Heat Wave Instant Reusable Heat Packs provide therapeutic pain relieving heat in seconds and are reusable for a lifetime. All Heat Wave products are portable, so they can be used at home or office, while driving or commuting, or while attending or playing sports. The medium and large rectangular packs can be used for: Ankle, calf, knee, thigh, and back pain General muscle soreness and menstrual cramps EcoFriendly = Cost Savings We tested Heat Wave Instant Reusable Heat packs and stopped counting at 500! You never need to buy another heat pack. If you have been using disposable hand warmers or wraps, you only get one use. Then what? They get tossed in a landfill, with iron metal shavings making their way into the ground water. Then what? You need to continue to shell out more cash every time you need one. Not with Heat Wave. One purchase and you are set for life! Heat Wave Instant Heat Wraps reach a soothing 130 degrees. Instant heat and reusable Large pack heats up in under 60 seconds Lasts several hours under clothing or fabric Reusable 500 times and more Eco-Friendly - reusable - never has to be disposed