Sport drinks

It is already mentioned that ingesting carbohydrate before, during, and following exercise can benefit performance by ensuring adequate fuel for energy production during exercise and for replenishing glycogen stores following exercise. While selecting a wise diet can provide for most of the athlete’s nutritional needs, nutritional supplements can also be of great value. In addition, adequate fluid intake is necessary for preexercise hydration, hydration during exercise, and rehydration following exercise. Sport drinks are uniquely designed to meet both the energy and fluid needs of the athlete. Performance benefits from these drinks have been clearly documented, not only in endurance activities, but in burst activities as well.

Composition of sport drinks

Sport drinks differ from one another in a number of ways besides in taste. Of major concern, however, is the rate at which energy and water are delivered. Energy delivery is primarily determined by the concentration of the carbohydrates in the drink, and fluid replacement is influenced by the sodium concentration of the drink.

Energy delivery – the carbohydrate concentration

A major concern is how rapidly the drink leaves the stomach, or the rate of gastric emptying. In general, carbohydrate solutions empty more slowly from the stomach than either water or a weak sodium chloride(salt) solution. Research suggests that a solution’s caloric content, a reflection of its concentration, might be a major determinant of how quickly it empties from the stomach and is absorbed in the intestine. Since carbohydrate solutions, increasing the glucose concentration of a sport drink significantly reduces the gastric emptying rate. For example, 400 ml(14oz) of a weak glucose solution(139 mmol/L) is almost completely emptied from the stomach in 20 min, but emptying a similar volume of a strong glucose solution(834 mmol/L) can require nearly 2h. However, when even a small amount of a strong glucose drink leaves the stomach, it can contain more sugar than a larger amount of a weaker solution simply because of its higher concentration. But, if an athlete is trying to prevent dehydration, this would deliver less water and thus be counterproductive.

Most sport drinks on the market contain about 6 to 8g of carbohydrate per 100ml( 3.5oz) of fluid(6% to 8%). The carbohydrate source is generally glucose, glucose polymers, or a combination of glucose and glucose polymers, although fructose or sucrose has also been used. Research studies have confirmed enhanced endurance performance with use of solutions in this range of concentration and with these sources of carbohydrates when compared to water. Carbohydrate solutions above approximately 6% slow gastric emptying and limit the immediate availability of fluid. However, they can provide a greater amount of carbohydrate in a given period of time to meet the increased energy needs.

Rehydration with sport drinks – the sodium concentration

Just adding fluid to the body during exercise lessens the risk of serious dehydration. But research indicates that adding glucose and sodium to sport drinks, aside from supplying an energy source, stimulates both water and sodium absorption. Sodium increases both thirst and palatability of the drink. Recall that when sodium is retained, this causes more water to be retained. For rehydration purposes, both during and following exercise, the sodium concentration should range between 20 mmol/L and 60 mmol/L. There is an important loss of sodium from the body with sweating. With high rates of sweating and large volumes of water intake, this can lead to critical reductions in the sodium concentration of the blood and possibly lead to hyponatremia.

What works best?

Athletes will not drink solutions that taste bad. Unfortunately, we all have different taste preferences. To further confound the issue, what tastes good before and after a long, hot bout of exercise will not necessarily taste good during the event. Studies of taste preferences of runners and cyclists during 60 min of exercise showed that most chose a drink with a light flavor and no strong aftertaste. But, will athletes drink more if given a sport drink as compared to water? In one study, runners ran on a treadmill for 90 min and then recovered while seated for an additional 90 min. Both exercise and recovery conditions were controlled in an environmental chamber at a temperature of 32°C(86°F), 50% humidity. Three trials were conducted, two with two different sport drinks(6% and 8% carbohydrate) and one with water. Subjects were encouraged to drink throughout each trial. The volume consumed during exercise was similar for all three drinks; but during recovery, the runners drunk about 55% more of each of the two sport drinks than water.

The athlete’s diet

Vegeterian diet

In an effort to eat a healthy diet and increase their carbohydrate intake, many athletes have adopted vegetarianism. Vegans are strict vegetarians who eat only food from plant sources. Lactovegeterians also consume dairy products. Ovovegeterians add eggs to their vegetable diets, and lacto-ovovegeterians eat plant foods, dairy products, and eggs.

Can athletes perform well on a vegetarian diet? Athletes who are strict vegans must be very careful in selecting the plant foods they eat to provide a good balance of the essential amino acids, sufficient calories, and adequate sources of vitamin A, riboflavin, vitamin B12, vitamin D, calcium, zync, and iron. Adequate iron intake is of particular concern in female vegetarian athletes because of the lower bioavailability of iron in plant-based diets and because of women’s greater risk for anemia and low iron stores. Some professional athletes have noted significant deterioration in athletic performance after switching to strict vegetarian diets. The problem usually is traced to unwise selection of foods. Including milk and eggs in the diet decreases the risk of nutritional deficiencies. Anyone contemplating switching to a vegetarian diet should either read authoritative material on the subject written by qualified nutritionists or consult a registered dietitian or sport nutritionist.

