Flexibility vs. Mobility
This is Part 1 of a 2-part blog examining flexibility, mobility and stability.
by Isabelle Barter
The terms flexibility, mobility and stability are being thrown around a lot these days. If you’ve been keeping up with the blogosphere you’ve probably seen headlines along the lines of “stretching is out, mobility is in,” or “flexibility is bad, it’s all about stability now!” But as with most things, it’s a little more nuanced than that.
Flexibility and mobility both fall under the umbrella term range of motion, which describes the degree of movement that occurs at a joint. There are many different technical definitions, so for our purposes we will define flexibility as the ability of the tissue(s), or muscle(s) to yield. Think of a rubber band being gently pulled on one side. If the rubber band is new it will yield and stretch, before returning to the starting position. We would call this flexible, or in physics terms “elastic” (Jules Mitchell does a great job explaining this more in depth here). If the rubber band is dried out, or has been damaged and glued back together, it won’t yield as easily and is therefore less elastic.
If flexibility is about muscles, mobility is flexibility plus the range of motion available in the joint itself. We have many different types of joints in the body, from ball and socket joints like the hip and shoulder that allow for the greatest range of motion, to hinge joints like the knee that permit less range, and there’s even a classification called synarthrosis, meaning without movement, like where your teeth connect to your jaw (yes that’s a joint too!). We refer to a joint like the hip as more mobile, and the teeth as less mobile (hopefully). While pop fitness may be encouraging you to mobilize, mobilize, mobilize! we need to consider the appropriate range of motion for each joint. The knee for example, on average, should be able to achieve around 140 degrees of flexion , and extend to “straight” or neutral at 0 degrees. If we aren’t able to move through those ranges, then we say the joint is lacking mobility.
Mobility then, is a measure specific to the particular joint, and your ability to move can be significantly affected by many factors like joint structure, bone spurs that can develop over time, the elasticity of surrounding muscles, the skin covering the joint or even a big scar.
Then, there’s hypermobility, or as Katy Bowman clarifies in her blog here, what should actually be called hyperlaxity. Hyperlaxity occurs when ligaments surrounding a joint become slack. As Katy points out, people are not hypermobile, joints are. So some joints can exhibit excessive range of motion, while a neighboring joint that is designed for lots of range may be restricted. Despite how we might think it looks when someone displays excessive range of motion (as in this image), it does not mean that the muscles around that joint are flexible. Often the stretch bypasses the muscles surrounding the joint and is transferred to the ligaments themselves. Read more about Jill Miller, creator of Yoga Tune Up® and The Roll Model® Method, pictured in this deep forward fold, and her thoughts on over-stretching here and here.
There are several reasons why a joint may become lax. For instance, Ehlers-Danlos Syndrome (ED) is a genetic disorder characterized by faulty or reduced collagen (Collagen is a protein found in large quantities in connective tissues like tendons and ligaments, and it acts like a rubber cement in the joints, providing tensile strength and structure). Many of us have developed hyper laxity in the elbows or knees because of repetitively pushing ourselves past neutral or normal ranges. Unlike muscle, ligaments are not elastic, and once stretched beyond a certain point, they are unable to return to optimal tautness. So while we might think a big backbend is impressive or beautiful, we should pause and consider what this excessive range of motion demands of our spinal ligaments.
Just like a hyperlax elbow joint, the ligaments won’t be returning to normal length any time soon. This change in the spine is not as obvious, however, because we can’t see to the level of the spinal ligaments. Professional gymnasts, dancers, contortionists, etc. may need certain ranges of motion that aren’t optimal for most people. If you need the range you also need to “own” the range, meaning you are able to get in and out of the position with muscular control, not just flopping into the shape. How do you develop this muscular control? Check back soon for Part 2 of this blog where we will discuss how to create stability and motor control.