If your hip flexor muscles are not flexible, you will not even be a good athlete in your sport(s)! Tight hip flexors also lead to some painful injuries (like pulls and strains) that are hard to recover from. There's a reason that cornerbacks in the NFL make loads of cash: they have great speed/quickness and the best swivel hips around!
Do hip ins, outs and holds before and after your workouts to help loosen those tight hip flexor muscles!
In, Out and Hold. Lean forward, with your hands on a wall for support. Swing the right leg out and in 3 times and hold for 6 sec after the third swing. Start by swinging the right leg out away from the body. Then swing it back across the body and out again. Hold in the "out position". Next, swing the leg inward across the body, back out and in again. Hold in the "in position". Do 3 reps with each leg.