Contrast Fomentation (Hot and Cold).

Cold bag use in acute injury because in internal inflamation, swelling ,and redness is inside,and hot bag use in chronic.

Hello Cold pack is suggested when the injury is acute for initial 3days because there is internal swelling, inflammation and redness inside .so applying cold back helps in reducing all these initial symptoms and pain . Hot pack is applied in chronic cases  or if internal injury is more than 3 or 4 days. Muscle spasm and tightness is reduced only with heating.

Cold pack for acute injury and Hot pack for chronic injury.Hope This will help

It's always confusing, and my suggestion is whenever there is swelling and the goal is to reduce it, use cold pack. If there is need of increasing blood flow in particular region, use hot pack.