Precompetition meal

For years, many athletes have eaten the traditional steak dinner several hours before competition. This practice might have originated from the early belief that muscle consumes itself to fuel its own activity and that steak would provide the necessary protein to counteract this loss. But we know that the steak is probably the worst food an athlete could eat before competing. Steak contains a relatively high percentage of fat, which requires several hours for full digestion. During competition, this would cause the digestive system to compete with the muscles for the available blood supply. Also, nervous tension is typically high before a big competition, so even the choicest steak cannot truly be enjoyed at this time. The steak would be more satisfying and less likely to disturb performance if the athlete were to eat it either the night before or after the competition. But if steak is out, what should the athlete eat before competing?

Although the meal ingested a few hours before competition might contribute little to muscle glycogen stores, it can ensure a normal blood glucose level and prevent hunger. This meal should contain only about 200 to 500 kcal and consist mostly of carbohydrate foods that are easily digested. Foods such as cereal, milk, juice, and toast are digested rather quickly and won’t leave the athlete feeling full during competition. In general, this meal should be consumed at least 2h before competition. The rates at which food is digested and nutrients are absorbed into the body are quite individual, so timing the precompetition meal might depend on prior experience. In one study of endurance cyclists, a prolonged cycling exercise trial to exhaustion at 70% of the subject’s VO₂max was performed under two different conditions, with 14 days between trials: 100g of carbohydrate breakfast fed 3h before exercise(Fed) and no feeding before exercise(Fasted). Subjects tested under the Fed condition exercised 136 min before reaching exhaustion compared with 109 min in the fasted trial, indicating the importance of the precompetition meal.

A liquid precompetition meal might be less likely to result in nervous digestion, nausea, vomiting, and abdominal cramps. Such feedings are commercially available and generally have been found useful both before and between events. Finding time for athletes to eat is often difficult when they must perform in multiple preliminary and final events. Under these circumstances, a liquid feeding that is low in fat and high in carbohydrate might be only solution.

Muscle glycogen replacement and loading

Earlier was established that different diets can markedly influence muscle glycogen stores and that endurance performance depends largely on these stores. The theory is that the greater the amount of glycogen stored, the better the potential endurance performance because fatique will be delayed. Thus, an athlete’s goal is to begin an exercise bout or competition with as much stored glycogen as possible.

On the basis of muscle biopsy studies conducted in the mid-1960s, Astrand proposed a plan to help runners store the maximum amount of glycogen. This process is known as glycogen or carbohydrate loading. According to Astrand’s regimen, athletes should prepare for an aerobic endurance competition by completing an exhaustive training bout seven days before the event. For the next three days, they should eat fat and protein almost exclusively to deprive the muscles of carbohydrate, which increases the activity of glycogen synthase, an enzyme responsible for glycogen synthesis and storage. Athletes should then eat a carbohydrate-rich diet for the remaining three days before the event. Because glycogen synthase activity is increased, increased carbohydrate intake results in greater muscle glycogen storage. Training intensity and volume during this six-day period should be markedly reduced to prevent additional muscle glycogen depletion, thus maximizing liver and muscle glycogen reserves. Originally, an additional intense training bout was performed four days prior to competition.

This regimen has been shown to elevate muscle glycogen stores to twice the normal level, but it is somewhat impractical for most highly trained competitors. During the three days of low carbohydrate intake, athletes generally find training difficult. They are also often irritable and unable to perform mental tasks, and they typically show signs of low blood sugar, such as muscle weakness and disorientation. In addition, the exhaustive depletion bouts of exercise performed seven days before the competition have little training value and can impair glycogen storage rather than enhance it. This depletion exercise also exposes athletes to possible injury or overtraining.

Considering these limitations, many proposed that the depletion exercise and the low carbohydrate aspects of Astrand’s regimen be eliminated. Instead, according to Sherman and colleagues, the athlete should simply reduce training intensity a week before competition and eat a normal, mixed diet containing 55% of the calories from the carbohydrate until three days before competition. For these days, training should be reduced to a daily warm-up of 10 to 15 min of activity and accompanied by a carbohydrate-rich diet. Following this plan, as seen in the figure below, glycogen will be elevated to nearly 200 mmol/kg of muscle, the same level attained with Astrand’s regimen, and the athlete will be better rested for competition.

