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.
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. 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.
Other posts in this series
Copyright Joseph Miller