Every athlete has been told to stretch at some point in their career. Most of us grew up in a sport where static stretching was (unfortunately) a part of our “warm-up.”  As a circus/acrobatic PT, I’m often asked about stretching’s role in flexibility gains & how/why it works. It’s actually not too complex of a topic & once you grasp the basics, you’ll be better prepared to build your own program. So, settle into a pancake and let’s tackle the physiology & theory of stretching!

Since the early 1980’s (yup, almost 40 years ago now), several theories have been proposed to explain increases in muscle flexibility observed after intermittent stretching. Most of these early theories advocate a mechanical increase in length of the stretched muscle. This is what I grew up being taught, and I’ll admit was even advocated as late as 2009 when I graduated from PT school. The rehabilitation world suggested that increases in muscle extensibility observed after intermittent stretching involve an increased mechanical length of the stretched muscle. Unfortunately, these theories (viscoelastic deformation, plastic deformation and neuromuscular relaxation) didn’t stand up to the research literature & were ultimately disproven.  Let’s look at why…

 Viscoelastic Deformation: Debunked Theory #1

In this theory, the main idea was that skeletal muscles were thought to be viscoelastic. This means that when stretched for a short period of time, there would be a gain in length that eventually returns to “normal” once the stretching force was removed. Viscoelastic is a mechanical term – meaning the theory is built around a physical change in length of the fibers. Basically, with this theory, muscles should demonstrate elasticity by resuming their original length once tensile force (aka stretch) is removed. Yet, like liquids, they also behave viscously because their response to tensile force is rate and time dependent (think of how water pushes back harder the faster you try and move). So, theoretically, an immediate increase in muscle length would occur due to the viscous behavior of muscles whenever they undergo stretch of sufficient magnitude, duration and frequency.

Ok…yikes! So, in more basic terms the idea was this:  You go to an aerial class and spend a large portion of it stretching, working active flexibility & moving through pose after pose. You leave class feeling more flexible, because your muscles “got longer temporarily.” You hop in your car for the drive home. Once there, you decide to show your Instagram followers a selfie of the full split that you showcased in class only to realize that your split is no longer the full 180. Proponents of this theory would argue that your muscles are exhibiting “viscoelastic deformation” and your muscle’s length temporarily increased, after which it returned to “normal.”

So, what did the research say? In human studies, viscoelastic deformation and recovery were extensively been tested on hamstring and ankle plantar-flexor muscles. The results refute viscoelastic deformation as a mechanism for increases in muscle length and extensibility. All of these studies consistently showed viscoelastic deformation of human muscle to be false. While there was a temporary gain in flexibility after stretching, there was not actually a change in length. So, no mechanical change in the muscle was actually seen…something else was causing the gains.

 Plastic Deformation: Debunked Theory #2

Another popular theory suggested that increases in human muscle extensibility observed immediately after stretching were due to “plastic,” or “permanent” deformation of connective tissue. The classical model of plastic deformation would require a stretch intensity sufficient to pull connective tissue within the muscle past the “elastic” limit and into the “plastic” region of the torque/angle curve so that once the stretching force was removed, the muscle would not return to its original length but would remain permanently in a lengthened state. Basically, you stretched so hard your muscle would never be the same...Ha! I can hear you chuckling already. If you’re anything like me you’ve stretched “hard” only to injure yourself and make no real gains (other than getting to visit your favorite ACRO PT).

But still, let’s look at this in a little detail. The theoretical physiology of this model would go like this:

Your muscles are made up of microscopic fibers, the two main ones called actin & myosin.  These fibers overlap (think pieces of string running in parallel). When a muscle contracts, there are little “arms” known as cross-bridges that pull the actin and myosin into a more “overlapped” position. When the muscle releases, these little “arms” let go and there is again less overlap. In this model, it was thought that when you stretched, the amount of overlap was lessened over time, muscle building blocks were added in series to make things longer and thus the muscle was “plastically deformed” (think overstretched hair-tie/rubber band).  This would mean that at each muscle cell, there was now more length and so, there would be more flexibility.  Basically, you’d stretched the rubber band so well it would never return to its original state but was now longer and somehow better for it.

