Some commercial heat packs are designed to fit specific parts of your body. Hot tubs or whirlpools are also useful. Heat creams and ointments are popular but don't provide heat very deeply into muscle tissue. The massaging effect of putting the cream on is helpful. Avoid getting these creams into your eyes or on sensitive skin.
Heat increases the blood flow to an injury and can worsen swelling.
Heat packs that are too hot or left in place too long may cause burns.