This rehab secret has been known since 18941, but it’s still rarely used.
It’s almost like magic.
Actually, it’s better than magic.
You can make something out of nothing.
It’s one of those few moments where it sounds too good to be true, but it actually lives up to the hype.
Let’s talk about the cross-education effect.
What is it?
Cross-education of strength is when you strength train only one side of your body and those strength gains transfer over to the other side.
Yes, it’s real.3
If you lift weights on only your right arm, your left arm will get stronger.
Cross-education can happen with a variety of qualities, including strength, hypertrophy, and skill performance.
Here we’re focusing on strength.
Why does it matter?
Having an arm immobilized in a sling for only 3 short weeks can reduce your strength by 20%.7
Having weak quads is associated with reinjury risk following ACL reconstruction, but you’re limited on how much strengthening you can do on the injured side during the early phases of ACL rehab.2
That’s where cross-education can be so valuable.
Training the healthy arm can reduce any strength loss in the injured arm during those 3 weeks in the sling7 and strength training the healthy leg in the early phases of ACL rehab makes the injured leg much stronger later on.2
If an athlete is injured, training the opposite side of the body can be a cheat code for a better return to performance.
How does it happen?
Well… we don’t really know exactly how it happens.
But, we have some thoughts.
There’s a strong consensus that the cross-education of strength is strongly driven by changes in the nervous system.4, 5, 6, 7
Our nervous system serves as our body’s software. It’s the programming language that tells the rest of the body what to do.
Cross-education relies heavily on improving that software efficiency.
There are a few different areas along the nervous system that are thought to be altered with cross-education. The nervous system runs from the top down. It connects the brain with the spinal cord, and the spinal cord with the muscle. Research suggests that little to no cross-education changes in the nervous system occur at the muscular or spinal cord level.4
It looks like these changes stem from the top; the cortical level.
Cortical mechanisms4
The overarching concept of why we think cross-education of strength happens is motor irradiation. Motor irradiation suggests that when a nervous system signal is happening in one area of the brain, it can spread to other areas of the brain and activate those other areas.
The main areas of focus are the connections between the two halves of the brain (corpus callosum) and the nerve pathways that are leaving the brain (corticospinal tract).
Corpus callosum4
Our brain is divided into two sides, called hemispheres. The right side of our brain controls the movements on the left side of our body and vice versa. The area where movement is controlled in each hemisphere is called the motor cortex.
The corpus callosum is a bridge that connects the two halves of the brain. Through the corpus callosum, there are connections between the right and left motor cortex. The active motor cortex can send signals that increase activation of the inactive motor cortex, while also sending signals that reduce inhibition of the inactive motor cortex.
Increased excitation signals and decreased inhibition signals have the same net effect: more activity on the side of the brain that controls the injured limb.
Corticospinal tract4
The brain sends signals down from the motor cortex to tell the muscles what to do. This signal is sent down through a pathway called the corticospinal tract.
We already know that the right side of the brain controls the muscles on the left side of the body. That happens because the nerves in the corticospinal tract cross from the left side of the body to the right side in an area of the brainstem called the medulla.
Simply, as the nerves are going from the top down, they cross over to the opposite side of the body. The fancy word for that crossing over is decussation.
However, a few nerves in the corticospinal tract that come down from the brain do not cross over to the other side. We call those ipsilateral corticospinal pathways (ipsi = same, lateral = side). These ipsilateral pathways are thought to play a role in cross education.
They receive the activation signals from the active motor cortex, but stay on the same side as the injured limb.
Increased activity in the nerve pathways that control muscles of the injured limb results in more strength gained/preserved over time.
How can you apply it?
1) If you’re injured, train the opposite limb.
This one is as straightforward as it gets.
If you’re injured, train the heck out of the other limb.
There’s no reason to go to physical therapy and just do 3 sets of 10 single leg squats on your injured leg.
Get after it on the opposite side of the body.
2) Train hard and heavy.
The evidence shows that the amount of strength gained in the injured side is directly proportional to the amount of strength gained in the healthy side.
The most important training variables will always be consistency and intensity. Cross-education is no different.
You can’t just go through the motions. You need to load it up and let it rip.
If your healthy limb isn’t being smoked, the strength transfer to the injured side won’t be optimal.
Fun Fact:
The transfer of strength is higher when you injure your non-dominant limb and can train with your dominant side.5
References:
1. Green LA, Gabriel DA. The cross education of strength and skill following unilateral strength training in the upper and lower limbs. J Neurophysiol. 2018 Aug 1;120(2):468-479. doi: 10.1152/jn.00116.2018. Epub 2018 Apr 18. PMID: 29668382; PMCID: PMC6139459.
2. Harput, G., Ulusoy, B., Yildiz, T.I. et al. Cross-education improves quadriceps strength recovery after ACL reconstruction: a randomized controlled trial. Knee Surg Sports Traumatol Arthrosc 27, 68–75 (2019). https://doi-org.wa.opal-libraries.org/10.1007/s00167-018-5040-1
3. Manca, A., Dragone, D., Dvir, Z. et al. Cross-education of muscular strength following unilateral resistance training: a meta-analysis. Eur J Appl Physiol 117, 2335–2354 (2017). https://doi-org.wa.opal-libraries.org/10.1007/s00421-017-3720-z
4. Contralateral effects of unilateral strength training: evidence and possible mechanisms. Timothy J. Carroll, Robert D. Herbert, Joanne Munn, Michael Lee, and Simon C. Gandevia Journal of Applied Physiology 2006 101:5, 1514-1522
5. Hendy AM, Spittle M, Kidgell DJ. Cross education and immobilisation: mechanisms and implications for injury rehabilitation. J Sci Med Sport. 2012 Mar;15(2):94-101. doi: 10.1016/j.jsams.2011.07.007. Epub 2011 Sep 15. PMID: 21924681.
6. Ruddy KL, Carson RG. Neural pathways mediating cross education of motor function. Front Hum Neurosci. 2013 Jul 29;7:397. doi: 10.3389/fnhum.2013.00397. PMID: 23908616; PMCID: PMC3725409.
7. Pearce AJ, Hendy A, Bowen WA, Kidgell DJ. Corticospinal adaptations and strength maintenance in the immobilized arm following 3 weeks unilateral strength training. Scand J Med Sci Sports. 2013 Dec;23(6):740-8. doi: 10.1111/j.1600-0838.2012.01453.x. Epub 2012 Mar 19. PMID: 22429184.