Debra Porter demonstrates a great hip flexor stretch. She opens up the hip of the raised leg and stretches the front of the leg muscles on that leg, which can tighten and lead to lower back pain. The benefit of using the Bosu for this stretch is that she gets some extra upper body stretch, as well as a little more rotation than normal through the hip area because of the extra height provided by the Bosu. Loosening up the hip flexors can also improve running stride and increase lung capacity by allowing the front of the body to open up.
To perform this stretch, lean forward into the Bosu, allowing some of your body weight to be supported by your arms.
Lift one leg high to stretch the hamstrings of the supporting leg.
As you relax into the arms and stretch the hamstrings, bend the lifted leg towards the opposite hip to stretch the hip flexor.