Today’s blog is a challenging one and I’m not sure where to start. I feel a little background is necessary so I will go with that.
Two weeks ago, I submitted the first draft of my research on the science of stretching – a full 150 pages covering all the topics surrounding stretching and flexibility, from tissue mechanics to proprioception and stretching tolerance. When I created this blog earlier this year, my plan was to provide content that resembled the flow of the book – beginning with about Chapter 3 (biomechanics) and continuing sequentially through the last chapter (effectiveness of various stretching protocols). To date, the blog is still hovering around Chapter 3 and 4.
Recent events, however, have urged me to jump right to the end. I’ve had some interesting twitter conversations, I’ve been interviewed by Brooke from Liberated Body (link will be updated with interview when it is posted in late July or early August 2014), and I’m leaving for Europe today to film 18 classes for Udaya which apply my research to the practice of yoga and will eventually be packaged into a DVD set. So, how do I leap from Chapter 3 to Chapter 10 while maintaining the ease of the blog format?
Anyone? Anyone? Right – so here goes.
Proprioceptive Neuromuscular Facilitation (PNF) describes a stretching style that recruits muscle activation of the target muscle somewhere during the stretching process, usually right before. There are at least 9 different methods with slightly varying techniques and durations of contractions (concentric, isometric, and eccentric), holding times and subsequent relaxation/stretching procedures. A detailed review of the methods is too much for this post, so just understand the PNF involves muscle contraction followed by muscle relaxation – often referred to as “contract relax”.
The most common method of PNF stretching that I have encountered in the world of yoga comes in the form of an “adjustment” by the teacher. For example, during a supine single leg hamstring stretch (Supta Padangusthasana or Recline Big Toe Pose) the teacher provides resistance by firmly holding the leg while the student strongly contracts her hamstring for 6 seconds, pushing leg against the direction of the stretch. The teacher then instructs the student to relax and the teacher passively stretches the student into a new and greater range of motion. Everyone is impressed by how effective the method is – the teacher is brilliant and the student is pleased by her sudden flexibility.
Unfortunately, the momentous increase in range of motion is transient. It is temporary. Within hours ROM will be what it was. The acute improvement of ROM during PNF stretching is really just an illusion. No biomechanical adaptations to the tissues occured instantaneously. Short tissues did not suddenly become long. So what happened?
To put it simply (very very very simply), the nervous system allowed the extra range of motion. The nervous system is a powerful limiter in flexibility, only allowing you to perform joint positions that it trusts you won’t injure yourself in. Flexibility is actually much more a factor of stretching tolerance than biomechanical properties.
Stretching tolerance is an issue of sensation, not mechanics. Improvements in range of motion (increased flexibility) are due to greater tolerance than muscle or connective tissue length (range for all you regular readers).
I understand this is a difficult concept to digest. It is not what we learned in yoga teacher training. We learned:
1. Stretch your hamstrings.
2. Make them longer.
3. Put your foot behind your head.
Boom.
As a biomechanist, I really didn’t want to accept stretching tolerance. I wanted to reject and dispute it. But the evidence was there and I was getting nowhere in my writing as long as I refused to move forward. But when I finally was willing to unlearn what I knew, stretching science started to make a lot more sense.
For example, if stretching tolerance is the determinant for ROM, then passive stretching methods like the classical yoga version of the above hamstring stretch and the PNF method are essentially the same thing. They have similar and comparable outcomes when it comes to improving range of motion. Both require time and frequency to gain substantial improvement.
The difference between the two lies in something other than range of motion: muscle activity and motor control. The Minshull (2014) study, compared the effects of PNF stretching with passive stretching. After 8 weeks of training, subjects in the passive group showed greater electromechanical delay (the time between muscle activity measured by electromyograph (EMG) and change in joint position), compromising swiftness and efficiency of movement while the PNF group maintained electromechanical delay. Passive flexibility training comes at a cost. If PNF stretching can preserve neuromuscular performance, particularly at end ranges of motion where delays in force transmission are a risk for injury, then it may be considered a more effective stretching method.
To be clear, I am not saying you should abandon all passive stretching in your yoga practice (I am actually huge proponent of restorative yoga with very deliberate prop placement). What I am saying, or rather asking you is this: What are you going to do with this new range of motion? If you don’t have strength and optimal muscle function at the new range of motion, how does it serve you?
My concern with yoga is that the community, as a whole, is so focused on stretching for flexibility that we forget about stretching for function and performance, which must involve neuromuscular activity.
