I am a firm believer in stretching hip flexors every practice. Virtually everyone is tight in the hip flexors because we sit so much. Add repetitive movement exercise (like running and biking) and you get short, tight, hard hip flexors.
Tight hip flexors pull the pelvis into an anterior (forward) tilt. When the pelvis is misaligned, so is the spine, and there is an ensuing domino effect through the body. Standing up straight with tight hip flexors overworks the lower back muscles (since they are pulling against that anterior pelvic tilt), and is one of the leading causes of lower back pain. You will never have good posture with tight hip flexors!
So, open your hip flexors. Often.
What Exactly Are the Hip Flexors?
Hip flexors work together to bring the knee closer to the torso. There's actually quite a few of them. Here are the four major hip flexors:
How Do You Stretch Hip Flexors?
The most common way to stretch hip flexors is a simple lunge (vanarasana). It's one of those postures people assume is a no-brainer, but there are actually several necessary refinements you should know and practice if you want your lunge to be safe and effective, so be sure to review lunge basics before you learn the hack.
And now, the hack. All you need is a wall, a yoga mat, and a block. (If you have sensitive knees, you will also want a blanket for padding.) It does two great things: first, it begins by anchoring the top of the hip flexors and then stretching back and away from that anchor. Then it anchors the bottom of the hip flexors and stretches forward and away from that anchor. So the muscle fibres get a very deep opening in both directions, creating balance.
Second, this hack -- how can I say it delicately? -- it keeps us from cheating. When you're done, notice how much deeper your stretch was than your usual lunge (and it will be if you do the hack correctly). You will have a new level of wisdom in the cells of your hip flexors, and you should make it your new normal. Don't fall back into the routine of a ho-hum lunge.