PNF stretching
So what is PNF stretching?
PNF stands for “proprioceptive neuromuscular facilitation”. It is a concept in which the body uses its proprioceptive system to encourage (facilitate) or discourage (inhibit) certain movements (muscle contractions).
And what is proprioception? Proprioception is the body’s ability to be aware of itself in space. In other words, it’s your body’s way of knowing that if you put your arm up in the air, it’s up in the air (relative to the rest of your body). Proprioceptors are found throughout the body in muscles, tendons, and joints. Combine proprioception with information from your vestibular (balance) system, and your brain has the ingredients for spatial awareness, movement, and changes in speed.
A little about the history of PNF
In the 1930’s and 1940’s, a neurophysicist named Herman Kabat was working with individuals with neurological conditions (such as cerebral palsy). He found that stimulating distal body parts (ie, the ankle relative to the hip) allowed proprioceptors in the more proximal body parts (ie, the hip relative the ankle) to engage.
For the population he was working with, this was huge, as many had deficits in proprioception from their neurological conditions. It allowed things that were otherwise seemingly asleep to wake up. Through his work with these individuals, he determined that moving limbs and body parts in concert (using the whole leg, for example) versus in isolation (flexing just the hip) produced the desired results (helping the muscle lengthen).
These concepts of the distal affecting the proximal and moving the body in concert versus isolation also apply to those of us who are healthy. It’s a more functional way of moving, as the body rarely works in isolation. And for those with an interest in (or awareness of) functional training, PNF is the basis for it. That being said, how exactly does it work? I’m glad you asked.
Mechanisms of PNF stretching
PNF is based on two concepts: autogenic inhibition and reciprocal inhibition. Here’s the science of it. Autogenic (self-generating) inhibition (restriction or hindrance) is a reflex that occurs when a muscle is under high tension. Such a tension might occur when a muscle is being stretched towards its end range (for example, the hamstring).
When the body (specifically golgi tendon organs) senses this increased tension, it allows some slack in the muscle to prevent it from tearing. Another name for this reflex is the inverse myotatic stretch reflex.
Reciprocal inhibition is the concept that, as one set of muscles act on a joint (for example, the quadriceps contract to straighten the knee), the opposing muscles (in this case, the hamstrings) lengthen to allow this action to occur without injury.
Both of these can be utilized to lengthen a specific muscle group. One reflex that stands in the way is the myotatic stretch reflex. This reflex causes the muscle being stretched to tighten, or contract, to resist injury, with the goal of trying to keep the muscle the same length (not having it stretch too far). Think about having your reflexes tested at the doctor’s office. This is an example of a myotatic stretch reflex.
So you have one reflex that allows the muscle to relax and lengthen to prevent injury (autogenic inhibition or the inverse myotatic stretch reflex), and another that contracts and tightens that very same muscle to prevent injury (myotatic stretch reflex). How can both be true?
The answer is that both of these reflexes work together.
You don’t just get one or the other (at least, not in healthy individuals). Think about how out of balance you would be if only your quadriceps worked to straighten the knee, but never the hamstrings to bend it. You need to be able to both end and straighten your knee, just as you need to have reflexes in your body that oppose each other.
In order to have a healthy, balanced body, you need your reflexes to react in a balanced way that allows you to safely move through space, lift, bend, twist, run, jump, and stretch. How much and for how long these reflexes work depend on the specific motorneurons that regulate these reflexes in the spinal cord.
In healthy individuals, they determine a safe balance and which side (if any) eventually wins out. Some have a longer latency, meaning they take longer to set in. Other reflexes occur quite quickly in response to a stretch (ie, the myotatic stretch reflex). So how long you hold a stretch or whether or not you contract a muscle will affect these different reflexes.
Static stretching (see last week’s blog post) invokes the inverse myotatic stretch reflex to work its magic. PNF stretching utilizes specific durations and muscle contraction intensity to target the assistance of autogenic and reciprocal inhibition to achieve its goals. And dynamic stretching (which we’ll discuss next week) bypasses certain reflexes based on the duration of the hold.
How to perform PRN stretching
There are three main ways to perform PNF stretches. Since I’ve done most of this blog referring to the hamstrings, I will continue to use them for my examples.