How does that look in practice? Resistance stretching. There are many ways to go about it and will make great content for a future blog post, a class or a workshop, or an upcoming podcast. 🙂 For now, I hope I have left you with plenty to consider.
I also have a lot more to say about PNF stretching, particularly about the proposed, yet dubious, neurophysiological theories involving stretch receptors that we are taught in yoga. But because an education in tissues mechanics is essential to stretching science, my next post will go back to Chapter 4. Maybe.
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Konrad, A., & Tilp, M. (2014). Increased range of motion after static stretching is not due to changes in muscle and tendon structures. Clinical Biomechanics. doi:10.1016/j.clinbiomech.2014.04.013
Marshall, P. W. M., Cashman, A., & Cheema, B. S. (2011). A randomized controlled trial for the effect of passive stretching on measures of hamstring extensibility, passive stiffness, strength, and stretch tolerance. Journal of Science and Medicine in Sport, 14(6), 535–540. doi:10.1016/j.jsams.2011.05.003
Minshull, C., Eston, R., Bailey, A., Rees, D., & Gleeson, N. (2014). The differential effects of PNF versus passive stretch conditioning on neuromuscular performance. European Journal of Sport Science, 14(3), 233–241. doi:10.1080/17461391.2013.799716
Sharman, M. J., Cresswell, A. G., & Riek, S. (2006). Proprioceptive Neuromuscular Facilitation Stretching Mechanisms and Clinical Implications. Sports Medicine, 36(11), 929–939.
Weppler, C. H., & Magnusson, S. P. (2010). Increasing muscle extensibility: a matter of increasing length or modifying sensation? Physical Therapy, 90(3), 438–449. doi:10.2522/ptj.20090012
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Thanks for this Jules – it’s great! You write “The most common method of PNF stretching that I have encountered in the world of yoga comes in the form of an “adjustment” by the teacher.” but what about students activating PNF themselves by repeating the poses against their own resistance? This is the hallmark of Desikachar’s Viniyoga approach. Can you comment on this?
Yes, of course. It does not matter if you student creates her own resistance, uses a prop or piece of equipment, or accepts the assistance of another. The principles are the same. I only gave that example because it’s common and many people don’t know it’s called PNF.
Yes, yes, a thousand times yes. Thank you for this. So many of my clients perceive the ‘fix’ for their posture to be a massive static/passive stretch in the opposite direction, without consideration of their body’s ability to actually maintain that range functionally. I actually used the phrase “I don’t want to poop on your parade” yesterday to try and explain this to a client. Next time I will direct them to your blog 🙂
Oh yeah, Sarah!!! Lovely to see you comment here. Thank you so much! I will continue with this topic in the future so there will be more. 🙂
Excellent blog, Jules! I love that you’re challenging assumptions, and diving into gray areas of yoga anatomy. While static stretching may have little to no effect on resting muscle length, I would love to hear your thoughts on the effects of static stretching on ligaments and fascial tissue, and whether you believe there is a benefit.
Hi David! Thanks for commenting. I am totally working my way to a very long conversation about your inquiry. I have addressed it in my book, which should be available next year. I will tell you this….the obsession with static stretching on ligaments must end. 🙂 Keep reading the blog and I will eventually get to explaining. There are so many things to consider and some education in tissue mechanics is necessary to effectively make my point.
Hi Jules , all u are expressing sounds great . One Question ? How does someone who doesn’t have the proper training in these modalities / concepts apply this rationale ? ( devils advocate ) .
“No biomechanical adaptations to the tissues occurred instantaneously. Short tissues did not suddenly become long.”
Now Jules you have hit the nail on the head, once again. But only once. Let me hammer this point home a little more as you are so correct. Tap , tap ,tap…!!! Bang..bang….bang!!!!! The problem is with the “S” word and with the “L” word. Most in the general population and most of those who get some sort of training with a certificate to teach some modality of movement therapy and of manual therapy as well, are taught that “stretching” means that the procedure thang we do (“stretch”) means that we are increasing our body’s flexibility to go further into a deeper range of motion at say a particular joint. Fine, we can live with that, somewhat, I guess.
What is not made crystal clear is that this is not designed nor meant to make the tissues physically longer. Sure you can make tissues physically longer by tearing them and depending on what source you read and the tissue tested and under what conditions, that ranges from somewhere between 4-12% beyond resting length. Literally stretching tissue beyond this amount results in injury and tissue damage and also you get the added benefit of scar tissue possibly accruing during the so called healing process, no fun! But wait! Then you get to learn to stretch scar tissue! Way more not fun!
