A strain is a stretch or tear of a muscle or tendon. The hip flexor muscles allow you to lift your knees and bend at the waist. So with a hip flexor strain it will be painful or impossible to do these motions

Hip flexor strain occurs from overuse of the muscles in your hip. Bicyclists, athletes who jump or run with high knee kicks, athletes like soccer players who do forceful kicking activities, and ninja’s get this the most.

Pain in the groin region, where the thigh meets the pelvis Pain with hip flexion This would hurt with a hip flexor strain 

Putting ice packs on the injured area for minutes for 2-3 days, or until the pain goes away Taking anti-inflammatory medications Doing rehabilitation exercises to help you return to your activity When you are recovering from your injury, you will need to change your sport or activity to one that does not make your condition worse. For example, you may need to swim instead of bicycling or running.

As an athletic trainer you can also help out the athlete by wrapping them with an ace bandage.  Like so…

Everyone recovers from injury at a different rate. Return to play will be determined by how soon the injured area recovers, not by how many days or weeks it has been since your injury occurred. In general, the longer that you have symptoms before you start treatment, the longer it will take you to get better.

You have full range of motion in the injured leg. Your leg has regained normal strength. You are able to jog, run, or sprint straight ahead without limping You are able to cut at full speed without pain You are able to jump on both legs without pain You are able to jump on the injured leg without pain