Functional Range Conditioning: PAIL’s and RAIL’s

By Skyler Zarndt MS, ATC, CSCS

So far in this Functional Range Conditioning review, we have discussed stretching, progressive adaptation, Controlled Articular Rotation’s (CAR’s), and now we will talk briefly about PAIL’s and RAIL’s.

PAIL’s and RAIL’s are terms that are unique to FRC, such as CAR’s is unique as well.  PAIL’s and RAIL’s stands for Progressive Angular Isometric Loading and Regressive Angular Isometric Loading, respectively.  The purpose of PAIL’s/RAIL’s is to:

  1. Teach the nervous system how to control progressively larger ROM’s (Principle of Specificity)
  2. Prepare the body’s tissues to function in newly acquired ranges (Progressive Adaptation)

In previous posts, I have discussed the importance of acquiring mobility, but that this mobility is not acquired through simple stretching.  For a quick review, click:

PAIL’s and RAIL’s “combines stretching with isometric loading at progressive articular angles in order to simultaneously expand ROM, as well as strengthen & produce tissue adaptation in the newly acquired ranges.”  In other words, what we are trying to do is to find end or near-end ranges of motion, apply a neural drive (isometric muscle contraction), override the stretch reflex, and obtain control of a NEW range of motion.  Simple, right?!

So why isometrics and not just static stretching?  Well, we’ve already discussed that the Central Nervous System runs the show, and our central governors basically dictate the “safe” ranges of motion for our body.  So we must learn to reset those systems.  This requires some sort of muscle unit contraction, and isometrics have been shown to be safe and effective, especially at end ranges.  Isometrics allow us to:

  • Override or bypass the stretch reflex
  • Efficiently and Effectively activate motor units
  • Build strength and progressive tissue adaptations in new ranges
  • Do this in a SAFE manner

Let’s look at these bullet points a little closer.  Performing an isometric contraction while in a stretched or shortened position TRICKS the CNS into thinking it’s in a safe position, or position which it can control.  This little trick is often used in Proprioceptive Neuromuscular Facilitation (PNF) stretching, as well.  It allows roughly 10-15 degrees of new motion by altering the stretch reflex!

Secondly, in order to teach the nervous system to fully use the new ranges, we must create some sort of neural drive component.  That is, we must activate Motor Units (a single motor cell and all the individual muscle fibers it innervates) as best as possible.  Research has shown that during isometric contractions, since contraction is limited by load, the nervous system is forced to maximize Motor Unit activation.

Third, in order for isometric training to improve mobility, there needs to be a strengthening effect.  Research has shown that isometric strength training can improve strength, but only within 10-15 degrees of the angle at which the exercise was performed.  So if I perform, a squat for example, at a knee flexion angle of 90 degrees, with a weight that’s so heavy I cannot move the weight…then I will get stronger from the isometric contraction, but only 10-15 degrees AROUND that 90 degree of knee flexion mark.  But, since we are searching for mobility here, the end ranges of motion are where we are searching for strength and control, so the system should work well.  We don’t need to use PAIL’s/RAIL’s throughout a full range of motion necessarily, but perhaps only where we LACK motion.

It should also be noted that the isometric contraction needs to be performed at an intensity that is >80% of Maximal Voluntary Contraction.  So the contractions are intense and taxing on the nervous system.

Remember that strength and stretching prescribed simultaneously results in optimal improvement in flexibility…aka – stretching while applying neural drive creates better movement!

Finally, the isometric loading PAIL’s/RAIL’s is SAFE.  Isometrics are the safest of the 3 types of contraction (concentric and eccentric being the other two).  Also, isometric contraction’s lead to maximal contractions and motor unit recruitment WITHOUT the wear and tear on joints due to shear forces of the other contractions.  It should also be noted that isometric contraction does NOT increase inflammatory mediator release.  So we wont see the same muscle soreness that one would from say, heavy squats or bench pressing.  Granted, we are trying to accomplish different goals, but I am simply making a point on the safety of isometrics.  Especially since we are working at end ranges of motion where our nervous system isn’t exactly comfortable!

strenght tension curve

A quick look at the Length-Tension Curve will better help to explain what exactly the PAIL’s/RAIL’s system is trying to achieve.  Since our muscles develop the greatest tension (at more or less) mid length, and less at shorter/longer lengths, we are trying to expand the length at which we can develop tension.  So in this example, take the blue line further down the x-axis.  It will allow us greater muscular control across a broader range of muscle length.

So in review, the PAIL’s/RAIL’s system combines stretching and isometric contraction to expand our usable range of motion by spreading the length-tension curve.

Here is a Youtube video of Dr. Spina explaining his system:



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