Physiology of Long-Distance Swimming

In September 2013 Diana Nyad completed a 53 hour, 110 mile swim from Cuba to the Florida Keys. After a number of failed attempts she finally set a world record in marathon swimming at 64 years old.

Diana Nyad routes

The sport of long distance swimming is populated by many interesting athletes who have a drive unlike some other sports. It’s one thing to swim for hours in a pool, but these athletes swim for days on the open ocean, dealing with sharks and jellyfish as well as the physiological challenges of strenuous long duration exercise. Setting records like these takes a special kind of determination. In this article we are going to examine some of the physiological aspects of long-distance swimming.

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 Long distance or Marathon swimming as it’s called, requires that swimmers face a number of challenges, some unique to the sport, including physiological demands, environmental factors and even psychological challenges. Swimming these distances in the open ocean poses a series of challenges not encountered in a swimming pool. For any activity this intense, clearly a long distance swimmer will burn through a large number of calories. Studies show that even in short events, blood glucose levels rise as the liver mobilizes sugars used in muscle metabolism faster than the muscles can process them.

High blood sugar levels then flood the muscle cells where sugars are turned into ATP using the oxygen requiring process of aerobic respiration. When oxygen levels fall in the active cells, they switch over to the less efficient anaerobic fermentation, leading to the production of lactic acid. Blood lactate levels then rise indicating high levels of muscle metabolism.

A key component of long distance swimming success is a nutrition plan that provides a steady stream of calories, typically in the form of carbohydrates as well as electrolytes and fluid replenishment. Swimmers use various liquid concoctions that include a variety of sugar combinations, some protein powder and electrolyte mixes. This provides the necessary nutrients and fluids lost during such rigorous exercise. Swimmers report they consume 300 to 500 calories per hour and must maintain a scheduled feeding regimen during the course of the swim.

Making this more difficult, swimmers may experience salt mouth or a buildup of salt in the mouth and throat from ocean spray or swallowing mouthfuls of salt water. This leads to inflammation of the mucous membrane surfaces making swallowing very painful. Such a state makes it hard to take in nutrients and fluids. This can lead to dehydration. Cold water has a diuretic effect which further complicates the dehydration issue. In cold water, the blood vessels in the extremities tend to vasoconstrict. With reduced blood flow in the periphery the blood pressure can suddenly increase. The easiest way to remove the excess fluid is to increase glomerular filtration rate and send the fluid through the kidneys and into the bladder. This becomes another mechanism of lost fluids, leading to dehydration.

One study showed that swimmers in cold waters will not suffer from the effects of hypothermia provided the time spent in the water is relatively short, less than 30 minutes. The heat generated during exercise tends to counteract the effects of being in cold water, but only for a period of time. Longer exposure would result in a lowering of core body temperature to possibly dangerous levels. Simple immersion in cold water impairs respiratory function leading to a loss of breathing control as well as a racing heart rate and hypertension. A more substantial reduction in core body temperature or hypothermia, leads to heart arrhythmias, pulmonary edema and a slowing down of the activity of the central nervous system. Even the kidneys are affected and will shut down.

To avoid hypothermia, most swimmers try to set records in warmer waters like off the coast of Florida or California and locations throughout the Mediterranean. Many swimmers cross the English Channel but only during warmer months. It is possible to acclimatize to swimming in colder water by training in such conditions.

Recent studies indicate that part of the acclimatization process involves two different types of adipose tissues that help the body to deal with cold water swimming. Brown fat and white fat play a significant role in acclimatizing to swims in cold water. White fat cells store energy in the form of fatty acids which are released when metabolism increases. Such fatty tissues also has an insulating effect though most long distance swimmers are quite lean.

Most adults also have a small amount of brown fat cells that actually metabolize fatty acids into ATP, releasing heat in the process. White fat cells just store energy while brown fat cells undergo metabolism much like muscle cells and generate heat along the way, which helps to counteract the effects of hypothermia. Exposure to cold helps activate the growth of brown fat cells, a part of the larger acclimation process.

Another difficulty faced by long distance swimmers has to do with the time it take to complete these marathon swims. Depending on the distance to cover, weather and currents, these athletes may be in the water for 12 to 24 hours or more without rest or sleep. Surprisingly, sleep deprivation is not considered a factor in the physical performance of the task of swimming. While there are few athletes who have made swims over 24 hours, some studies suggest that sleep deprivation does not impair the physical act of swimming. It does however play a role in the mental state of the person. Unsuccessful marathon swims happen when the athlete is either physically or mentally unable to continue.

Swimming in the ocean also poses risk of interaction of the creatures that make it their home. Some swimmers have used shark cages to protect them from attack, though the sport of marathon swimming considers this to be a cheat and is officially frowned upon. Other less scary but equally dangerous creatures are swarms of jellyfish found in warm waters where many try to set records.

Snorkeling with Jellyfish

Some species are venomous, causing severe cramps and vomiting. Box jellyfish thwarted the attempt by an Australian swimmer to traverse the Florida straits. The stings were so severe she complained of having paralyzed legs and was hospitalized for days. Diana Nyad wore a special suit and mask that prevented the stings and she was able to complete the epic swim. However, other members of the sport claim this violated the strict rules and traditions of marathon swimming.

Contributed by Michael Troyan. Michael has spent 20 years teaching non-majors biology and microbiology and currently works as an online instructor at Penn State University. He can be reached at mbt102@psu.edu

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