Yeah, so this theory never really got off the ground. In 10+ studies (between 1983-1998) that researched plastic, permanent, or lasting deformation of connective tissue as a factor for increased muscle extensibility, none of the cited evidence was found to support the model. The term “plastic deformation” was considered only to be a synonym for deformation that is permanent in nature.  So basically, it was a way of describing the changes clinicians saw with repeated stretching, but there was no evidence to link it to any true mechanical change.  Yup…we rehab professionals were just taught to describe it that way to patients to simplify the true reasons you get more flexible with time. (Yes, please allow me to apologize for PTs everywhere as I was in on the lie when I was a student).   

 Neuromuscular Relaxation: Debunked Theory #3

Alright, so nothing mechanical is actually changing when you stretch. So…the change must exist in another system.  Researchers moved their thoughts onto the neuromuscular system (brain & muscle interaction).

This theory was built around the idea that stretching could retrain the nerve receptors in muscle and ultimately change how they function reflexively. (Yup. Changing reflexes seems unlikely – but stay with me for a moment). In all of our muscle fibers there are nerve/neuromuscular areas that sense stretch & force. This feedback is then used to protect the muscle by engaging more fibers to increase the contraction or by shutting the muscle off to prevent rupture. The rehabilitation world had previously suggested that involuntary contraction of muscles due to this neuromuscular “stretch reflex” could limit muscle elongation (aka flexibility) during static stretching procedures. That was the theory at least. It was hypothesized that in order to increase muscle extensibility & bypass this “reflex”, slowly applied repetitive static stretch could stimulate alternate neuromuscular reflexes that induce relaxation of muscles undergoing static stretch (rather than force protective contraction). This was why ballistic (or bounce) stretching has been discouraged for years! Some authors even suggested that neuromuscular reflexes adapt to repeated stretch over time, which enhances the stretched muscle’s ability to relax and results in increased muscle extensibility. So, basically, the more slow, consistent and gentle your stretch, the more consistent the change in your brain/muscle connection with the outcome being that the relaxation reflexes are more often stimulated than the contraction reflexes.

Drum roll please… Well, the experimental evidence did not support a neuromuscular reflex change as the cause of any of these assertions.  Stretch reflexes have been shown to activate primarily during very rapid and short stretches of muscles that are in a mid-range position (not an end-range position like a stretch).  They also only produce a muscle contraction of very short duration – not a long, resting muscle hold.  These studies actually did not demonstrate any contraction/relaxation reflexes in stretched muscles. (So, the brain and neuromuscular system reflexes weren’t engaged with stretching at all).

So, now that you’re an expert on useless flexibility theories (here’s to a possible Jeopardy category), let’s talk about what the rehabilitation world currently believes is actually occurring..

Sensory Theory for Increasing Muscle Flexibility: The “current” supported research theory

This theory proposes that increases in muscle extensibility are due to a modification of sensation only. Basically, what you feel while stretching changes, so you are able to stretch further. No mechanical change, no re-programming of your reflexes! Even better – the current studies support that increases in muscle extensibility observed after a single stretching session and after short-term (3- to 8-week) stretching programs are due to modified sensation.

The simple description is that when you stretch, you experience the sensation of stretching. Initially, this is perceived by your body as a “potentially noxious (painful)” or unpleasant feeling and your body reacts by setting what it perceives as a safe endpoint to the stretch. Now, let’s say you hold the stretch for a minute or two. During that time the body begins to perceive less and less of the stretch (aka desensitization) as the sensation your muscle is providing is not new or useful information.  Your brain is trying to minimize the number of things that your body has to react to such that your sensory system is more efficient, so it “turns down the volume” on what you’re feeling. Let’s use a common example: Think of the sensation that you get when you put on tight socks first thing in the morning. Initially you notice them on your feet, but then after a few minutes, you no longer feel the socks. They’re still on, but not registering with your sensory system. Your body recognized this was extraneous, unneeded information and simply “muted it.”

