What happens when you stretch a muscle? (part 1)


“What happens when a muscle stretches?” This frequently asked question — a perennial query during anatomy trainings — deserves a few blog posts, especially as the mechanism is often misunderstood.

Stretching targets muscles in two ways. First, it directly affects muscle and connective tissue itself. As you’ll see in this post, those effects are temporary and short-lived. 

Stretching also indirectly affects muscles by affecting the nervous system. That will be the subject of the last post in this series.

Some substances are elastic; like rubber, they stretch and spring back. Others stretch and stay stretched, like taffy. Materials
that exhibit both characteristics – including muscles and connective tissues – are called viscoelastic. (In fact, taffy is actually viscoelastic. If you stretch it a lot, it stays stretched, but if you stretch it just a little, it springs back elastically.)

Technically, viscosity is resistance to flow. In unscientific terms, it’s gooeyness. Ketchup is viscous, as are honey and molasses. Many apparently solid substances are also viscous — they just flow very slowly. Even glass is viscous. Over centuries, it flows downward with the pull of gravity, accounting for the wavy appearance of very old windows.

When you stretch a muscle slowly, you take advantage of the tissue’s viscosity, allowing it to lengthen gradually. Stiffness is resistance to a change in shape. In the short term, stretching reduces the stiffness of muscle tissue. Stretch a muscle too far, too fast, however, and you’ll injure it or tear it, in much the same that if you stretch taffy too fast, you’ll snap it.

Heat increases viscosity. Warm honey flows more easily than cold honey. Likewise, warm muscles lengthen more easily and safely than cold muscles. It’s the internal temperature of the muscle that counts, not the temperature of the room. Muscles heat up when they work. That’s one of the beauties of vinyasa – it warms up your muscles from within. Of course, if the room is too cold, your muscles will lose heat faster than they can build it up, but you don’t need a 100 degree studio either.

What happens when you stop stretching a muscle? Basically, its elasticity reasserts itself. It returns to its normal, pre-stretched length, like a rubber band.

You probably already know this from your yoga practice. Let’s say that after an hour of sweaty, stretchy vinyasa, you can sink into full hanumanasana (splits pose), while an hour later, you’re not even remotely close to doing the pose. What happened? Your post-class muscles reverted to their normal stiffness. This means, that, for practical purposes, you don’t really own that level of flexibility. If your feet suddenly slide on a patch of ice on a cold day, you won’t effortlessly drop into full hanumanasana. You’ll more likely tear your hamstring.

Studies have consistently shown that the short-term effects of stretching on muscle tissue are, in fact, short term. Although stretching acutely reduces muscle stiffness, tissues return to their normal stiffness within 15 minutes. Thirty minutes after stretching, joint range of motion is back to what it was before stretching. That’s a good thing, actually. Muscles need to maintain an appropriate length and stiffness to work effectively. If they were as gooey as taffy, they wouldn’t be able to move your bones. Permanently lengthened muscles would just droop uselessly.

For years, athletes were told to stretch before working out or competing. However, recent studies have shown that static stretching immediately before a game causes performance to suffer. Stretched muscles are weaker and less powerful. (By the way, this only matters if you stretch right before a workout. Waiting even 15 to 30 minutes will allow your muscles to return to their normal stiffness and strength. And, obviously, you still need to warm up properly before working out.)

So, does this mean that you can’t increase your flexibility? Of course not. If you’ve practiced yoga for any length of time, you’re almost certainly more flexible than when you started. But it does mean that lasting changes in flexibility have nothing to do with changes in muscle and connective tissue itself.

This was demonstrated in this Danish study from 1996. Researchers compared the effects of three weeks of single-leg hamstring stretching on both muscle stiffness and range of motion. Although participants significantly increased their range of motion in the stretched leg, hamstring muscle stiffness did not change at all. What happened? Basically participants became more tolerant of the sensation of stretching, allowing them to go farther before reaching the point of pain. This seems to be the primary mechanism for long-term flexibility changes. 

The next post in this series will look at how those long-term changes happen.

Other posts in this series

Part 2


Magnusson SP et al. A mechanism for altered flexibility inhuman skeletal muscle. J Physiol. 1996 November 15; 497(Pt 1): 291–298

Magnusson P & Renstrom P. The European College of SportsSciences position statement: The role of stretching exercises in sports. Euro JSport Sci. 2006 June; 6(2): 87_91

Mizuno T et al. Viscoelasticity of the muscle-tendon unit isreturned more rapidly than range of motion after stretching. Scand J Med SciSports. 2013 Feb;23(1):23-30 

Weppler CH & Magnusson SP. Increasing muscle extensibility: a matter of increasing length or modifying sensation? Phys Ther.2010 Mar;90(3):438-49 

Copyright Joseph Miller

Photo: By lululemon athletica (Flickr: Yoga Journal Conference) [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

3 thoughts on “What happens when you stretch a muscle? (part 1)

  1. […] they must lengthen or that the tissue must become more pliable over time. Yet, as we’ve seen, the evidence doesn’t bear that […]

  2. […] 日本語訳 Yo Miyamura 原文リンク What happens when you stretch a muscle? (part 1) Joe Miller […]

  3. […] Part1はこちら 筋肉をストレッチした時に何が起こっているのか?(Part 1) […]

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.