High lunge yoga pose can be used to stretch the hip flexors.
The hip flexors work to close the front of the hip joint. With high lunge you focus on opening the front of the hip joint by sinking the pelvis down.
So that you also stretch the psoas major muscle, you can focus on lengthening your torso upwards, and even on bending your ribcage backwards as you hold this yoga pose.
Starting with your back knee on the floor, you can have the top of the foot on the floor or you can tuck the toes forwards.
I like having the top of the foot on the floor since when I lift my knee I then get a stretch along the whole front of the leg, including the front of the ankle. However I also like to practice with toes tucked under.
Focus on pressing your back foot into the floor as you inhale. Imagine kicking the floor with your foot. As you do so notice the tension in your thigh as you increase the pressure. See if you can increase the pressure to the point where your back knee is just about to leave the floor. Then gradually decrease the pressure as you exhale.
If you focus on feeling your back knee as well as your foot you can notice the point at which your knee is about to leave the floor. The knee will feel "empty." (I've included an in-depth explanation of "empty" in my book Balance Basics.) If you relax the leg the knee becomes "full". Try the following four part exercise. Do each part slowly and smoothly.
I used to teach this next part first but I found that students unfamiliar with their body got confused when I added the knee lift after. So I teach beginners the knee lift first and then add the upper body.
With torso upright, focus on first lengthening your neck. Pull your head back and up, and your chin inwards and downwards. Make the back of your neck feel long. Then let your head move forwards and relax your neck as you exhale.
Then, as you lengthen your neck lift and open your chest. Then relax both together while exhaling. Now add the knee lift. As you lengthen your neck and open your ribcage, press your foot down and smoothly lift your knee. Then lower your knee and relax your leg as you relax your spine and neck.
Initially you can have both hands on your front knee to help push your ribcage up and back. Once you are comfortable with lengthening your spine and lifting your knee you can add the arms. On a single inhale, reach the arms forwards and then up as your lift your knee and lengthen your spine. For smoother synchronization I like to reach the arms forwards as I make the back knee empty, then I reach the arms up as I lift the back knee. Reverse these steps to come down.
As you get more comfortable with high lunge you can try bending your spine backwards. I'd suggest that you try lifting your back ribs to give yourself more room to bend. You may also find it helpful to internally rotate your back leg as you do so. But keep your back knee pointing down.
You can also do High lunge with your back shin against a wall.
Use blocks for your hands or keep your hands on your front knee.
For this version, as you inhale you can press your back foot into the wall. Relax as you exhale.
From here you can work at moving your front foot forwards so that you work towards front to back splits.
Keep the torso upright to keep this as a hip flexor stretch.