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Flexibility vs. Mobility: What’s the Difference?

back pain dr. ravi mobility Jun 20, 2019


I recently attended a continuing education course called Functional Range Conditioning (FRC). It was one that has been on my list for quite some time and it was awesome to finally check it out.

In this blog post, I’m going to expand upon some of the principles and techniques I learned and how you can start to implement this in your daily movement practice.

First, let’s define a few words. What is flexibility? What is mobility? Are they the same thing? We hear these words used interchangeably. However, they are in fact different.

  • Flexibility = Passive Range of Motion (PROM)


  • Mobility = Active Range of Motion (AROM) = (Flexibility + Strength/Control)

The foundation of the FRC system is based on the acquisition and maintenance of functional mobility and articular health. It is very dependent on your passive and active range of motions.

Basically, the goal is to make your AROM and PROM the same. PROM is the prerequisite which will allow you to improve your AROM.

FRC utilizes a concept called “bioflow.” While I don’t get too caught up in systems or their coined terms, I’m cool with this one. It basically talks about tissue continuity (gross tissue --> cellular --> intracelluar) calling it STUFF. Stuff being cells, fibers, and ground substance. Composition of these components dictate the type and physical properties of a certain tissue whether it's bone, fascia, ligament, tendon, muscle, capsule etc. Cell signaling and progressive adaptation is how these cells change into these different structures. Think about an ACL graft that is harvested from a patellar tendon – do you think it stays a tendon over time or evolves to becoming a ligament just like the initial ACL? Yeah, science is pretty cool.

I could geek out on this stuff all day, but let’s move on to the application of improving your mobility – there’s a few techniques used to start working on making your passive movement more active.

Insert Controlled Articular Rotations (CARS) - Active, rotational movements at the outer limits of articular motion. There’s 3 levels for CARS which are related to isolated blocking, external resistance and amount of irradation. Irradation simply put is the amount of tension you create throughout your body – in nerdy science terms this is also called Maximum Voluntary Contraction (MVC) often expressed in percentages.

The best example of irradation is to give someone a hand shake. First, squeeze using your hand, then hand and forearm, then hand, forearm and shoulder, etc. Your grip gets stronger and stronger the more musculature you recruit. The more irradation, the more force you exert. You can use this to dial in higher levels of recruitment while doing your CARS or other FRC techniques. “Force is the language of cells” – one of my favorite quotes at the course.

CARS can be implemented different ways whether that is by focusing specifically on a certain joint or you can take part in the morning CARS routine to give all your synovial joints in your body some love each day.


The next step to continue to work on improving your joint integrity and control is via PAILS and RAILS. PAILS and RAILS are isometric contraction efforts (sometimes combined with stretching) used to communicate with both the connective tissue & neurological systems.

2-3 minutes of stretching to build stretch tolerance, then:

  • PAILS: Progressive Angular Isometric Loading
    • Combines stretching with isometric loading/training at progressive articular angles in order to simultaneously expand ROM, as well as strengthen & produce tissue adaptation in the newly acquired ranges.
    • Hold for 5-10 seconds, then work into newly acquired range, then go into RAILS.
  • RAILS:
    • Combines stretching with isometric loading/training at regressing articular angles in order to strengthen, and induce beneficial tissue adaptations in progressively shorter angles.
    • Hold for 5-10 seconds, then work into newly acquired range, then go into passive 1-2 minute stretch, then repeat PAILS/RAILS as needed.
    • Stretched tissue: PAILS
    • Shortened tissue: RAILS

This is a great video by Joe Gambino from Par Four Performance going over the Hip 90/90 PAILS/RAILS.

I see PAIL/RAILS as a way to safely acquire and create control into these newly stretched positions without movement. Basically isometric holds to own a position with increased stretch tolerance.

The next and my most favorite part of the course and system is the End-Range Control techniques. End range is where we see a lot of injuries and tissues breaking down. Why? Well, from a physics standpoint, we’re just not able to produce as much force at these end ranges due to length-tension relationships. Another big factor is because we rarely go there. And when we do, we typically aren’t ready for it and are pushed there by accident – which is why we need to train these end ranges. It allows us to build better tissue resilience and reduce the risk of injury. Here’s how we break down end-range control:

End-Range Control: PALS/RALS

  • PALS: Progressive Angular Loading
  • RALS: Regressive Angular Loading

Passive Range Holds

  • Find end-range similar to PAILS and then hold position with irradation for 5-20 seconds - all based on intensity of contraction and exertion.


​Passive Range Lift-Offs

  • Go to end-range, then back off about 10 degrees then lift off and then hold position with irradation for 5-20 seconds - all based on intensity of contraction and exertion.



  • Go to end-range, then back off as needed to set up different hover obstacles - think about seated hip flexion with multiple kettlebells as you abduct. Same with shoulder flexion.


​End-Range Rotational Training

  • Same as Passive Range Lift-Offs but you hold and do small circles at that end range around a small object (i.e. clinician hand or safety bar of a squat rack).

My suggestion is don’t get too caught up on the wording of these different techniques, but understand the conceptual framework and you’ll be able to implement this immediately. We all know that we have certain aspects of our joints where our active and passive is not the same. If you’re wanting to improve your squat or overhead position, or if you just want to build up resiliency in different tissues, then give your joints some love with some of these different techniques.


Dr. Ravi Patel, PT, DPT, CSCS

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