Mobility and flexibility are widely used terms, sometimes interchangeably, to describe the process of attempting to achieve more range of motion in a certain body segment. You may have seen or performed the following scenario: “I’m going to go work on my mobility”. Then, you proceed to grab a foam roller to roll the body part(s) you feel need the work or bend down to touch your toes to stretch your hamstrings. Is there anything wrong with this? Absolutely not. But, at the end of the day, the effectiveness depends on your goals. So, first off, let’s dive into what mobility and flexibility mean.
Mobility describes an individual’s ability to actively move through a given range of motion whereas flexibility describes how far a joint (a place where 2 bones meet) can be passively moved through a range of motion. Put another way, flexibility asks if you have the potential range of motion in a body part while mobility asks if you have the same range of motion accessible during an activity. Here is an example of a movement that can be done actively or passively:
Passive: pulling on the bank to raise my leg
Active: I use my muscles to raise my leg
Usually, you have more flexibility than you do mobility because at some point your muscle will become limited in strength due to its shortened position as demonstrated by the length-tension relationship.
Note that the active tension (think muscular tension) is at its lowest when the muscle length is in shortest or longest position. Passive tension, the tension created by non-contractile elements (i.e. ligaments, joint capsule), begins as the muscle reaches its longest anatomical position. This anatomical position is different for everybody and is likely contributed by bony geometry, previous injury, nerve tension, other structural abnormalities etc.
Now that we have some perspective on the difference between what flexibility and mobility may look like, let’s talk about some of the internal and external contributions to each. We’ll start with our internal hardware. The bony shapes that make up the joints in your body can affect the range of motion your body allows you to get into – you can thank your parents for that. For example, within your hip joint (femoroacetabular joint), there are many different angles that can classify your own geometry. For example, you’ll note that the differences in shape between these hip joints may allow one person to move smoothly through a given range of motion compared to another with all else being equal.
The names of the angles and the normal values aren’t necessarily important to memorize, but these should illustrate the variance between hip joint shapes.
A person having a slightly more than normal anteverted hip socket and anteverted femoral head may have an easier time getting into the bottom of a squat compared to someone with a retroverted hip joint. In other words, not all squats are created equal!! Interestingly, however, bony adaptations, or changes, can take place when continuous levels of stress are placed on them. For example, the humeral head (humerus = arm bone) on the throwing arm for competitive baseball players may adapt to give them more motion for throwing. Research lends support to this: (https://www.jospt.org/doi/pdf/10.2519/jospt.2007.2449). Thus it’s possible for some degree of change to take place over a lengthy period of time.
Short of surgery or trauma, it’s difficult to change our bony geometry in a short period of time. We thus have to look at other tissue structures and their role in flexibility and mobility. The muscles surrounding the joint is likely the next go to for how to achieve greater flexibility or mobility. How exactly do we create length in our muscles to improve our flexibility or mobility? Do we add more muscle cells to the end of our muscles every time we static stretch? (hint: no) What’s the process? Look out for part 2!
By: Henry Mercier