Been working on this ever since I started Foundation (got Foundation around June, only seriously started working on it September?)
Some helpful tips to those who are still struggling with Swivel Hips:
1. This is a hip exercise (hip internal/external rotation). You should feel it in your hips rather than on your knees. Putting too much pressure on the knees isn't good for your ligaments.
2. If you feel it in your knees, either you're performing exercise incorrectly or you need more work on your knee. At least have the SLS progressions mastered past SLS/PE3. The strength elements helps your hip adductors (especially your gracilis) which benefits this exercise and the integrated mobility helps the knee internal/external rotation.
3. Stretching the piriformis will be very helpful in the hip internal rotation aspect (or even hip external rotation). Trigger points could also be present in piriformis, it's even possible for the trigger points to have referred pain in the knees.
4. When massaging piriformis, common mistakes is to massage it in the middle of the butt, to properly massage it, you should be doing it more laterally (rather than medially). If massaging by yourself, I find lacrosse ball to be the deepest.
5. If piriformis doesn't help, go after the hip internal rotators (TFL, gluteus medius, gluteus minimus). Massage the gluteus medius and gluteus minimus. Stretch/massage the TFL.
6. Posture actually pays a big part in Swivel Hips. Believe it or not, this was actually the one that caused the biggest increase in range of motion. Your back should be arched. The lumbar spine shouldn't be rounded in this exercise.
For those thinking of skipping this exercise:
A major sticking point in Swivel Hips is the lack of hip internal rotation. Gluteus medius is one of the muscles responsible for hip internal rotation. Swivel Hips helps to ensure you have the required mobility (such as gluteus medius) before moving on. From sPL/PE2>iM and onwards, it's all about building strength in the gluteus medius muscle in the mobility you build up in Swivel Hips.
Gluteus medius is a frontal plane muscle so it is responsible for stabilizing the hips (especially in single leg stance) and side-to-side movement (eg. abduction). It helps in all straddle positions because in a straddle position the hip is abducted.