It is possible to increase carbohydrate stores rapidly after even a very short near-maximal-intensity bout of exercise. In a study of seven endurance athletes, scientists found that cycling for 150s at 130% of VO₂max followed by 30s of all-out cycling and 24h of high-carbohydrate intake was sufficient to nearly double muscle glycogen stores in just one day.

Diet is also important in preparing the liver for the demands of endurance exercise. Liver glycogen stores decrease rapidly when a person is deprived of carbohydrates for only 24h, even when at rest. With only 1h of strenuous exercise, liver glycogen decreased by 55%. Thus, hard training combined with a low-carbohydrate diet can empty the liver glycogen stores. A single carbohydrate meal, however, quickly restores liver glycogen to normal. Clearly, a carbohydrate-rich diet in the days preceding competition will maximize the liver glycogen reserve and minimize the risk of hypoglycemia during the event.

Water is stored in the body at a rate of about 2.6g of water with each gram of glycogen. Consequently, an increase or decrease in muscle and liver glycogen generally produces a change in the body weight of from 0.5 to 1.4kg(1-3 lb). Some scientists have proposed monitoring of changes in muscle and liver glycogen stores via recording the athlete’s early morning weight immediately after rising – after emptying the bladder but before eating breakfast. A sudden decrease in weight might reflect a failure to replace glycogen, a deficit in body water, or both.

Athletes who must train or compete in exhaustive events on successive days should replace muscle and liver glycogen stores as rapidly as possible. Although liver glycogen can be depleted totally after 2h of exercise at 70% VO₂max, it is replenished within a few hours when a carbohydrate-rich meal is consumed. Muscle glycogen resynthesis, on the other hand, is a slower process, taking several days to return to normal after an exhaustive process bout such as the marathon(see figure below). Studies in the late 1980s revealed that muscle glycogen resynthesis was most rapid when individuals were fed at least 50g(about 0.7g/kg body weight) of glucose every 2h after the exercise. Feeding subjects more than this amount did not appear to accelerate the replacement of muscle glycogen. During the first 2h after exercise, the rate of muscle glycogen resynthesis is much higher than later in recovery. Thus, an athlete recovering from an exhaustive endurance event should ingest sufficient carbohydrate as soon after exercise as a practical. Adding protein and amino acids to the carbohydrate ingested during the recovery period enhances muscle glycogen resynthesis above that achieved with carbohydrate alone. 

Influence of dehydration on training performance

Variables	Dehydration
Physiological function
Cardiovascular
Blood volume/plasma volume	Decreased
Cardiac output	Decreased
Stroke volume	Decreased
Heart rate	Increased
Metabolic
Aerobic capacity(VO2max)	No change or decreased
Anaerobic power(Wingate test)	No change or decreased
Anaerobic capacity(Wingate test)	No change or decreased
Blood lactate, peak value	Decreased
Buffer capacity of the blood	Decreased
Lactate threshold, velocity	Decreased
Muscle and liver glycogen	Decreased
Blood glucose during exercise	Possibly decreased
Protein degradation with exercise	Possibly increased
Thermoregulation and fluid balance
Electrolytes, muscle and blood	Decreased
Exercise core temperature	Increased
Sweat rate	Decreased, delayed onset
Skin blood flow	Decreased
Performance
Muscular strength	No change or decreased
Muscular endurance	No change or decreased
Muscular power	Unknown
Speed of movement	Unknown
Run time to exhaustion	Decreased
Total work performed	Decreased
Wrestling simulation test	Decreased

Electrolyte balance during exercise

Normal body function depends on a balance between water and electrolytes. When large amounts of water are lost from the body, as during exercise, the balance between water and electrolytes can be disrupted quickly. Our focus will be on the two major routes for electrolyte loss: sweating and urine production.

Electrolyte loss in sweat

Human sweat is a filtrate of blood plasma, so it contains many substances found there, including sodium(Na⁺), chloride(Cl^-), potassium(K⁺), magnesium(Mg²⁺) and calcium(Ca²⁺). Although sweat tastes salty, it contains far fewer minerals than the plasma and other body fluids. In fact, sweat is 99% water.

Sodium and chloride are the predominant ions in sweat and blood. As indicated in the table below, the concentrations of sodium and chloride in sweat are about one-third those found in plasma and five times those found in muscle. Each of these three fluids’ osmolarity, which is the ratio of solutes(such as electrolytes) to fluid, is also shown. Sweat’s electrolyte concentration can vary considerably between individuals. It is strongly influenced by genetics, the rate of sweating, the state of traiing, and the state of heat acclimatization.