The “L” word and the “E” word (read “lengthening” and “elongating”) should most likely be dropped from all movement and manual therapies. I know, I know….SOME modalities (I am told and have personally heard those words used) claim to “elongate” your muscles and “lengthen” them by their methods. You see this is the difference between marketing (read “selling it”) and science (read “researching it”.) Is it not strange how human connective tissue of all types behaves differently depending upon whether you are selling something or researching something? Why you would almost tend to conclude that human connective tissue has the properties of a chameleon, depending not on what it is under, but on who is working with it.
(Ok so I was being snarky and bordering on sarcasm. Don’t worry! Just like increased ROM from stretching, it won’t last long.)
“Stretching tolerance is an issue of sensation, not mechanics. Improvements in range of motion (increased flexibility) are due to greater tolerance than muscle or connective tissue length (range for all you regular readers).”
In one word, well actually five words, when it comes to the “why” of increased ROM from stretching:
YOU GET USED TO IT! (GASP!)
And that is why you have a greater tolerance, not a change in the properties of the connective tissue being worked with (notice I did not use the “S” word.)
“It is not what we learned in yoga teacher training. We learned:
1. Stretch your hamstrings.
2. Make them longer.
3. Put your foot behind your head
Boom.”.
Well, what was missing in yoga teacher training and most other so called trainings was step 4:
4. Call 911. (If you haven’t fainted.)
“But when I finally was willing to unlearn what I knew, stretching science started to make a lot more sense.”
One must give up what one thinks one knows in order to see something new. It is not what one looks at that matters, but what one sees.
“What are you going to do with this new range of motion? If you don’t have strength and optimal muscle function at the new range of motion, how does it serve you?”
There are two factors which combine to form one functional concept: One factor is stability, the other factor is dispersion/compression. Together these form the boundaries of the functional concept called adaptability.
Your body has to have some amount of stability. It has to also have some degree of dispersion/compression of the forces both that come from the outside and/or from the inside. Take the blow up tubular signs you see at some oil change stations. You blow air up the advertising tube and it magically goes up and then collapses. Blows up, comes down. Blows up, comes down. Blows up… well you get the idea by now. Your kids love it. But it’s just an advertising gimmick. I mean it doesn’t do anything other than that. At least an elevator delivers stuff when it goes up and comes down. Stability? It’s a loser!
Dispersion/Compression? I don’t think it could produce even a really quiet burp or a silent fart.
Come to think of it that is one thang that flexible people have in common with advertising tubes…they get attention. But getting attention will not get you through life; it will just get you more attention. And those who do super flexible asanas in order to impress their teachers are just,,, you guessed it, brown posers!
So every body has to have some stability (so you don’t collapse like the advertising tubies) and some way to accommodate dispersion/compression forces while maintaining said stability. And those together amount to a degree of adaptability and adaptable people survive far better than flexible ones.
I think movement and manual therapists should take a vow to not use the “S” word upon penalty of surrendering our certificates or at least attending hours of rehab CEU’s and training. But that (the training) was where the problem started in the first place wasn’t it?
Personally I really like the words stretching and flexibility.
And I like increasing flexibility.
But I also like increasing/improving controllability.
Sometimes I work on increasing how far apart I can get my feet then I follow it with “controllability.” Other times I work on controllability at the same time as I work on increasing the distance between my feet.
One of the, for me, interesting things about passive stretching, even in a non-restorative context, is that it can help with body awareness. And that is something that should go hand in hand with controllability. Want better control, improve sensitivity at the same time.
And as for the difference between sensitivity and perception, the two can be calibrated so that perception better matches the actual position of the body.
Thank you for this Jules. I am not even remotely in the fields of biomechanics nor yoga, please know that. I read this post today after having listened to the podcast interview you did on this subject with the lovely woman from Liberated Body. I am a mom of 3 who has dealt with diastasis recti since my first pregnancy over 18 years ago. I have been Providencially nudged in the direction of biomechanics, and I’m so thankful for people like you who are educating me on how my body moves and how my brain connects to what my body can and cannot do! I look forward to reading more on your site 😉
Love this!! As an CYT who also works in Physical Therapy, I’ve realized there are numerous nerve gliding techniques (used in physical therapy) that just so happen to be the same movements performed in many Yoga asanas. Since our nerves glide and stretch independently of our muscles, this is worth noting in addition to the proprioceptive neuromuscular components. Would love to see something written on this concept in the future! Great article on PNF here! 🙂