Hi, I am the physical therapist who manages Sk8Strong. I feel the need to weigh in on this topic! You CAN have both a flexible back and a strong core. Back flexibility has more to do with the mobility in the joints and the joint capsule of each vertebrae in the spine, not flexibility of the muscles. When you bend backwards each vertebrae extends back on the one below it. If I person does not have good back extension, it should not not be forced, as you are just 'jamming' the joints into extension. This can lead to inflammation in the facet joints, and lower back pain. In regards to hip position, you are genetically born with your hip socket in a certain position/angle. The amount of turn-out is also determined by the position and mobility of the joint capsule, not necessarily the muscles. You can only improve hip turn-out so much with flexibility, as you don't want to over-stretch the capsule.
Every skater needs a strong core to be able to be a good skater. Those with hypermobility in their back need even more core strength to stabilize vertebrae that may move too much. The same applies to a person with too much hip mobility. An extra mobile hip capsule in conjunction with hip weakness (which is very common) can lead to hip, knee, or back problems if not addressed. All skaters need a strong glute medius.
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