- Hold-relax: Have a partner raise your leg into the air with your knee straight until a stretch is felt in the back of the leg (not pain). Make sure your other leg is straight and flat against the floor or mat table. It’s easiest to prop your ankle onto your partner’s shoulder for support. Durations vary for how long to hold the stretch here from as little as 7 seconds to as much as 20 seconds, depending on the study. After this initial stretch phase, push your leg down onto your partner’s shoulder (not a maximal contraction, as this is not necessary per Feland et al, but closer to 20-50% maximal contraction). Hold this submaximal isometric contraction for 6-10 seconds (Rowlands et al). Then relax your leg, and have your partner stretch your hamstring further (passively). Repeat this cycle of holding in a relaxed state, contracting your hamstrings, and relaxing to stretch further for 3-5 cycles. This stretch primarily utilizes autogenic inhibition.
- Contract-Relax: This is almost identical to the above; however your partner should hold your leg in the air with his/her hands versus propping your leg on his/her shoulder. This is because your leg will go through an isotonic (same muscular tone) contraction versus an isometric (same muscle length) contraction. In an isotonic contraction, you will push your leg all the way back down to the bed or mat table while your partner offers resistance. Again, 20-50% of maximal contraction should be sufficient here. The contraction times and stretch times should be the same as above. Repeat this cycle 3-5 times. This stretch primarily utilizes the inverse myotatic stretch reflex.
- Contract-Relax-Agonist Contract (CRAC): The set-up for this stretch is almost the same as for the Hold-Relax technique; however you begin by actively performing a straight leg raise to lift your leg straight into the air until a stretch is felt in the back of the thigh. Then, rest your ankle on your partner’s shoulder. The hold times from above will apply here, as well. The hamstring contraction you perform is isometric (versus isotonic), just as in the Hold-Relax technique. After your first isometric contraction, you now actively lift your leg (potentially off of your partner’s shoulder) into a greater hamstring stretch for 7-10 seconds, with your partner coming to meet your leg in the new position. Repeat this cycle 3-5 times. This technique primarily utilizes reciprocal inhibition.
PNF stretches can be used with other muscle groups, as well, such as the quadriceps, plantar-flexor muscles of the calf, biceps, triceps, and pectoralis muscles, to name a few. Having a partner to help you is recommended, although there are some creative ways to perform some of these techniques on your own.
For example, you can lie in a doorway with your leg propped up on the wall for a hamstring stretch. When going further into the stretch (after the contraction), just slide your bottom closer to the wall while keeping your leg straight. You are limited to either the Hold-Relax or the CRAC technique here, but it’s nice to have options for PNF stretching when you don’t have a partner.
Evidence
I found no articles that determined PNF stretching did not result in some increased range of motion or flexibility gains. Some showed that a longer contraction time resulted in more gains (Rowlands et al). Others showed gains with PNF versus static stretching after 60 minutes of exercise (Funk et al). Another revealed increased range of motion with the CRAC over the Hold-Relax technique, although both yielded positive results (Nagarwal et al).
As with most studies, there are some drawbacks, such as all-female or all-male groups, small sample sizes, and/or particular age ranges that limit generalizability. But again, I found none that said that PNF stretching didn’t work.
What this means for you is that there may be a different way for you to stretch that increases your gains or that you find more pleasing. Everyone is different, and having different tools to reach the desired outcome is paramount as a healthcare practitioner, and as a dancer or athlete. That being said, I encourage you to try some of these techniques, and see what works for you and what doesn’t.
Next week, we’ll conclude our series on stretching with dynamic stretching. Stay tuned!
Resources:
https://en.wikipedia.org/wiki/Proprioception
https://en.wikipedia.org/wiki/PNF_stretching
https://en.wikipedia.org/wiki/Autogenic_inhibition_reflex
https://en.wikipedia.org/wiki/Reciprocal_inhibition
http://www.healthline.com/health/fitness-exercise/pnf-stretching#2
http://bjsm.bmj.com/content/38/4/e18.full Feland et al
http://www.worldacademicunion.com/journal/SSCI/sscivol04no01paper03.pdf Nagarwal et al
http://www.danielfunk.com/published-work/ (scroll down to 2003)