Instant Heat Packs are ideal for use in the treatment of pain and muscle spasm, in circumstances where you don't have access to an external heat source. Ideal for applications such as neck pain, arthritis, back injuries and shoulder injuries. Instant Heat Packs can be carried on day trips and away matches where conventional hot packs are not practical. Instant Hot Packs are activated by squeezing an internal compartment within the pack. This starts a chemical reaction that instantly makes the pack warm.