My last post introduced viscoelasticity.  Today’s post will cover one of the phenomena of viscoelasticity: creep.

Creep is defined by an initially rapid increase in strain (deformation) followed by a slower increase in strain at a constant stress (load) over time .  Simply put, some materials continue to elongate when stretched even when you don’t continue to increase the force of the stretch.

For example, gummy worms exhibit creep. I have referenced these candies before in my post on stretching range because of their viscoelastic properties.  If you stretch a gummy worm, at first it elongates quickly, but then it continues to elongate even if you don’t pull any harder.  Said another way, the rate of elongation slows, but it still continues to elongate.   For math and science peeps, this means the relationship to strain (deformation) over time is non-linear.

creep

Creep is a reversible phenomena.  Once the load is removed, the original  shape (or length in this case) is recovered.  This is called…recovery.  Recovery is not instantaneous and is also a function of time.  After you stop stretching a gummy worm, it will begin its recovery and eventually return to resting length.

Unless you’ve stretched it beyond its elastic capacity

Collagen fibers (a primary constituent of our connective tissues), because they are viscoelastic [1], also exhibit creep and recovery.  That means that under a constant load, like gravity, the fibers will continue to elongate.  When the force is removed, the fibers will recover.

Unless the fibers have elongated into the plastic region

I often hear yoga teachers talking about the importance of stretching tendons and ligaments to improve flexibility.   It provides a false notion that stretching these tissues will somehow make them longer.  When elongated, tendons and ligaments will either creep and recover (i.e. not get permanently longer) or get long enough to plastically deform (i.e. damaged fibers).

But that does not mean that these tissue should not be stretched.

Now is a good time to define what I consider to be a stretch.  A stretch is a tensile load.  Period.

An elastic material can be under a tensile load and respond with varying amounts of temporary elongation – some materials are stiff and resist deformation, others are compliant and readily deform.  When it comes to human connective tissues, it is the load and not the deformation that counts.  An applied tensile load triggers a complex cellular process that results in an adaptation.  When you stretch a ligament, the cells are signaled to produce more collagen to increase the capacity to withstand the load.  When you over-elongate a ligament you end up with reduced capacity to withstand load.  Plastic deformation (tissue damage) occurs at 4-8% elongation, causing tearing, inflammation, and eventually scar tissue formation.  So stretching connective tissues by applying a tensile load is one thing, but stretching with the goal of making them longer is quite another.

As a yoga teacher, it is important you make this distinction clear.  I lie awake at nights worried that there is too much emphasis on elongation and compliance in the yoga community.  I’m not kidding.  I also lie awake at nights wondering how to convey the message without getting too scientific.  Again, not kidding.  I continue to lie awake worried about how an easy to understand example or analogy I may give completely distorts the science and how the scientific community will denigrate me.  Again, not kidding.  Then I cry to my husband/boyfriend who tells me I take myself too seriously and I finally fall asleep while he lies awake worrying about me and all of the above. And before you ask, just be totally clear, the “husband/boyfriend” is one man, not two men.  We just haven’t come up with a one word term for what we are.

Let’s get back to creep though and wrap up this blog.  I have a sleepless night to get on with.

A recent study on creep in the muscle-tendon unit of actual living humans showed that the steepest slope of the creep curve has been measured to occur within the first 15-20 seconds [2].   After that, elongation continues but at a slowing rate.  Since we don’t have any studies on yoga postures and creep, we are left to make educated choices.  It is my opinion that if we plan to be in passive stretches for 3-5 minutes, we can reduce the potential of too much  elongation by providing proper support.

forward fold

In the photo above, I have placed a bolster under my head to minimize the creep under the constant load (gravity).   The back side of my body is now getting a continuous stretch (tensile load) but is not under continuous elongation.  After several minutes in the pose, I can slowly come up and recover.

The moral of the story…use yoga props wisely when holding passive stretches.

When and why you should or should not hold passive stretches is another topic entirely.

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[1] Svensson, R. B., Hassenkam, T., Hansen, P., & Magnusson, S. P. (2010). Viscoelastic behavior of discrete human collagen fibrils. Journal of the Mechanical Behavior of Biomedical Materials,3(1), 112–115. doi:10.1016/j.jmbbm.2009.01.005

[2] Ryan, E. D., Herda, T. J., Costa, P. B., Walter, A. a, Hoge, K. M., Stout, J. R., & Cramer, J. T. (2010). Viscoelastic creep in the human skeletal muscle-tendon unit. European Journal of Applied Physiology, 108(1), 207–211. doi:10.1007/s00421-009-1284-2

 

 

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