When you train a particular body area with weights, stretching, mobilizing, or some other form of therapy, what exactly adapts to make an improvement? Is it something in the local area, like bigger muscles? Or longer muscles? Or smoother, more vibrant, luxuriously healthy fascia? Or is the adaptation centrally located, that is, in the central nervous system or brain? One interesting source of insight into these questions comes from studies where they train just one limb and then see whether the performance of the untrained limb changes in some way. There are some interesting results, so let’s take a brief look.
It is well known that resistance training on one side of the body will increase strength on the opposite side as well, even if that side is untrained. This is called the “contralateral strength training effect” and it usually results in the untrained side getting half the strength gains as the trained side.
Interestingly, unilateral resistance training will also reduce the severity of delayed onset muscle soreness (“DOMS”) on the untrained side. Not that surprising in theory I guess, but cool.
This is strong evidence that at least part of what makes you stronger (and more resistant to DOMS) after weight training is changes to the brain and central nervous system. These central adaptations are probably in the nature of learning a skill - e.g. the skill of firing the right muscle fibers at the right time in the right amounts to overcome the resistance. Despite the unfortunate term “muscle memory”, these skills reside in the brain not the muscle, and can therefore be transferred to the opposite side of the body.
Can the benefits of stretching transfer from one side to the other? Well, I’m not sure there are any benefits to stretching. Maybe we should consider its detriments. It has been shown many times that static stretching will tend to reduce power output. But can this power outage transfer to the non stretched limb? The answer appears to be yes! Now you can get the power reducing benefits of stretching in half the time! Static stretching, now sucking bilaterally!
Pain and Coordination
I don’t have any studies showing that unilateral interventions to address pain or coordination will have contralateral effects. However, I suspect that there are some relevant studies out there somewhere, and until I find them here is some speculation based on the logic gleaned from the studies above.
I would guess that coordination training on one side of the body will have at least some transfer to the other side. If strength transfers from side to side because it is a skill that resides in the brain, then surely coordination would transfer as well. To use a simple example, I am pretty confident that Rafael Nadal would not exactly be starting from scratch if he started playing tennis with his right arm.
So if joint mobility drills on your right shoulder alter its movement in some beneficial way, you can expect some improvement in your left shoulder mobility as well.
Interestingly, Moshe Feldenkrais often gave one sided movement lessons based on the assumption that if the brain learned anything useful on one side, it would apply that knowledge to the organization of the other side as well.
In regard to pain, I would guess that once the brain “learns” that a particular activity or therapy affects pain for better or worse at a particular joint, this learning will affect how the brain responds to similar stimuli at the opposite joint.
So, for example, if you develop right knee pain from running, perhaps this would be an independent risk factor for left knee pain. And inversely, if you do some form of therapy to help with your right knee pain, this may have some independent benefit for the untreated side. Of course this is speculation, but if anyone has a good study on this issue, or some anecdotal evidence, please share in the comments.
So what can we do with this interesting information?
First, remember that the target for your exercise is to some extent located more centrally than the parts doing the physical work.
Second, keep these rules in mind next time you have an injury and want to keep training. Anything that makes you stronger, more coordinated and more painfree on one side is likely to have at least some similar benefits on the other side, even if it cannot currently be trained.