There’s two types of straddle split ability factors:
1. Constant: depth, location and direction of the acetabulum, type of the femoral head (long/short) and its provision in the joint.
2. Factors you can change: elasticity of muscles, tendons and ligaments, temperature of body and air, muscle strength, ability of muscles to contract and relax, neural regulation of muscle tone, time of the day, and even your psychologic status.
So, if the acetabulum is narrow, deep and facing forward, the neck of the femur rests on the acetabulum edge even in turnout position, diaphysis femur is thick, pelvis is broad and massive – there is a possibility you can’t perform a straddle spit by anatomic reasons. However, it’s extremely rare situation, I asked coaches who trained flexibility for decades (gymnastics, ballet, etc.) and they knew max. 1-2 people with this trouble in their entire practice. I still never met them.
If you can’t do splits with proper stretching and breathing exercises for a long time (regularly!! 2-3 times per week),if you don’t have any diseases and injuries of hip joints, injuries of muscles and ligaments in this area and your progress stopped, and attempts to force stretch brings acute pain instead of an ordinary discomfort or drawing pain, you should think about this anatomical obstacle. Front splits doesn’t count =)
How do you know for sure? If you are a fitness enthusiast, you should ask yourself if you really need a straddle split. The way you do it definitely have benefits. If you want it badly, it’s recommended to do MRI of hip joint, and discuss the results with an experienced sports medicine physician or traumatologist.