My Inner Nerd: PNF Stretching

So as part of my school work this past semester, in one of my classes I needed to write a final research project on basically any topic having to do with motor learning (which is nearly anything in the realm of body/movement) and relate it back to the early influential theories that we learned in class. For my topic I chose to do some research on PNF stretching, since I’ve been interested for a while in the theories behind that stretching technique.

I posted on Facebook about how excited I was to delve into the topic and got a bunch of feedback that people were interested in learning a little more about the science of stretching, so I decided I’d share some of the findings I found most interesting. I’m going to try to explain this as clearly as possible, but I’ve been swimming in research papers about PNF for the past month or so, so it’s hard to see this with fresh eyes. If you’d like clarification on any of this, feel free to ask! You can also feel free to compliment my ridiculously awesome drawings, which were custom done for this entry on my iPad.

PNF is a type of physical therapy that was developed in the 1950′s, and PNF stands for “proprioceptive neuromuscular facilitation”. Built into our bodies are special types of receptor cells (called proprioceptors) that help us to maintain awareness of our bodies in space—which is also called “proprioception.” For instance, your sense of proprioception tells you where your hand is in relation to your foot, even if you are holding them out in the air. Proprioception is what allows you to close your eyes and touch your finger to your nose.

This physical therapy technique was later adapted to help people who were working on increasing their range of motion in movement, and flexibility and is referred to as PNF stretching. This basically translates into the idea that you can take advantage of proprioceptive cells to facilitate the connection between mind and body and increase range of motion and strength.  One of the most common PNF stretching techniques is known as the “hold-relax” stretch, which is illustrated with naked stick figures below (yeah, I forgot to draw them with clothes on, but we’re all adults here right?). I’m using a hamstring stretch for this example.

• Step 1 would be finding a comfortable passive stretch, bringing your leg towards your face while keeping both hips square and your butt on the floor.
• Step 2 is contracting your hamstring to bring your heel down towards the floor (with a straight leg) as resistance is applied. The resistance can be from a partner, as shown, who is pushing against your heel as hard as you are pulling it down, or you can apply the resistance yourself, using your arms to pull back on the leg. The idea is that the resistance is equal to the force you’re exerting in the contraction so that the leg stays in place even though it is doing work. This is also called an isometric contraction.
• Step 3 is relaxing the hamstring. Assuming you find an increase in flexibility from doing steps 1-2, you would move your leg closer to your face until you find the new “comfortable” passive stretch, and then repeat steps 1-3 again.

The way that PNF stretching supposedly works is by taking advantage of a certain type of proprioceptive cell, called the golgi tendon organs, that live in the tough, somewhat stretchy tendons that connects muscle to bone. One of the function of the golgi tendon organs (GTOs for short) is to sense when the muscle that they’re attached to is being stretched. In my awesome drawing below, I’ve circled the GTO so you can see more clearly where they live on your body. The red is muscle, the white is bone, and the little circled thing is the tendon that the GTO is inside of.

So, if a GTO senses too much tension on the muscle (while the muscle is working and contracting), then as a safety mechanism, they force the muscle to relax, which causes lengthening and stretch. An example of this is that if you’re lifting too much weight for your muscles to actually handle, they will relax, forcing you to drop the weight, so that the muscles don’t risk tearing. This is called “autogenic inhibition” and you may hear personal trainers and gym rats toss this wordage around.

The theory behind the hold-relax stretch, in particular, is that by forcing your muscles to contract, and applying the resistance, you are increasing tension and causing the golgi tendon organs to freak out, and go, “HEY! HEY! STOP THAT! WE’RE GOING TO MAKE YOUR MUSCLES RELAX NOW!” and then they do, so that when you relax your hamstring, it’s more relaxed then you could make it consciously, and you get a deeper stretch. This is the theory that this whole PNF stretching this is based on. Now, what people have found, and what the current research tells us, is that… drummmmroll! This isn’t actually the case. There’s a couple of studies that support this conclusion, and I won’t get into too much detail here as to how they disprove the GTO theory (because you might all fall asleep), but basically something else is going on that is causing PNF stretching to be effective in increasing flexibility.

I found this incredibly interesting: we don’t actually know HOW it works. We can’t really explain it. One of the front running theories is that by causing yourself major discomfort through the hold-relax cycle, you are actually just increasing your tolerance for pain and discomfort in the stretching phases. There was one awesome study that  supports this theory pretty strongly. They measured flexibility in a leg and then had subjects do some PNF stretches on their arm. Then, they measured the range of motion in their leg afterwards. And found that even though the PNF stretches hadn’t been done on the leg, flexibility increased in the leg! So this theory is that your flexibility increases with PNF stretching because you condition your brain to deal with the pain/discomfort signals.

Beyond the “how,” the research on this topic, and stretching and flexibility in general, is all over the place in terms of what is recommended, what is considered effective, and what is the “best” way to stretch. The reason for this is that there are a LOT of variables to try to keep constant when you run an experiment. For example, people like ballerinas, who tend to be very flexible, won’t have the same flexibility gains as people who have never stretched a day in their lives. Another is that PNF stretching has a lot of variables built into it, which makes it difficult to compare against other types of stretching in large studies: how long do you hold each step? How hard are you fighting the resistance? Should you be working as hard as possible and shaking because you’re resisting and contracting so much? How many times a week should you stretch, and how many hold-relax cycles should you be doing each time? What joints are you looking to grow your flexibility in?

What I’ve found through the majority of the research I did is that if you are considering adding PNF stretching to your regular flexibility training regimen, these are some things that you may want to think about:

• I hope that most of you know this already, but you should NOT train for flexibility prior to pole- this has been found by many studies to reduce explosive power in the muscles that are stretched, and that reduction in strength lasts for at least 15 minutes (Franco, 2012). If you are going to be working tricks that require flexibility and don’t have full range of motion when you’re cold, use dynamic stretch to warm up instead
• Most studies seem to agree that if you are doing 4-5 reps of PNF 3x a week for 30 days, you will increase your flexibility just as much as if you did more repetitions. To maintain flexibility, studies recommend doing at least 1 session a week (of 4-5 reps), but to increase flexibility, you should maintain 3 sessions a week
• Holding each step for 3 seconds is just as effective as holding each step for longer
• Holding step 2, the isometric contraction phase, at about 20% of complete effort, is just as effective as using more effort

I found that there were a lot of studies that disagreed on whether or not PNF stretching was more effective than static stretching, where you simply hold a stretch with no resistance. I was pretty surprised by that, since in my experience, people find PNF stretching to be really, really effective. I would say that no matter what type of stretching technique (or combinations of them) that you actually use, you should definitely do some research and experiment a little to see what works for you, and what you feel most comfortable with. More then a few studies did suggest that risk of injury and possibly pulling muscles is likely to be more related to stretch rate (or how quickly you go to maximum range of motion) and not necessarily technique, so no matter what type of stretching that you do, it’s wise to be gentle with yourself as you ease further and further into your full range.

I hope that was interesting for some of you guys! Feel free to leave questions/thoughts in the comments, and if I can help to clarify any of this stuff, I absolutely will.

Tomorrow’s post: Thursday Tricks…