Hip flexor strength is extremely important yet it is often overlooked. All too often the hip flexors are blamed for being too tight. BUT have you considered the consequences of the hip flexors being weak? If that's the case, stop stretching that weak muscle and let's strengthen it!
Training hip flexion at end range is especially important for sprinters, hikers, and hurdlers. A great drill to train the hip flexors is demonstrated here using a stool. However, there is a correct and incorrect way to do this exercise.
A correct version is demonstrated on the left. The individual should focus on keeping their trunk upright and using their hip flexors to elevate their foot off the stool without compensation.
An incorrect version is demonstrated on the right. Another way you can lift your foot off the stool is by compensating by basically doing an ab crunch in a standing position. You can appreciate the amount of spine flexion happening in this demonstration compared to the other.
Demonstrated last is a progression of this exercise by simply raising the height of the stool! Same rules can be applied to regressing the exercise by lowering the surface. Another alternative is to provide support for one or both arms if you're having difficulty balancing. [P]rehab those hip flexors!