Medicare Instant Hot Pack for Powerful Heat Therapy. Relax and loosen sore, tired and stiff muscles with the Medicare Instant Hot Pack. This Hot Pack provides form-fitting, flexible hot therapeutic relief for arthritis, muscle spasms, menstrual cramps and more. Hot Therapy can also be used beforeor after exercise to improve mobility by loosening muscles and increasing joint elasticity. Heat packs are particularly popular among people who experience lower back pain.