Work One Arm and Watch the Other Get Stronger Without Doing a Thing

Doug Joachim
Feb 13, 2013
4 min read

Updated: Sep 16

Picture this: you're nursing a tweaked shoulder from an overly ambitious weekend warrior session, but you're itching to get back to the gym. Good news—you don't have to sit on the sidelines twiddling your thumbs. Enter the contralateral strength training effect, or as I like to call it, your body's built-in cheat code.

The contralateral (opposite side of the body) strength training effect is fascinating stuff. When you work out one side of your body, the opposite side—the one that's just hanging out, maybe scrolling through Instagram while you're grinding—actually reaps some benefits too. Meta-analysis shows that an individual may gain up to "half the increase of strength of the trained side" in their nonworking side. Pretty amazing, right? I certainly think so.

Here's the practical gold: the next time you injure your shoulder, ankle, or any other body part, you can still hit the gym and train the uninjured side without losing too much ground. It's like having a fitness insurance policy. But don't get your hopes up too high—while you may notice increases in strength, lean muscle mass adaptation is pretty much insignificant. Your biceps aren't going to magically grow from single-arm training alone.

The Science Behind the Magic

There are many theories about why one-sided training increases strength in the non-working side, but the precise physiological mechanisms are still a bit of a mystery. The smart scientists think the gains are likely due to neuromuscular adaptations and an increase in the neural drive to the untrained side. Translation? Your nervous system pathways to the muscle get better at their job, even without direct training.

A study from the University of South Wales put this to the test. Ten participants did only right-handed wrist exercises for 4 weeks while another 10 served as the control group (aka the couch potatoes who didn't exercise either wrist). The experimental group showed improvement in wrist strength in both the right hand and the untrained left hand, whereas the control showed no significant change. Many peer-reviewed papers have shown similar results, so this isn't some fluke finding.

The Ugly Stepchild: Unilateral Training

Here's the thing—unilateral (one-sided) training is often the ugly stepchild of many workout routines, especially for the bigger muscles. When was the last time you did single-arm dumbbell chest presses? Or a single-leg weighted deadlift? (By the way, that's one of the best butt exercises out there.)

I think many people skip single-sided exercises because they take more time to complete. But here's where that thinking gets flipped on its head—one could argue that one side is mostly resting when the other is working, so you can switch sides without a traditional rest period and finish at roughly the same time. Besides, I always feel like I'm wasting time during rest periods when I could be working. There are no bad exercises, just missed opportunities.

The Plot Twist: Bilateral Deficit

Now here's where it gets really interesting. Research has shown there's something called a "bilateral deficit" present in many people. This means the total amount of weight both limbs can lift together is actually less than the sum of what each limb can lift individually. Furthermore, integrated muscle tests using EMGs suggest the prime movers in bilateral work are less active! Your muscles are literally slacking off when working as a team.

Bilateral Deficit Example:

2-Arm Barbell Curl: 90lbs total
1-Arm Dumbbell Curl: 50lbs each arm (100lbs total)

Mind-blowing, right?

Studies show some trained individuals are immune to the bilateral deficit, and most Olympic weightlifters actually have a bilateral increase in strength—probably due to the specificity principle since all their competition lifts are bilateral movements.

The Bottom Line: Context Matters

There's an argument to be made that unilateral training might be better than bilateral work in some contexts. But before you go down that rabbit hole, you need to ask yourself: what does "better" mean for your specific goals? If you're like most people, you'll benefit more from a hybrid training style that integrates both modalities.

In certain situations, unilateral work may be needed to correct imbalances and strengthen weak links. It's not about choosing sides—it's about being smart with your programming and understanding that your body is more adaptable than you think.

Practical Takeaways

So what's the real world application here? Don't panic the next time you get injured. Train around it, not through it. Use the contralateral effect to your advantage, mix in some unilateral work regardless of your injury status, and remember that variety is the spice of life when it comes to training.

Your body is resilient, adaptable, and full of surprises including the ability to get stronger on one side while the other does the work. Pretty cool trick, if you ask me.

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Sources: 1. Appl Physiol Nutr Metab: Greater Bilateral Deficit 2. Clinical Neurophysiology 3. Maximal voluntary force of bilateral and unilateral leg extension