This sensory theory was observed in several studies that looked at both single stretching sessions & 3-to 8-week stretching programs. In both cases, the force of the stretch (aka the intensity) was not increased over time, rather the “torque” on the muscle was held consistent. The only instructions given to the subjects were “move until you feel a comfortable stretch and consistently hold.” The most common time interval analyzed was 90-seconds, but gains were seen with as few as 10-second intervals. In the studies that covered several weeks, joint angle increases of 17 degrees were seen within an individual while the “perceived amount of stretch” (on a 0-10 scale) that they reported remained constant!

What does this mean for athletes looking to gain flexibility?

Ok, so now what? In the end does it really matter why stretching works if you’re not a PT/physiology nerd? I’d argue that it does.  If you build your stretching program around this idea of sensory habituation, you’ll see gains – even in the very short term (aka one aerial class/one stretching session).  

Here are the key points to consider when building your stretching protocol:

1. What motions are you targeting? (Note that I didn’t specify muscles.)  Think of stretching as functional.  You need to touch your toes to set up a skill on fabrics, but you can’t stretch that far yet. Does it matter which muscle is the most limiting? Unless you’re injured, the answer is a resounding NO! If you stretch the motion, the most limiting muscle will get the brunt of it and you’ll see that sensory adaption & gains.

2. Where is your baseline? (Take a photo so that you can set reasonable goals).

3. How much time do you have to devote to stretching daily/weekly? Be honest with yourself so you stick to your routine!

4. What amount of stretch feels “comfortable” or “reasonable” to you? Pushing past this will likely work against you, since too much pain will cause a guarding response from the muscles that is counterproductive.

Have answers to those questions? Great! Now write out your stretches, your frequency (how often/how long a hold) & your timeline for re-evaluation (I’d recommend 4-6 weeks for that follow-up photo just to allow you time to build the habit). 

When you’re stretching:

1. Focus on relaxing & breathing through the stretch. Think “mindfulness” and work to be calm in the moment. Be aware of the sensation you’re feeling. Give it a numerical rating (mild range please) and stay constant.

2. Make a routine of it! Same time of day, same setting, etc. You want this to be a comfortable part of your fitness regimen – so you stick to it.

3. Avoid comparing yourself to others! If you’re competitive (like me) this will lead you to injury rather than success.

The goal of this post was to take the “mystery” out of stretching. Sure, you can learn the anatomy & target specific muscles, but the point is that you don’t have to!  You can target the poses, positions and skills and grow as an athlete all on your own. That said – hiring a coach or seeking help from a PT is never a bad option! When in doubt, ask a professional – that’s how you learn & grow too!

References:

CH Weppler, SP Magnussun; Increasing Muscle Extensibility: A matter of increasing length or Modifiying Sensation? Physical Therapy, Volume 90, Issue 3, 1 March 2010, Pages 438–449.

de Weijer VC, Gorniak GC, Shamus E. The effect of static stretch and warm-up exercise on hamstring length over the course of 24 hours. J Orthop Sports Phys Ther. 2003; 33: 727–733.

Folpp H, Deall S, Harvey LA, Gwinn T. Can apparent increases in muscle extensibility with regular stretch be explained by changes in tolerance to stretch? Aust J Physiother. 2006; 52: 45–50.

Hortobágyi T, Faludi J, Tihanyi J, Merkely B. Effects of intense “stretching”-flexibility training on the mechanical profile of the knee extensors and on the range of motion of the hip joint. Int J Sports Med. 1985; 6: 3173–3121.

Liebesman J, Cafarelli E. Physiology of range of motion in human joints: a critical review. Crit Rev Phys Rehabil Med. 1994; 6: 131–160.

Magnusson SP. Passive properties of human skeletal muscle during stretch maneuvers: a review. Scand J Med Sci Sports. 1998; 8: 65–77.

Magnusson SP, Simonsen EB, Aagaard P, et al. Viscoelastic response to repeated static stretching in the human hamstring muscle. Scand J Med Sci Sports. 1995; 5: 342–347.

Ryan ED, Beck TW, Herda TJ, et al. The time course of musculotendinous stiffness responses following different durations of passive stretching. J Orthop Sports Phys Ther. 2008; 38: 632–639.

Willy RW, Kyle BA, Moore SA, Chleboun GS. Effect of cessation and resumption of static hamstring muscle stretching on joint range of motion. J Orthop Sports Phys Ther. 2001; 31: 138–144.

Additional references available upon request*