The Difference Between Mobility and Flexibility (And Why It Matters For Throwers)

Flexibility is defined as “the ability of a muscle to lengthen”.

Mobility is defined as “the ability of a joint to move actively through a range of motion”

There are many contributing factors that go into a joint’s mobility: muscle tissue, joint health, motor control, and the fascial system.  Additionally, the entire kinetic chain has a significant impact on mobility.  For example, spinal function and scapular function can greatly impact glenohumeral (shoulder) mobility.  Similarly, an ankle injury can impact knee function which in turn impacts the hip joint.  Everything is connected.

When looking at a thrower’s shoulder, one of the movements I look at carefully when assessing them is glenohumeral internal and external rotation.

Brewers v White Sox

Throwing creates crazy layback positions (demonstrated by Chris Sale here) and this impacts the shoulder’s mobility greatly.

It should be noted that this is only one piece of the puzzle when it comes to assessing an overhead athlete.  However, in my experience, the two biggest red flags as predictors of pain and/or injury from throwing are a lack of shoulder flexion ROM and glenohumeral internal rotation ROM.  For today’s post, I’m going to strictly talk about internal rotation.

On a broader scale, if you aren’t assessing, you’re just assuming.  Many throwing athletes are injured but just don’t know it yet (dysfunctional but asymptomatic), so a solid assessment can pave the way for subsequent proper exercise prescription.  From there, the training program as a whole, can improve function so that, hopefully, painful symptoms never come to be.  In the context of this post, if an athlete has limited internal rotation during his assessment, it should not be ignored even if he’s never had any issues with throwing up to this point.

 If you think about the biomechanics that happen during throwing, if someone doesn’t have adequate internal rotation mobility they’re on a crash course to getting hurt down the road.  The reality is throwing is the most explosive movement in all of athletics – on a 90 mph fastball, the head of the humerus rotates internally at 7000 degrees/second.  However, proper assessment and exercise prescription can safeguard an athlete from getting hurt, and allow the athlete to perform at a high level for a long time.
If you looked at an athlete who had limited internal rotation ROM and only tried to attack the issue by working on the glenohumeral joint, you’re missing the forest for the trees.  The issue could be neural, joint related, or there could be fascial restrictions.  It could even be impacted by breathing patterns.  Realistically, it would most likely be a combination of all of the above.  That’s why prescribing static stretches (the sleeper stretch in this case) isn’t going to produce the desired result.
In over 200 throwing athletes in the past five years, I’ve never come across one, older than sixteen, who didn’t lack at least a little bit of internal rotation ROM in their initial assessment.  In my next post, I’ll show a few different ways to improve (or at the very least maintain) that mobility.