What happens when you stretch a muscle? (Part 2)

In the previous post in this series, I described how stretching temporarily affects muscle tissue. This segment begins a discussion of long-term changes in flexibility.

Ultimately, lasting gains in flexibility have more to do with your nervous system than with your muscles. More on that in the next post, but first, let’s look at an alternative hypothesis, namely that stretching increases muscle length due to the addition of sarcomeres within muscle cells.

Muscle cells, or fibers, are made up of bundles of thin strands called myofibrils, which run the length of the cell. Myofibrils are in turn composed of smaller units called sarcomeres. Linked end to end, like railroad cars, sarcomeres make up the length of the myofibril. Sarcomeres are the contractile units of a muscle. When the nervous system tells a muscle to contract, protein filaments within the sarcomere slide across one another, shortening the sarcomere. As each sarcomere along the chain shortens, the entire muscle cell shortens.

Structure of skeletal muscle

In experiments in which a lab animal’s muscle is immobilized in a shortened position for up to several weeks, those  fibers lose up to 40% of their sarcomeres, shrinking the muscle’s length.

Likewise, when a muscle is immobilized in a stretched position for a long time, extra sarcomeres are added to the ends of the myofibrils. The result is a longer muscle, although the changes tend not to be as dramatic as when muscles are fixed in a shortened position.

In any case, when the muscles are released from immobilization, they tend to quickly return to their normal length.

It’s not known whether this happens in humans, because such experiments would never be approved for people. However, it’s a reasonable guess that if sarcomerogenesis, as the process is known, occurs in mice, human muscles would probably react the same way.

However, it seems unlikely to me that sarcomerogenesis has anything to do with the changes that happen with voluntary stretching. Sarcomerogenesis requires muscles to be immobilized for long periods, and even the most ardent yogi isn’t going to hold paschimottanasana for a week or two to get longer hamstrings.

On top of that, it’s hard to see what advantage there would be in increasing the actual resting length of a muscle. Muscles need to be a certain length to contract effectively. In fact, that’s probably why muscles add or subtract sarcromeres when they’re immobilized; it allows them to contract more effectively. The length of a muscle is determined by the distance between its attachment points to your bones. Making the muscle longer would serve no purpose.

My guess is that if sarcomerogenesis plays a role in increasing flexibility outside of the lab, it’s only as part of the recovery process after prolonged immobilization of a limb. For instance, if your arm is placed in a cast for several weeks with your elbow bent, your elbow flexors will probably adapt by removing sarcomeres. When the cast is removed and you regain your normal range of motion, they probably add sarcomeres to return to their previous length. But I don’t think it plays much of a role in the kinds of increases in flexibility that you’d normally see with regular yoga practice.

So, if muscles don’t actually lengthen when you get more flexible, what does happen? I’ll take up that question in the next post.

Other posts in this series

Part 1

References

Gajdosik RL. Passive extensibility of skeletal muscle: review of the literature with clinical implications. Clinical Biomechanics.2001 Feb;16(2):87-101<

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