Given time, proper training, and the wish to succeed, would anyone be able someday to do a 180 degree split? Or are people out there that for some reason would never be able to do it, no matter how much effort they invest in it?
Tom Kurz says it's a misconception that most people are physically incapable of training up to a full side split:
There are simple tests that show you that for the great majority of people the structure of joints and the length of ligaments are not the obstacle for doing splits. You can use these test to determine if you have the potential to do front and side splits before you start your stretching program. (Yes, there are people who cannot do side splits because of hip joint deformity called coxa vara.)
Here is the side split test: Stand beside a chair or table and put your leg on it as shown below. Make sure that both your hips and your raised leg are all in one line. Repeat this test with your other leg.
If you think that the length of your muscles and structure of your hips will not let you do side splits, try this test… The leg resting on the chair is in the position it would have in a split
Now, what have you done? You have done “half side splits” with both your legs!
You have proved to yourself that both your hip joints have all the mobility (range of motion) needed for a full side split! You have also proved that the muscles of each of your legs are already long enough for a side split. You know that no muscle or ligament runs from one inner thigh to the other (or, if you don’t know it, you can ask your doctor). So, what keeps you from doing the whole side split with both legs spread sideways at the same time? Your nervous system, that’s what!
Yes everyone can eventually learn to do the splits. Most people weren’t born flexible (like me) but I can do the splits on both legs. You just have to put some effort into stretching and try every day. Unless you’re naturally flexible, most likely you won’t get the splits in one day. It might take weeks. Remember take stretching slow. Don’t force yourself down because you might get injured. Also don’t hold a stretch for more than 30 seconds.
I think it is extremely difficult for some people to the point it's not possible. Personally for me I believe it would be near impossible for me to do the front splits despite being able to do side splits. The curvature of my spine doesn't allow for the stretches needed to practice and perform the front splits. it depends if your spine is naturally more suited to flexion or extension (the shape of your spine being suited to bend back or forward). if your back isn't suited for it then it has to bend in the weak position twice as much as normal and depending on the angle of your hips it may not be feasible.