In one word, VERY! The kettlebell swing is a powerful movement with amazing health benefits that increase strength and flexibility. However when performed incorrectly it is also a movement that can create back, hip, or knee injuries when the form is incorrect. Always be aware of your form and periodically have a certified trainer evaluate your swing. You can also do this yourself using a smart phone or video camera. Be sure to squeeze the glutes and quads every time you swing tighten the abdominal muscles as if you are bracing hard for a punch. Swinging correctly will make your stronger and more flexible than ever before, however incorrectly performing the movement can create or increase back strain or pain.