Image via Runner's World
You see this stretch being done everywhere by everyone. It's undoubtedly one of the most popular stretches in the world. Whether you lifts weights, do yoga, or run, chances are you've probably at least tried this stretch before. Although this stretch can be good for loosening the hip flexors, most people get it slightly wrong - unfortunately, it's the small details that can make the difference between effectively stretching a muscle and reinforcing harmful imbalances. Let's explain the reasoning behind the stretch and its ubiquity first.
This stretch is mainly for a muscle group referred to as the hip flexors. These muscles work together to bring the knee toward the chest, the motion of which is called hip flexion. Now, when the hip flexors shorten, they bring the knees up toward the chest, and when they lengthen, the opposite happens. As you can see, with this stretch we're trying to lengthen the hip flexors by bringing one knee at a time away from the chest. Since so many of us sit for prolonged periods of time (and the hips are flexed when sitting in a chair), this stretch is often used to combat the chronic shortening of our hip flexors. As such, it's prescribed by everyone from meatheads to physical therapists.
When most people do this stretch, they simply get into a kneeling lunge and push themselves forward as far a possible to get into the "stretch", like this:
What not to do.
However, simply pushing yourself forward like this is not enough to truly lengthen the hip flexors. It looks like I'm getting a pretty good range of motion here, but I'm not actually getting a very good stretch in my hip flexors. Here's why - check out the lower back. See how it's arched forward? Now look at the structure of the iliopsoas group - the most powerful of the hip flexors. These muscles run from the femur (thigh bone) to the lower back, and therein lies the key to an effective hip flexor stretch.
Image: Beth Ohara via Wikipedia
When you allow the lower back to arch forward during a hip flexor stretch, the pelvis tips forward with it. This is called an anterior pelvic tilt, which can be a postural result due to tightness in the hip flexors. Basically what's happening is that, during this stretch, your tight hip flexors are literally pulling your spine forward with them. This results in a suboptimal stretch, if any at all. That's right - if you're not careful, you might not be stretching your hip flexors whatsoever. All that's happening is that you're jamming the head of your femur into the front of the hip capsule.
Here's what to do instead: as you go into the stretch, imagine that you're:
Tucking your tailbone under you - this keeps your pelvis from going into an anterior tilt.
Flattening your lower back the entire time - don't let it arch.
You'll probably find that if you keep these things in mind, your range of motion will be (much) shorter, but you'll feel a much more intense stretch in the muscles of the hip flexors. Bonus tip: squeeze/tighten the glute on the side you're stretching - this will create extra stability in the hip joint as you stretch the flexors.
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