The hip flexor muscle group is a very important functional component of the body. This group is instrumental in maintaining a strong core and optimal movement. When these muscles get tight or weak they will interfere with function and cause pain and tightness in the lower back and hips (especially while standing), poor posture and gluteal pain. The most common cause for tight or weak hip flexor muscles is prolonged episodes of sitting or bending over. It is important to maintain flexibility and strength in these muscles, they contribute to overall stability of the spine.
Take a few minutes to read through the attached articles; you will discover some great exercises to do to stretch them out and strengthen them. You will also learn the correct and recommended way to do a sit up (or should I say crunch?).