I do hip flexor exercises at the gym by bringing knees to chest on the apparatus shown above. Usually three sets of twenty to thirty repetitions, with visits to a couple of other stations in between each set. (That's not me in the picture). Bent legs. Some people do it with straight legs but not me. Or alternating straight, bent, straight, bent etc. But not me, because of my lower back. I find it a very easy exercise; being a runner my hip flexors are strong to begin with. Does work the lower abdomen a bit as well though and I feel it there afterwards.
