The Hot or Cold Shoulder Sleeve goes from microwave to freezer to deliver hot or cold therapy for sore muscles. Simply heat it in the microwave for heat therapy in seconds. Or, store it in the freezer for cold therapy any time you need it. The unique gel interior stays flexible, even at -20 degrees F. The adjustable strap fits securely around your chest to relieve minor aches and pain for up to 40 minutes. To determine which sizes may best fit you, you use your shirt sizes.