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.

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.


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