Once Inspired

Shortness of breath is related to the fact that body heat is dissipated approximately two to four times as fast in cool water as in air of the same temperature. During exercise in cold water, skin heat loss can be 70 times greater than in air of equal temperature.

On immersion in cold water, exercise performance may be significantly impaired at first, as a result of the initial physiological responses to cold. The respiratory response may cause swimming failure within a few minutes, as high breathing rates make the synchronization of breathing and swim stroke impossible, thus resulting in the inhalation of water. It has been reported that compared to thermo-neutral water, cold waterincreases oxygen demands during moderate exercise requiring oxygen consumptions up to 2.0 l/min, but not intense exercise at 3.0 l/min.

Approximately three minutes after cold-water immersion, peripheral cooling may begin to affect performance. The hands are particularly susceptible to cooling, largely because of their high ratio of surface area to mass and, to a lesser extent, their low level of local heat production and variable blood supply. The effects of peripheral cooling are primarily due to alterations in muscle and nerve function. Low muscle temperatures affect several chemical and physical processes at the cellular level, including metabolic rate, enzyme activity and calcium and neurotransmitter release from nerves. In addition, there is increased muscle tone of both protagonist and antagonist muscle groups.

The rate of conduction of nerve impulses is slowed in water temperatures below 20 degrees C. The cooling of peripheral motor and sensory nerves leads to dysfunction equivalent to peripheral paralysis. The result of these alterations is that performance is reduced, as maximum power output is reduced by 3 percent per degree fall in muscle temperature and mechanical efficiency is also reduced.

By contrast, muscle temperatures above 27 degrees C are associated with greater ability to sustain muscle contractions. This may be due to slower production and accumulation of the metabolites that cause fatigue. At muscle temperatures below 25 degrees C muscle fatigue occurs earlier as cooling begins to impair neuromuscular function in peripheral muscle fibers, leaving a smaller number of fibers to produce the same amount of force. The changes in neuromuscular function resulting from cooling explain, in part, the reduction in work capacity in cold water. Other reasons for this decline are alterations in central circulation and reduced core body temperature.

The shivering evoked by cold-water immersion increases oxygen consumption. This effect becomes increasingly pronounced as more and moremuscle groups are recruited and as skin and deep body temperatures continue to fall.There is progressive involvement of the muscles of the neck, torso and finally the extremities. It is possible for shivering to occur during exercise but this reflex is progressively centrally inhibited with increasing exercise intensity. Shivering has been reported to be 80 percent suppressed by exercise at an intensity of 50 percent VO2max and totally suppressed at workloads requiring oxygen consumptions exceeding 1.2 l/min.

Higher fitness levels may protect against the long- and short-term responses to cold-water immersion. If all else fails, the initial responses can be attenuated by slowing the rate of entry into cold water, while swimming performance is significantly improved if the initial ventilatory responses are allowed to subside before swimming is commenced.A fitter individual will also have a greater capacity to cope with the increased work requirements associated with exercise in cold water. With less fit individuals, the combination of the reduction in body temperature and the increased oxygen requirements for sub-maximal work in the cold may seriously impair physical performance. Maintenance of normal levels of muscle glycogen in largemuscle groupsmay be an important factor in protection against the cold. It has been reported that low skeletal muscle glycogen levels are associated with more rapid body cooling during coldwater immersion in humans.

Commercial and recreational divers, long-distance swimmers and triathletes who spend long periods in cool or cold water may experience considerable bodyheat loss. However, if exercise is performed at a high intensity, it is theoretically possible that normal body temperature could be maintained. Most swimmers in long distance endurance events wear a wetsuit for thermal insulation in cool/cold water. However, at least two studies have shown that wearing a wetsuit in moderately cold water does not prevent a decline in body temperature. For example, 1.5 hours of water immersion (14 degrees C, 57 degrees F) performed by a wet-suited breath-hold diver caused rectal temperatures to fall from 37.7 to 36.9 degrees C (99.9 to 98.4 degrees F). Of course, triathletes typically spend less time than 1.5 hours in cool to cold water.

Finally, with regard to thermal comfort, heat flux and maximal swimming performance, the optimal pool temperature for competitive short-distance swimming is 28 to 30 degrees C. For longer distances, water temperature should be lowered to about 25 degrees C, as this will enable thermal balance to be maintained. Outdoor, long-distance swimmers are advised to train in water at temperatures they expect to encounter in competitive events.

Taken from http://triathlete-europe.competitor.com/