A person with little to no flexibility training can (usually) stand on their left leg while supporting their right leg on a table such that the two legs form a 90-degree angle. The same person can mirror that action by standing on their right leg and lifting their left leg to a 90-degree angle. So if someone has the range of motion to move each leg individually 90 degrees, then...
Why can't they do a split?
Is it because there is some physical connection between the two legs? Are there muscles, tendons, or ligaments that tie the two legs together so when they simultaneously stretch out their range of motion is limited? No. The reason one can't do a split is neurological, when they attempt to move a muscle or group of muscles past what their nervous system believes to be their maximum range, their muscles start contracting to prevent the stretch. Stretching further requires removing unnecessary muscle contractions, not increasing the length of tissue.1
This is the premise of Pavel Tsatsouline's book, Relax Into Stretch. Mr. Tsatsouline gives a list of different techniques and stretches to "trick" the nervous system into allowing your body to realize its full range of motion. I cannot verify Tsatsouline's claims myself, as I have not yet attempted his methods. But guitar has taught me that learning to relax muscle groups can lead to an incredible increase in performance.
- I have seen other sources argue that it is indeed possible to increase the length of tissue, by adding length to muscle fibers via adding "sarcomeres" - the contractile unit of the fiber. The field of sports "science" seems to be wrapped in pseudoscience. I certainly don't know whether Tsatsouline's theory is correct. [↩]
"when they attempt to move a muscle or group of muscles past what their nervous system believes to be their maximum range, their muscles start contracting to prevent the stretch." - the 90-degree phenomenon seems much more simply explained that the pelvis tilts to divide the angle between the two leg joints, thus the range of motion of each could just be some 50 degrees. Not that there aren't purely-neurological constraints, e.g. try fully extending the ring finger while keeping the rest in a fist; it can typically be done without pain but only by relaxing the fingers indeed and holding them in place externally. But if this were the case with splits, you'd expect the legs could be arranged in the same way without special training and books on the subject.
p.s. "relax muscle groups" link is broken.
Thanks for making this point. I went ahead and tested this out in the following way: I tried to isolate the "pelvic tilt" by moving my hips left and right while keeping myself otherwise straight with feet planted ~ shoulder-width apart. After playing around a bit I found how I could tilt to make the stretch easier (i.e. cheat and thus refute Tsatsouline's argument.)
I figured that, if pelvic tilt was a non-factor, I should be able to push my hips slightly to the left as I held my right leg to 90 degrees. I tried this and was _unable_ to do so. (This exercise of pushing the hip to the opposite side felt great and I wouldn't be surprised if doing it regularly is a good way to build up to a split.) So I guess yes, Tsatsouline's reasoning/argument is flawed although his conclusion that inflexibility comes primarily from an over-restrictive nervous system may still be correct.
With this said, I've been following his advice to hold stretches for a long time "until the muscles are so tired that they relax/give up and stop contracting, thus allowing you to stretch further" and have made measurable progress within a week. (For the first time since I played soccer in high school I can sit on the floor with one leg crossed and the other straight and reach and touch my toe.)
And ty, I fixed the link.