The muscles that make up the hip flexors include:
Many of the exercises being promoted as "ab exercises" actually work the hip flexors more than the abs. The hip flexors are strong powerful muscles that often overtake the abdominal muscles when performing some variations of abdominal exercises. In order to isolate the abdominals you need to minimize the involvement of the hip flexors and maximize the contraction of the abdominals.
One example of an ab exercise that actually focuses on the hip flexors includes the full sit-up exercise, especially when the feet are held down. This movement primarily involves the hip flexors and may cause the lower back to arch. This could increase the risk of back pain, particularly if you have weak abdominal muscles. Therefore, the full sit up is not recommended for beginners.
Another example of an ab exercise that works the hip flexors is any leg-raising exercise done in a supine (laying face up) position. Again, this movement works the hip flexors far more than the abs and shouldn't be done until you have good abdominal strength.
Keep in mind that the best way to isolate the abs is by minimizing the involvement of the hip flexors while doing your ab workout.