the heart and activity monitors. - I just recently gave one to my son in the red color. Seems a waste. But instead JBL used a more powerful single driver. The boys love it! They are the perfect fit and they do not slide out in the back like sketchers. My daughter gets super hot since her foot sweats a bit with very cushiony slippers but there?s many people that would love this. Straps fit my narrow ankles which sometimes is an issue for me. 5 in shoes, but the Rainbow size Large / 7. I havent had a headset die on my this fast.