in a half-squat grew larger as movement velocity increased, with concurrent changes in vastus lateralis and vastus medialis activation.¹⁰ Samozino and colleagues linked this to the underlying force-velocity properties of the limbs during ballistic push-off, and Sarabon and colleagues confirmed that both contraction type and velocity are major sources of variability in reported deficit size across studies.¹¹,¹² Put simply, a slow, controlled bilateral squat and an explosive bilateral jump are not testing the same neuromuscular process, and a lifter can show a meaningful deficit in one context and almost none in another.

Who Shows the Largest Deficit

Individual differences are large. Yamauchi and colleagues compared young and older women during multi-joint leg extensions and found the elderly group carried a substantially larger bilateral deficit in both force and power output, consistent with age-related declines in neural drive capacity.¹³ Ye and colleagues reported sex differences in the deficit across proximal and distal upper-limb muscles, with the pattern varying by muscle group rather than following a single rule.¹⁴

Training history also matters, and this is where the finding becomes practically useful rather than merely descriptive. Janzen and colleagues ran eight weeks of either unilateral or bilateral leg press training and found that bilateral training reduced the bilateral deficit while unilateral training did not, indicating the deficit is at least partly trainable and task-specific rather than a fixed neurological ceiling.¹⁵ Botton and colleagues reported broadly similar neuromuscular adaptations to unilateral and bilateral strength training in women, reinforcing that both approaches build genuine strength even though the mode of testing changes what deficit shows up afterward.¹⁶ MacDonald and colleagues went further, showing that residual force enhancement techniques could partially offset the neural component of the deficit, evidence that it responds to intervention rather than being immovable.¹⁷

Does It Actually Matter for Performance

For most gym-based strength and hypertrophy goals, the bilateral deficit is a measurement curiosity more than a training problem. A squat, bench press, or deadlift is trained and tested bilaterally, so the deficit is baked into the numbers on the bar consistently across sessions and does not need to be corrected for. Where it becomes practically relevant is in explosive, single-effort actions and in athletes who both jump and sprint.

Bishop and colleagues found that the magnitude of the bilateral deficit measured during a countermovement jump was meaningfully associated with linear sprint speed and change-of-direction performance in trained athletes, suggesting the phenomenon is not just a lab artefact but tracks with tasks that rely on rapid unilateral force expression.¹⁸ This is one of the more direct arguments for including both bilateral and unilateral testing and training in athletic programs, since bilateral-only training may not fully prepare the neuromuscular system for the single-limb demands of sprinting, cutting, or landing.

What This Means in Practice

The practical takeaway is not that bilateral lifts are somehow deficient, or that unilateral work should replace them. It is that force, EMG amplitude, and rate of force development each tell a partially independent story about a bilateral effort, and none of them alone confirms how well a single limb is actually working. This is precisely the gap that single-muscle EMG monitoring is built to surface: instead of inferring one limb's contribution from a two-limb outcome, a sensor placed on one working muscle shows that muscle's own activation and fatigue pattern directly, on a bilateral lift or a unilateral one, without needing to assume symmetry between left and right.

For a lifter alternating bilateral squats with single-leg work, or comparing a barbell row to a single-arm row, that direct per-limb signal is the difference between guessing at balance and actually seeing it.

Key Takeaways

• The bilateral deficit describes reduced force, power, or rate of force development when two limbs contract together, compared with the sum of the same limbs contracting one at a time, typically in the range of 2 to 20 percent.

• The mechanism is thought to be primarily neural, involving reduced central drive and interhemispheric inhibition, but agonist EMG amplitude does not always fall in step with the force deficit, so the full explanation remains only partially resolved.

• The deficit grows with contraction velocity and shows up more reliably in fast, ballistic efforts than in slow or purely isometric ones, and it can appear in force while being absent in rate of force development on the same contraction.

• Age increases the deficit, sex and muscle group interact with it, and specific bilateral training can reduce it, meaning it responds to training rather than being a fixed trait.

