Especially in movement education, we hear a lot of talk about how flexibility training can give people longer muscles, like hamstrings for example. But is that actually what’s happening? Let’s zoom in and take a closer look at what flexibility training does.
Flexibility training can often include things like different types of stretches, active range of motion work, and massage. There are essentially 3 components to how flexibility training can achieve results (being, greater range of motion achieved). The order we go through them in below has no correspondence to importance or degree of impact. As we go through them we’ll also talk about the usefulness of stretching, active work and massage in each of the components.
Sarcomeres are the functional units of muscle cells. There are many tens-of-thousands of sarcomeres within a single muscle cell, and many thousands of muscle cells within a single muscle (just to give you an idea of how small these things are).
We have the ability to add more sarcomeres into the muscle cells that we already have. This is what sarcomerogenesis is – the creation of new sarcomeres. It can occur in parallel to create a thicker muscle cell, and therefore a thicker muscle as a whole, or it can occur in series which lengthens the cell. This is an essential component of growing taller, as you can likely imagine…
Research (like this) is finding that eccentric contractions of muscle can actually drive the process of sarcomerogenesis. It’s believed to occur somewhere between 10 days to 8 weeks of consistent eccentric training. It’s also important to note that these changes, just like almost all others in the body, are only temporary unless training is continued.
Worth highlighting, eccentric muscle contractions are where we contract AND lengthen the muscle at the same time. For example, the standard example is the downwards phase of a bicep curl. Also important to know, in the same vein, is that based on current evidence sarcomerogenesis does NOT occur as a result of passive stretching. (reference).
So yes, you can actually lengthen the muscle, literally. And this component of flexibility training is only found in active work – not in stretching or massage.
Nervous System Adaptations.
Disappointingly, much of the research on this particular component of flexibility training only attempts to measure neural changes during passive flexibility training regimens (which doesn’t make a whole lot of sense, if you ask me). This research tells us that passive stretching provides no measurable neural changes – which is logical. (this, for example)
When trying to find sources on neural changes resulting from an ACTIVE flexibility regimen, the research is scarce at the moment. This piece of research however, does just that.
Interestingly, the study above found the greatest benefits to flexibility occurred when using a conditioning program. This means that the training programming was more about general conditioning and lower intensity. In other words, using movement patterns that these individuals (dancers) were going to encounter during the rest of their training, was more beneficial than specific isolations, and also better than adding greater intensity (for example, extra load).
The proposed mechanisms for improved flexibility with active training include:
• Improved recruitment of the agonist muscle (more muscle units recruited with better neural organization of signalling).
• Decreased resistance from antagonistic muscles (decreased stretch-reflex sensitivity and therefore reduced limitation on the range of motion).
Whilst it hasn’t be directly measured with the comparison of flexibility, these neural adaptations contribute to increased strength as well. Using this reasoning, we are going to make the assumption that the neural changes/flexibility curve is similar to the neural changes/strength curve. This means that we tend to see most neural changes occurring within the first 4-8 weeks of training, beyond which, morphological adaptations often take over.
You can get both of these mechanisms rolling with strength training, and you can get assistance on the second piece (less sensitive stretch-reflex) from massage.
Connective Tissue Rearrangement.
Speaking of morphological changes, that’s what we’re about to discuss here. Connective tissues create and maintain the structure of our body. Some references will call this “fascia”, so use each term as you like when reading below.
Keep in mind this is a new area of investigation, we currently believe there are two ways in which the rearrangement of connective tissues influences flexibility.
Here they are:
1. As connective tissue is layered, the theory is that stretching (whether passive or active) initiates changes in how these layers are bound to one another – to how they are cross-linked. This hypothesis exists because we see that with repeated stretch applications longer than 10 seconds, we get a longer measure of a muscle cell, but the number of sarcomeres does NOT change that quickly, so that length cannot be due to sarcomerogenesis. (Reference). This leads us to believe that when the connective tissue network around our muscles and muscle fibers is stressed beyond a certain “normal” range (individual normal) it rearranges itself to allow greater elongation of each sarcomere. This rearrangement likely involves a decreased number of cross-links between layers.
2. Connective tissue tension on nerves can be reduced. Our connective tissue is also the support-network for all our nerves and blood vessels – as it, it holds it all. A recent (study) showed, that when the deep fascia of the medial head of the calf muscle (gastroc) was displaced, it increased the flexibility of the hamstrings. The researchers hypothesize that the displacement of the fascia lower in the leg caused a reduction in the tension/pull of the connective tissue on the sciatic nerve. With a reduction in the fascial pull on a nerve, the nerve itself can move more freely, and it’s responses to being stretched are also modified. (note: some nerves, like the special nerves that trigger the stretch reflex actually “fire” or depolarize in response to how much they’re pulled on – so if they’re starting with a little “slack” in the line so to speak, it makes sense that the muscle can now be taken to longer lengths without the reflex kicking in!)
These morphological changes can occur with either passive or active forces being placed on the connective tissues. The fascia therefore responds to stretching, active work and massage. These morphological changes are of course temporary, and the connective tissue is only going to hold onto those changes for the length of time you are regularly using this extra “gliding space” of the layers.
So the answer is, Yes?
Yes, it is, but it’s also not quite that simple, as you can see. It’s also worth noting that stretching only really contributes to one of the components of flexibility training, so you’ll get more bang for your buck if you spend more time moving actively, and getting massages 😉
Enjoyed that? If you did, you might also like this: “Yes, Your IT-Band Is Tight…But It Should Be.”
Have questions? Pop them below.