The science behind how ice baths promote faster recovery is largely based on how cold therapy treats acute injuries. As background, the body’s response to an injury has two stages: primary injury and secondary injury. Primary injury is the tissue damage from the initial trauma. Secondary injury is the cell death caused by a blockage of the oxygen supply to the injured area. If a cell’s oxygen supply is cut off, it dies. Dead cells release their contents into the surrounding area. The presence of these substances causes an inflammatory reaction to occur which clogs blood vessels and causes further cell death.
Cold therapy is concerned with treating this secondary injury by limiting cell damage. Cold therapy reduces the temperature of the treated tissue area up to 5 cm (2 in) below the skin’s surface. Cells react to the reduced temperature by decreasing their metabolism and, as a a result, decreasing the amount of oxygen they require to survive. By promoting the survival of more cells there is less tissue for your body to repair and less inflammation which can cause further cell death. This promotes faster recovery and is the most beneficial effect of cold therapy. Another beneficial effect of cold therapy is that is constricts local blood vessels which decreases the buildup of fluids at the injury site and improves the quality of muscle recovery by draining the lactic acid that accumulated in the stressed muscles.
Intense exercise like long distance work, interval training or racing may cause microscopic muscle damage (microtrauma) which produces the same physiological effects as an acute injury, although on a broader scale. The body has the same two responses to the trauma – primary injury and secondary injury. Ice baths, a form of cold therapy, are intended to limit the scope of the secondary injury in the same way described above.
If you're into using ice baths or interested in cold therapy as a recovery technique, consider using our cold leg wraps (they are much more comfortable than an ice bath!).
To learn more about cold therapy, check out chapter 5 of Chad Starkey's book titled, Therapeutic Modalities, 4th Edition, which is previewed in Google Books. This is probably the best online resource on cold therapy.