A hip flexor or glute medius stretch should always be part of a dynamic warm-up. These muscles are commonly tight from sitting and aggravated from lifting. Unfortunately, leg swings and walking lunges don't always do the trick.

The low back lunge, on the other hand, definitely stretches the hip flexor if the back leg is kept straight, in addition to stretching the glute and quadratus lumborum when the elbow is brought towards the foot.

Low Back Lunge

The extra trunk twist in the video is totally optional, but it's an easily incorporated addition that'll get rid of any T-spine tightness.

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Related:  The One Bar Warm-Up