• In trained athletes, a larger bilateral deficit during jumping is associated with better sprint and change-of-direction performance, making it relevant to programming for explosive, single-limb sports actions, even though it rarely matters for standard bilateral strength training.

References

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2. Škarabot J, Cronin N, Strojnik V, Avela J. Bilateral deficit in maximal force production. Eur J Appl Physiol. 2016;116(11-12):2057-2084.

3. Kawakami Y, Sale DG, MacDougall JD, Moroz JS. Bilateral deficit in plantar flexion: relation to knee joint position, muscle activation, and reflex excitability. Eur J Appl Physiol Occup Physiol. 1998;77(3):212-216.

4. Škarabot J, Ansdell P, Brownstein CG, et al. Corticospinal and transcallosal modulation of unilateral and bilateral contractions of lower limbs. Eur J Appl Physiol. 2016;116(11-12):2197-2214.

5. Beethe AZ, Ferland PM, Piucci G, et al. The Bilateral Deficit Phenomenon in Elbow Flexion: Explanations for Its Inconsistent Occurrence and Detection. Percept Mot Skills. 2022;129(1):47-62.

6. Oda S, Moritani T. Movement-related cortical potentials during handgrip contractions with reference to bilateral deficit. Eur J Appl Physiol Occup Physiol. 1995;72(1-2):1-5.

7. Buckthorpe MW, Pain MTG, Folland JP. Bilateral deficit in explosive force production is not caused by changes in agonist neural drive. PLoS One. 2013;8(3):e57549.

8. Girompaire L, Morel B, Boyas S, et al. Reduced cortical voluntary activation during bilateral knee extension. Hum Mov Sci. 2017;54:311-318.

9. Carr JC, Trevino MA, Grandjean PW. Bilateral deficit in strength but not rapid force during maximal handgrip contractions. Eur J Sport Sci. 2021;21(6):836-843.

10. Padulo J, Ardigò LP, Guágnano G, et al. Bilateral deficit magnitude increases with velocity during a half-squat exercise. J Sports Sci. 2022;40(11):1206-1213.

11. Samozino P, Rejc E, Di Prampero PE, Belli A, Morin JB. Force-velocity properties' contribution to bilateral deficit during ballistic push-off. Med Sci Sports Exerc. 2014;46(1):107-114.

12. Šarabon N, Kozinc Ž, Marković G. Factors influencing bilateral deficit and inter-limb asymmetry of maximal and explosive strength. Eur J Appl Physiol. 2020;120(7):1681-1688.

13. Yamauchi J, Mishima C, Nakayama S, Ishii N. Force-velocity, force-power relationships of bilateral and unilateral leg multi-joint movements in young and elderly women. J Biomech. 2009;42(13):2151-2157.

14. Ye X, Beck TW, Wages NP, Carr JC. Sex comparisons of the bilateral deficit in proximal and distal upper body limb muscles. Hum Mov Sci. 2019;64:329-337.

15. Janzen CL, Chilibeck PD, Davison KS. The effect of unilateral and bilateral strength training on the bilateral deficit and lean tissue mass. Eur J Appl Physiol. 2006;97(3):253-260.

16. Botton CE, Radaelli R, Wilhelm EN, et al. Neuromuscular Adaptations to Unilateral vs. Bilateral Strength Training in Women. J Strength Cond Res. 2016;30(7):1924-1932.

17. MacDonald GZ, Docherty D, Cook R, Zello GA, Chilibeck PD, Butcher SJ. Mitigating the bilateral deficit: reducing neural deficits through residual force enhancement and activation reduction. Eur J Appl Physiol. 2018;118(9):1911-1919.

18. Bishop C, Read P, McCubbine J, Turner A. Bilateral Deficit During Jumping Tasks: Relationship With Speed and Change of Direction Speed Performance. J Strength Cond Res. 2021;35(7):1833-1840.