What is the difference between PNF stretching and FRC's PAILs/RAILs?

Recently, there has been an explosion of practitioners using the Function range conditioning method (FRC) for gaining mobility. I am not exempt from this group, as I have taken both the primer weekend course and the full Level 1 FRC course in order to gain a more broad tool box for use with athletes and clients on how to minimize pain and optimize function.

Also recently, I was asked, what the heck is the difference between Proprioceptive Neuromuscular Facilitation stretching (PNF) and the Progressive and Regressive Angular Isometric Loading (PAILs/RAILs) technique that the course uses to gain strength and range at the end ranges of joints?

To dive deeper let's do a quick lesson in muscle physiology. First, how does a muscle contract?

A muscle is made up of many individual muscle fibres, depicted in the picture below as long tubules that are either red or blue. The muscle fibres (inside each packaging of one tubule), are all the same type of muscle fibre... Type 1 (slow twitch), Type 2a (fast oxidative fibres), and Type 2b (fast twitch fibres).

So, Motor units can be divided into three types based on their physiological characteristics

When a person decides to make a movement, their motor cortex in their brain sends a signal down the spinal cord, which activates Type 1 muscle fibres preferably, when the contraction is small, and not needing much force. When the muscle force required increases, more and more of the fast Type 2 fibres are activated in order to get more muscle tissue to aid in the contraction (this is essentially the Size Principle).

To add to the complexity, there are two protective mechanisms built into our muscles, one called a Golgi Tendon Organ (GTO) and the other called a Muscle Spindle (MS).

GTO's detect muscular tension through the pulling on the tendon that exists on either end of every muscle, via their location in the tendon. If the tension on the tendon is too great, there is a neural mechanism/pathway in place to cause the opposite muscle to contract, and the same side muscle to relax. Again, this is protective so that the person doesn't tear their quadriceps (in the example below!).

MS's are located within the muscle belly of each muscle, and detect a stretch in the muscle. The demonstration in the image below is of a tapping of the patellar tendon, a reflex test that is often done at the doctor's office, which causes a quick stretch in the quad, where MS exist. When the stretch occurs, MS's send information to the spinal cord, which directly synapses back to the muscle to cause the muscle to contract (the kicking of the knee jerk reflex as a response to the tap).

What is PNF stretching and PAILs/RAILs? How does all of this relate to stretching, and the difference between PNF and PAILS/RAILs style stretching? I've summarized them below.


⁃ Stands for proprioceptive neuromuscular facilitation

⁃ Involves stretching the person to current soft end range and asking them to relax, then getting them to kick against the stretch before relaxing again

- In the example image above, the individual is trying to get more range in their hamstrings, so they start by relaxing into the hamstring stretch (with a cord to pull them passively into a deeper stretch), then they contract their hamstring against the cord, followed by relaxing again and getting into a deeper hamstring stretch.

⁃ The kick of the hamstring in this example turns off the muscle spindle from firing (whose job is to detect too much stretch and make the muscle contract to prevent injury at end ranges)

⁃ The disadvantage to PNF is that it’s mostly passive throughout the stretch (besides the short period of hamstring contraction), but most activities and sports require you to be active throughout a range of motion

⁃ For this reason, no strength is built (the definition of mobility is strength in a range of motion), therefore it is possible that the gains in range of motion are more transient and less chronically achieved.


⁃ Stands for progressive and regressive angular isometric loading

⁃ Involves a relax, then push into the stretch (like PNF, to shut off muscle spindles) then a pull into the other range to pull into more ROM

⁃ Because you active motor units in the end range to create the ROM, you create more strength at end range that lasts (strength + ROM = superior)

⁃ Also, there a mechanism in the muscle called reciprocal inhibition, so when you pull the quad into deeper hamstring stretch with the RAILS, you activate this mechanism which is supposed to deactivate hamstrings from contracting

So, which one to chose?

Based on the principles that govern both PNF and PAILs/RAILs, I would recommend going the PAILs/RAILs route. This is because, not only do they work for increasing joint range of motion immediately (with a few exceptions), they use the antagonist muscle to pull the joint into a deeper ROM, which causes activation of motor units, and if done at a high enough intensity, can cause increases in strength in the joint.

For resources and explanations on PAILs/RAILs techniques, the best place to go is YouTube, in my opinion. I have found a multitude of videos and ideas on how to set up effective PAILs/RAILs stretches for specific joints, and many videos include a very detailed explanation.

Good luck with your mobility!


1. McArdle, W, Katch, F. I., Katch, V. L. (2015). Exercise Physiology: Nutrition, Energy, and Human Performance. Wolters Kluwer Health/Lippincott Williams & Wilkins

2. Jansson, E., & Kaijser, L. (1977). Muscle adaptation to extreme endurance training in man. Acta Physiologica Scandinavica, 100(3), 315–24. https://doi.org/10.1111/j.1748-1716.1977.tb05956.x

3. Wilson, J. M., Flanagan, E. (2008). The Role of Elastic Energy in Activities with High Force and Power Requirements: A Brief Review. Journal of Strength & Conditioning Research, 22(5), 1705–1715.

4. Staley, C. (2001). Physicall Incorrect: Charles Staley’s Approach to the Training Sciences.

5. McHugh, M. P., & Cosgrave, C. H. (2010). To stretch or not to stretch: The role of stretching in injury prevention and performance. Scandinavian Journal of Medicine and Science in Sports, 20(2), 169–181. https://doi.org/10.1111/j.1600-0838.2009.01058.x

6. Bishop, D. (2003). Warm up I: potential mechanisms and the effects of passive warm up on exercise performance. Sports Medicine (Auckland, N.Z.), 33(6), 439–454.

7. Fletcher, I. M. (2010). The effect of different dynamic stretch velocities on jump performance. European Journal of Applied Physiology, 109(3), 491–498. https://doi.org/10.1007/s00421-010-1386-x

8. Chew, M. (n.d.). Permanent Pain Cure - ELDOA: Breakthrough way to heal your muscle and joint pain for good.

9. Power, G. a., Dalton, B. H., & Rice, C. L. (2013). Human neuromuscular structure and function in old age: A brief review. Journal of Sport and Health Science, 2(4), 215–226. https://doi.org/10.1016/j.jshs.2013.07.001

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