Recently I have received questions about hip flexors a lot, so I decided to summarize some of my thoughts on this issue. Hip flexor injuries are very common, especially at the beginning of the season. Athletes spend the entire off-season developing the lower body kinetic chain, adding weight to the squat and deadlift. The lower body kinetic chain and its role in hip extension is very important, but the hip flexors are forgotten.
Let’s first take a look at the major hip flexors, there are only 3 worth mentioning. The one that plays the biggest role is the rectus femoris. It spans two joints and can flex the hips and extend the knees. The iliopsoas and tensor fascia lata (TFL) muscles play a supporting role. Interestingly, it is possible that a supporting muscle plays a major role during a certain phase of the movement. During hip flexion, when the rotation of the femur reaches approximately 90 degrees, the iliopsoas muscle replaces the rectus femoris and begins to play the main role. This is because the rectus femoris muscle has fully contracted at this time and can no longer produce enough force.
Try this simple test. Stand upright, with your hands behind your head and your elbows fully open. Raise one knee above hip height while maintaining your stance. At this time, people often make several compensatory actions. The first is that you cannot maintain this movement for a long time. Second, your TFL may twitch. This is because if your iliopsoas muscle is not strong enough, the TFL will try to help bear the load, and twitching will occur. The third is that your stance changes in order to lift your knees high enough. Finally, the hips tilted. That's because quadratus lumborum (QL) is trying to help.
Standing iliopsoas strength test Compensatory movements 
Difficulty raising the knee Stand Change (Hip Tilt)
This test utilizes Gray Cook's Hurdle Step Test from the Functional Movement List The principle of this. The occurrence of various compensatory movements indicates insufficient strength of the iliopsoas muscle. Good question.
In "Motion Deficiency Syndrome" by Shirley Sahrmann. In the book "Diagnosis and Treatment", she talks about the impact of secondary muscles on movements. Her point is that if the primary muscles are often injured, you should check whether the secondary muscles are weak. For example, if your hamstrings are frequently strained, your hip muscles may be weak. If your pecs are frequently strained, your subscapularis may be weak. If your hip flexors are frequently strained, your iliopsoas muscles may be weak. Maybe not enough power. So how to solve this problem?