There’s always debate about which has the better training effect: bilateral training or unilateral. In other words, is working with both legs/arms better than working with just one limb at a time? The answer you get depends on who you talk to. Let’s see what science tells us then draw conclusions from there.
If we’re training bilaterally, we’re training both limbs at the same time. Good examples of this would be squats, deadlifts, bench press, bent-over barbell rows, etc. Good examples of similar unilateral exercises, or single-limb exercises, would be split squats, 1-leg RDLs, 1-arm DB Chest Press, and 1-Arm DB Rows and all their variations and progressions. Both have their obvious merits. Proponents of bilateral training laud the increased load the body has to move while it’s opponents stress that this increased load leads to injury.
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There is such a thing as bilateral deficit. That means that the sum of the maximum amount of force you can produce with each limb is greater than the amount of force you can produce with both limbs. Taking bilateral deficit into account, you’d think that unilateral exercises would produce a greater training effect than bilateral. Not quite.
I’m going to go ahead and tell you without making you read further that unilateral exercises produce the same or a similar training effect as their bilateral counterparts (2). However, it’s important to understand what occurs during each type and know what kind of force the body is capable of moving.
Where there are holes in the research, common sense needs to fill in some blanks here and there until the science catches up. Exercise science has vastly improved over the last 20 years but we’re still playing catch-up. Most research examining the difference between bilateral and unilateral training examine single muscle groups in order to control for all variables. For example, if their looking at strength at the knee (quadriceps), they typically control for variables by examining just those muscles–wisely so… playing fast and loose with variables just leads to an invalid conclusion. However, as a result, we don’t see the whole picture. In some cases, they don’t even look at actual squat or deadlift variations but rather measure the difference with stable working conditions like leg extension machines (1). What the research tends to leave out in these studies is the effect greater load can have on the rest of the body… the risks and the rewards. So while single-limb exercises might get you the same or better results, they might also be missing a couple things.
Increasing the load on a back squat or a deadlift forces the body to recruit additional motor units from muscle groups elsewhere in the body to complete the lift. To put it into perspective, imagine lifting a 225lb barbell during a classic deadlift vs two 60lb dumbbells during a 1-leg RDL. Because you’re working on one leg, there’s no possible way you’ll be able to hold anything near 225 lbs and complete a single rep. Now holding 60 lbs in each hand for x reps isn’t anything to turn your nose up at but right away, we’re taking weight out of your hands and by extension limiting the training effect on grip strength. With the greater load in the deadlift, there also comes a greater need for scapular and core stabilization in order to protect the spine from herniated discs and related injuries. So there’s greater risk here but there are also big rewards to match. Proponents of bilateral training will call this more functional because when we pick up furniture or other heavy objects, we typically don’t do it on one leg.
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Now on the flip side, there are intrinsic benefits to working unilaterally. Looking at the 1-Leg RDL, there is an obvious need for greater medial/lateral knee stabilization. Balance comes into play as well. If only one dumbbell is used opposite of the standing leg, then there is a contralateral component thrown into the mix. The loaded side will have to resist being pulled down by the weight while the working side has to maintain balance while performing the hip hinge pattern of the RDL. Because of all of these elements, scapular and core stability plays a role in unilateral training, just for different reasons and different effects. The heavy anti-flexion core stability of the deadlift has been swapped for lighter anti-flexion and anti-rotation stability in the 1-Arm/1-Leg RDL.
So which is better?
It depends on your goals and where you fall on the spectrum between powerlifter and weekend warrior. Taken at face value there is a case to be made for each. Athletes for example have been proven to benefit from increased motor unit recruitment, the kind easy to find in heavy bilateral lifts. However, many movements an athlete executes, require the dynamic balance and force produced by unilateral exercises. Regardless, there doesn’t seem to be any reason to favor one over the other in your training cycle. If they both carry unique benefits, why would you? Body builders and strength athletes will pre-fatigue their target muscles with heavy deadlifts, paving the way for greater hypertrophy with low-load/ high volume unilateral accessory lifts. Functional lifters will find value in varying between bilateral and unilateral lifts.
Do you have back pain, hip pain, or knee pain? Most exercises can be modified to your unique movement pattern to offer safety and effective training stimuli without having to scrap them from your program. You can also just adjust the weight to accommodate your abilities. There is risk of injury in any strength and conditioning program in which you intend to get better. Find ways to mitigate the risk so you can keep improving.
For safety’s sake, there is a proper order of operations though. Because bilateral exercises can be loaded much heavier and require a greater tax on the body’s nervous system in order to be effective, it would be wiser to perform these first. Fatiguing yourself on endless single-leg/ single-arm exercises before trying to move big weight is just asking for injury and it might not be the kind you walk away from. Train the heavy bilateral movements first, then make further gains using low-impact unilateral exercises as accessory lifts.
- Botton, CE, Radaelli, R, Wilhelm, EN, Rech, A, Brown, LE, and Pinto, RS. Neuromuscular adaptations to unilateral vs. bilateral strength training in women. J Strength Cond Res 30(7): 1924–1932, 2016
- M. Jones, J. Ambegaonkar, B. C. Nindl, J. A. Smith, and S. A. Headley, “Effects of unilateral and bilateral lower-body heavy resistance exercise on muscle activity and testosterone responses,” J. Strength Cond. Res., pp. 1094-1100, 2012.