Choosing whether to use a hot or cold pack depends upon the type of injury.Use cold therapy when you have acute injuries, for instance, a knee sprain, any type of swelling, or post-surgery. Ice lessens the inflammation around the very area of injury, and can numb areas where pain exists. Use hot therapy for chronic conditions, like stiffness, and muscle discomfort. Application of this very therapy opens up the blood enabling blood to flow flawlessly to the affected area.