Not many people know this but living a sedentary lifestyle can really weaken your hip flexor muscle groups especially if you work a typical office job where you are sitting for more than 40 hours a week. I found this article on Train Your Hip Flexors – Hip flexors are muscle groups often neglected. Training this muscle group will ensure that your back and hip is healthy. really helpful. It goes in depth about the hip flexor muscles groups and how you can prevent injuries.