With the "3-Way Leg Swings" I can barely complete 10 reps (with a very modest range of motion). But the Gluteus medius on my standing leg quickly "burns" intensely. Also, I feel a "burn" in my swinging leg, around the Sartorius / Iliopsoas. The burn is less intense than with the standing leg, but still there. Any thoughts?
You can decrease the number of repetitions. You may start with 5 or 3 repetitions. Please make sure that you can feel that your glute muscle is working so that your hip flexors release. Keep in mind that you won't over fatigue it.