Cryotherapy is the use of cooling as a means of treating injuries, and may be used in different ways on both acute and chronic injuries. Much research has been carried out on the effects of cooling on damaged soft tissues, and although the benefits are now widely accepted, there are varying opinions on how long the application should be to gain maximum benefit. There are many athletes who believe a long soak in a hot bath after an injury is the best remedy to ease the pain.

In many instances, no matter how small the injury, tissues will have either been stretched or an impact received causing blood vessels to be torn or damaged. The extent of bleeding will depend on the vascularity of the tissues involved, and may be increased if injured during exercise. Blood will flow out until the vessels are restricted (vasoconstriction), so preventing further blood leaking into the tissues. It is important to stop bleeding into tissues as the blood will act as an irritant, increase inflammation, and must be cleared from the tissues before the healing process can properly commence. Cells starved of nourishment from the blood due to injury will soon die. These dying cells stimulate the release of histamine causing the blood vessels to dilate, thereby bringing increased blood supply and extra nutrients to help repair and rebuild the damaged tissues. During this phase of increased but slower and more viscous blood supply, the capillary walls become much more permeable and quantities of protein and inflammatory substances are pushed into the area causing swelling. Various reactions continue at a rapid rate, all contributing to the healing process. Muscle spasm may also occur causing the muscle to contract either voluntarily or involuntarily, helping prevent further movement. However, this may have adverse effects by further restricting blood flow and placing more pressure on nerve endings, leading to increased pain.

 source: radiologyinfo.org