Both Sides Now

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.

Resistance Training

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.

Swearingen sings Joni! With a perm!


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.

Third, it’s fun to make fun of static stretching, and Al Swearengen in a perm singing Joni Mitchell.

10 Responses to “Both Sides Now”

  1. Just before I read this post I was reading some thoughts by Robert Schleip PhD. on Idiopathic scoliosis. He speculated that at least some cases had their start in inaccurate proprioception. The possibility that our internal bodymap is playing such a large role in what our external body actually looks and feels like has to change the way we work on others and work out ourselves.
    One question I do have is how much working on one side would have on someone who has suffered a stroke. Definately more research is in order.

  2. Jeff,

    Thanks for the Schleip speculation and good questions. More research is needed …

  3. I was taught in massage school that if someone has an injury too fresh or painful to touch on one side, that work on the corresponding part on the other side would help ease the pain. Refreshingly, my teacher said, “I don’t know why this should work, but in my experience it does” and left it at that.

  4. Todd,

    I was curious about your ideas on stretching.

    You prove that static stretching prior to workouts will decrease performance, but if instead of static stretching someone were to utilize dynamic stretching it actually leads to an increase in muscle performance. (

    Alternatively, stretching preformed under a load can also be useful. An example would be to “pry” open the hips while preforming a barbell squat using moderate weight. This kind of stretching leads to increased muscle activation in such areas as the hips/glutes.(personal experience) The hip region is particularly benefited with stretching.( this article explains how a “compliant”, flexible hip flexor muscle is required for sprinting. To achieve a compliant tendon requires one to stretch pre and post workout.

    While stretching, it is important to distinguish between different methods including static, dynamic (body weight), dynamic (under load); also the muscle region being stretched is important. Static stretching for the thighs and lower legs will decrease performance as you have said, however statically and dynamically stretching the hip flexor region is extremely important.


  5. Todd,

    Moshe often used the example of writing, the learned pattern of the letters resides in the CNS not the dominant hand/limb. So we could all write with our non-dominant hand (or with a pencil tied to your nose) if need be, albeit messy until practice refines the movements.

  6. Also interesting to tie this in with your previous post on symmetry. So, if one is told they have a ‘muscle imbalance’ on one say the left side and they then strengthen the left side in hopes of ‘matching’ the right side, they will be chasing a moving target as the right side continues to gain strength on the coat tails of the left side 🙂

  7. “Interestingly, unilateral resistance training will also reduce the severity of delayed onset muscle soreness (“DOMS”) on the untrained side. ”

    When I first read this, I thought… well of course training in one side wont make the other sore! hahahaha

    Didn’t click right away.

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