The Roman Chair can help with hip flexor strengthening but it is also an exercise that is difficult for mastering good form. The key with the Roman Chair is to keep the low back flat against the backrest at all times. If your back arches at all, you are risking irritating the joints in the low back. So it's important to move slowly and with awareness during this exercise. It is also better to allow your knees to turn out slightly as you bring them up. That de-emphasizes the tensor fascia lata (TFL) which is often tight and over active in these types of muscle imbalances.
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2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS