How Does Stretching Work? A Guide to Flexibility.

Muscles attach to bones in order to move them. When we stretch a muscle, we are drawing two ends of the muscle away from each other. This can happen when one end is fixed in place and the other end moves away, or when both ends move away from each other. 

Many structures are affected when a muscle lengthens including the connective tissue, which in this case means ligaments (connecting bone to bone) and tendons (connecting muscle to bone).

Contrary to what one might think, stretching has nothing to do with how long or short our muscles are. Instead, the feeling of “tightness” has something to do with how our nervous system communicates with our tissues during a stretch that creates a sensation of tightness.

In other words, our depth of stretch is determined by our nervous system and not our muscle tissues. Once we know this, we can take the steps needed to understand how the nervous system communicates with the muscles to positively influence the degree to which we’re currently able to stretch. 

Let’s take a deeper look into these factors. 

Why do you want to stretch your body? 

Are you wanting to develop new ranges of motion? Get out of pain? Recover from the game of tennis you played yesterday? 

Clearly, there are lots of reasons to stretch. And when we know how stretching works, we can move in ways that help us get the results we’re looking for. I’ve felt and seen frustration in others when our hamstrings never seem to open over time. Or the outer hips never budge in pigeon, no matter how many years we’ve practiced it. There’s even the scenario where we’ve stretched our body so much that we become unstable and always on the verge of an injury (that was me about two years ago!). 

I don’t want to claim to be an expert in this area. Like everyone reading this article, I’m learning and evolving my understanding as I’m exposed to new models of thinking that make sense (and that utilize scientific evidence--very important to me these days). I’m trying things out, applying what I learn and monitoring my results. My goal with my movement practice is to feel better as I age, and I try to make movement choices that reflect that as best I can. 

When it comes to stretching, there are a few pieces of the puzzle that have helped me positively influence my body’s capacity. These puzzle pieces will be the topic of this article, and include: 

• Stretch Tolerance

• Active v. Passive Stretching

• Muscle Receptors 

Stretch Tolerance

I love the concept of stretch tolerance because it illustrates the point that stretching is governed by our brain and nervous system rather than the muscle itself.  

Stretch Tolerance is a feeling generated by the nervous system when we get the end range of a stretch. It is triggered by our history with movement and how confident our brain feels in our ability to do a particular movement safely. After all, the main goal of the nervous system is to keep our body safe. Stretch tolerance is part of this mechanism. 

At our end range of motion in a stretch we feel a sensation of discomfort or tightness. This is what reaching our stretch tolerance feels like, and is our signal to stop pushing deeper into a stretch. When we stretch and strengthen regularly, we are able to move into deeper end ranges. This is not because our muscles get structurally longer with practice, but because our nervous system doesn’t trigger our stretch tolerance until later in the process. 

Active V. Passive Stretching

Are you in a position where you stretch one part of your body every day but your stretch tolerance always triggers in the same place? Or, are you in a position where you’ve overstretched a particular area into chronic soreness or injury? 

When we’re not seeing consistent positive results from our stretching, active v. passive stretching may be the topic to pay attention to. 

Active Stretching is what happens when all of our muscles are working to create whatever physical position we’re in. On the flipside, Passive Stretching is what happens when we put our body into a position and let gravity or leverage facilitate our stretch. 

Passive stretching and active stretching are supported differently by the immune system. 

When we stretch actively, we contract our muscles to support our positioning which signals to the nervous system that we are in control of our movement. This helps our brain feel more confident in our movement and stimulates new strength adaptation in our muscles so that our brain continues to feel confident even after the stretch is over. (At some point, we’ll need to add load to continue our strength adaptation, but let’s assume that simply contracting our muscles in a given shape is enough to stimulate new strength at first.) 

Active stretching on a regular basis will help us develop new end ranges over time. Better yet, it will help us develop new end ranges that our muscles are able to actually control so that if, for whatever reason, we needed to perform a real-world task in a stretchy shape or lift something heavy in a certain position, we would be able to do that. That’s the sort of confidence our brain wants before it will adapt our stretch tolerance to go “deeper” into a stretch. Our brain wants our  body to be functionally mobile, not gumby and unstable. 

If you’ve been primarily practicing passive stretching--which is quite common in yoga--try making poses more active by using muscle contraction to create your shapes rather than gravity or leverage. 

Muscle Receptors

Muscles and joints have receptors within them that communicate directly with the nervous system. When we contract or stretch a muscle, these receptors detect changes in muscle tension and length and signal to the central nervous system. The nervous system responds by regulating the contractile state of the muscles (stretch tolerance), which in turn affects our range of motion (flexibility). 

Let’s take a look at the nature of some of these receptors. 

To start, we have Muscle Spindle Stretch Receptors in the belly of all skeletal muscles. These receptors respond to the biomechanical event of stretching a muscle and communicate with the nervous system as mentioned above. These receptors are designed to keep our body safe, which means they tend to fire as soon as they detect a change in muscle tension beyond what we normally do. Still, our tissues are designed to withstand a bit more than our stretch tolerance allows. The trick is to learn how our stretch receptors work so that we can, in turn, work with them to influence our stretch tolerance. Here are two ways we can influence these receptors to decrease their firing:

• Holding a stretch for more than a minute causes the muscle spindle to decrease its firing, and the muscle begins to relax.

• Backing part-way out of a stretch causes the muscle spindle to decrease its firing, relaxing the muscle and allowing some slack for a deeper stretch.

We also have receptors in the connective tissue--where the muscles and tendons are joined (muscles turn into tendons to attach to bones). When we contract a muscle, receptors in the connective tissue called Golgi Tendon Organs detect this change in tension and signal the muscle to relax. Similar to our stretch receptors, the Golgi tendon organs’ job is to keep our connective tissue safe from overload. Once we know that contraction stimulates these receptors, we can use contraction as a strategy to influence our stretching. This is called Facilitated Stretching, where we contract the muscle being lengthened to stimulate a deeper stretch.

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