You can still be “flexible” and have mechanically stiff tendons and ligaments.

If you read my first post on the topic of biomechanical stiffness, you may remember this statement:

The problem is that when we use stiffness in a casual manner, it doesn’t match up with the mechanical definition of stiffness.  This causes confusion and we end up thinking that the opposite of stiffness is flexibility (defined four our purposes as increased range of motion – ROM).  It’s not.  The opposite is more like compliance.

Stiff tissues resist deformation.  You can have stiff connective tissue and still have normal range of motion (flexibility).  Stiff really just means strong  and resilient.   A more thorough definition, also from my first post on stiffness:

In mechanical terms, stiffness is a measure of how much load a material (steel, for example) can take before it deforms.  In biomechanical terms, stiffness is a measure of how much load a tissue (tendon, for example) can take before it deforms.  Essentially, stiffness represents how strong and resilient a material is under stress.

Mechanical stiffness is the slope of the curve in the stress strain curve below, up until the yield point.  Mechanical stiffness is not the same as unyielding.  A tissue with high mechanical stiffness will still yield, but the amount of applied load it can withstand while remaining within the elastic (safe) region is greater.  Or said another way, the greater the mechanical stiffness, the greater the load bearing capacity.

stress strain logo


An example always helps.

Let’s consider the ankle region.  Without getting into the complex anatomy of the foot and ankle, we can all agree that we have many ligamentous structures and other connective tissue  materials holding these bones together.   If you go hiking (just as an example of some uneven terrain) greater stiffness will help you avoid injury.  How so?  When your ankle is under a load (your body mass and gravity) it will only maintain its structural integrity to the degree of stiffness that it has.  If your ankle ligaments don’t have enough stiffness, they will deform – and potentially deform beyond their elastic capacity or yield point.  Read:  tissue damage, injury.

This doesn’t mean you don’t have joint mobility.  You can have full range of motion in your ankle joints and still have a lot of stiffness.  Furthermore, stiffness can be increased through training, even low-load resistance training.¹  So reducing stiffness does not give you more flexibility (greater range of motion).  Reducing stiffness only gets you less load bearing capacity.  And who wants that?

Have you ever said, or heard someone say, “Oh, my hamstrings are so stiff, I need to stretch them out.”  Consider what that actually means.  Your connective tissues (which surround and are continuous with the proteins that make up what we call hamstrings) are supposed to be stiff!

If you are “feeling stiff” as in “your body aches” it means something different.  I probably just means you need to move and open some joint angles that have been closed for too long.

Or perhaps your muscles are unyielding.  In that case, a stretching protocol would serve you well as long as you targeted the tissue of mechanism causing the unyielding.  But not in an attempt to reduce or eliminate mechanical stiffness!

Our connective tissues – tendons, ligaments, and fascia; they all have stiffness.  It’s an unavoidable property that we should not try to do away with by stretching, massaging, or hot tubbing.  You cannot do away with stiffness.

Joint materials that lack sufficient stiffness are known to be lax.²  Joint laxity should not be confused with flexibility either.  Joint laxity is more a matter of an inability to regulate range of motion due to, wait for it…

…insufficient stiffness!

This where compliance becomes a factor.  More on that and range later. Right now, I’m going to go work on my stiffness.

Hey! Maybe we could get Fergie to change her lyrics to “I be up in the gym, just workin’ on my stiffness.”

Sorry, I couldn’t resist.


1  Kubo, K., Kanehisa, H., Miyatani, M., Tachi, M., & Fukunaga, T. (2003). Effect of low-load resistance training on the tendon properties in middle-aged and elderly women. Acta Physiologica Scandinavica178(1), 25–32. doi:10.1046/j.1365-201X.2003.01097.x

2  Kovaleski, J. E., Heitman, R. J., Gurchiek, L. R., Hollis, J. M., Liu, W., & Pearsall, A. W. (2014). Joint Stability Characteristics of the Ankle Complex in Female Athletes With Histories of Lateral Ankle Sprain, Part II: Clinical Experience Using Arthrometric Measurement. Journal of Athletic Training, 49(2). doi:10.4085/1062-6050-49.2.08