For best results, the back of your Apple Watch needs skin contact for features like Wrist Detect, the Taptic Engine, and the electrical and optical heart sensors. Wearing your Apple Watch with the right fit—not too tight, not too loose, and with room for your skin to breathe—keeps you comfortable and let the sensors do their jobs.

You may want to tighten your Apple Watch band for workouts, then loosen it when you’re done. In addition, the sensors will work only if you wear your Apple Watch on the top of your wrist.

Learn more about getting the best results when you use the Blood Oxygen app on Apple Watch Series 6.

If you have a Solo Loop or Braided Solo Loop, the band size should be comfortable, but snug against your wrist. Simply pull from the bottom of the band to stretch it over your wrist when you put it on and take it off.