It is clear that stretching doesn't actually make muscles permanently longer, experts agree. Instead, it may be that exercises such as reaching for your toes train the nervous system to tolerate a greater degree of muscle extension without firing off pain signals. ...
When animals are placed in casts that keep their muscles extended for a long time, their bodies do add additional sarcomeres, or the basic subunits of muscle fibers, but their muscles return to their original shape soon after the animal is removed from those constraints. And in those studies, it's not clear that the lengthened muscles improved the animal's flexibility.
In a June 2014 study in the journal Clinical Biomechanics, Tilp and colleague Andreas Konrad found no differences in people's muscles and tendons after six weeks of a static-stretching regimen.
So, if muscle fiber doesn't get longer as a result of stretching, why does stretching seem to increase people's flexibility? ...
The nervous system is the master conductor determining how far a person can stretch, said Brooke Thomas, a yoga instructor who discussed the science of stretching in a blog post on Breakingmuscle.com.
Nerve endings are dispersed throughout the muscle and tendon, and if a stretch doesn't feel safe for the muscle, those nerves will fire, registering pain and resistance, Thomas told Live Science.
These nerves "will say 'you better stop stretching, because if you stretch further, the muscle will maybe get damaged,'" Tilp told Live Science.
That's why a person under anesthesia, whose nerves are quieted, can be stretched through a full range of motion with no resistance. And healthy babies are born able to do the splits, because they haven't developed a blueprint for ranges of motion that feel unsafe, Mitchell said.
--Tia Ghose, Live Science, on flexibility being all in your head