Electrolyte concentrations and osmolarity in sweat, plasma, and muscle of men following 2h of exercise in the heat
Site	Na+	Cl-	K+	Mg2+	Osmolarity(mOsm/L)
Sweat	40-60	30-50	4-6	1.5-5	80-185
Plasma	140	101	4	1.5	295
Muscle	9	6	162	31	295
mEq/L = milliequivalents per liter(thousandths of 1g of solute per 1L of solvent), second column represents mEq/L of electrolytes

At the elevated rates of sweating reported during endurance events, sweat contains large amounts of sodium and chloride but little potassium, calcium and magnesium. Based on estimates of the athlete’s total body electrolyte content, such losses would lower the body’s sodium and chloride content by only about 5% to 7%. Total body levels of potassium and magnesium, two ions principally confined to the insides of cells, would decrease by about 1%. These losses probably have no measurable effect on an athlete’s performance.

As electrolytes are lost in sweat, the remaining ions are redistributed among the body tissues. Consider potassium. It diffuses from active muscle fibers as they contract, entering the extracellular fluid. This increase causes in extracellular potassium levels that it does not equal the amount of potassium that is released from active muscles, because potassium is taken up by inactive muscles and other tissues while the active muscles are losing it. During recovery, intracellular potassium levels normalize quickly. Some researchers suggest that these muscle potassium disturbances during exercise might contribute to fatique by altering the membrane potentials of neurons and muscle fibers, makin it more difficult to transmit impulses.

Electrolyte loss in urine

In addition to clearing wastes from the blood and regulating water levels, the kidneys also regulate the body’s electrolyte content. Urine production is the other major source of electrolyte loss. At rest, electrolytes are excreted in the urine as necessary to maintain homeostatic levels, and this is the primary route for electrolyte loss. But as the body’s water loss increases during exercise, urine production rate decreases considerably in an effort to conserve water. Consequently, with very little urine being produced, electrolyte loss by this avenue is minimized.

The kidneys play another role in electrolyte management. If, for example, a person eats 250 mEq of salt(NaCl), the kidneys will normally excrete 250 mEq of these electrolytes to keep the body NaCl content constant. Heavy sweating and dehydration, however, trigger the release of the hormone aldosterone from the adrenal gland. This hormone stimulates renal reabsorption of sodium. Consequently, the body retains more sodium than usual during the hours and days after a prolonged exercise bout. This elevates the body’s sodium content and increases the osmolarity of the extracellular fluids.

This increased sodium content triggers thirst, compelling the person to consume more water, which is then retained in the extracellular compartment. The increased water consumption reestablishes normal osmolarity in the extracellular fluids but leaves these fluids expanded, which dilutes the other substances present there. This expansion of the extracellular fluids has no negative effects and is temporary. In fact, this is one of the major mechanisms for the increase in plasma volume that occurs with training and with acclimatization to exercise in the heat. Fluid levels return to normal within 48 to 72h after exercise, providing there are no subsequent exercise bouts.

Replacement of body fluid losses

The body loses more water than electrolytes during heavy sweating. This raises the osmotic pressure in the body fluids because the electrolytes become more concentrated. The need to replace body water is greater than the need for electrolytes because only by replenishing water content can the electrolytes return to normal concentrations. But how does the body know when this is necessary?

Thirst

When people feel thirsty, they drink. The thirst sensation is regulated by the hypothalamus. It triggers thirst when the plasma’s osmotic pressure is increased. Unfortunately, the body’s thirst mechanism doesn’t precisely gauge its state of dehydration. It does not sense thirst until well after dehydration begins. Even when dehydrated, people might desire fluids only at intermittent intervals.

The control of thirst is not fully understood. When permitted to drink water as their thirst dictates, people can require 24 to 48h to completely replace water lost through heavy sweating. In contrast, dogs and burros can drink up to 10% of their total body weight within the first few minutes after exercise or heat exposure, replacing all lost water. Because of our sluggish drive to replace body water and to prevent chronic dehydration, we are advised to drink more fluid than our thirst dictates. Because of the increased water loss during exercise, it is imperative that athletes’ water intake be sufficient to meet their bodies’ needs, and it is essential that they rehydrate during and after an exercise bout.

Benefits of fluids during exercise

Drinking fluids during prolonged exercise, especially in hot weather, has obvious benefits. Water intake will minimize dehydration, increases in body temperature, cardiovascular stress, and declines in performance. As seen in the figure below, when subjects became dehydrated during several hours of treadmill running in the heat(40°C, or 104°F) without fluid replacement, their heart rates increased steadily throughout the exercise. When they were deprived of fluids, the subjects became exhausted and couldn’t complete the 6h exercise. Ingesting either water or a saline solution in amounts equal to weight loss prevented dehydration and kept subjects’ heart rates lower. Even warm fluids(near body temperature) provide some protection against overheating, but cold fluids enhance body cooling because some of the deep body heat is used to warm cold drinks to body temperature.

Hyponatremia

Fluid replacement is beneficial, but too much of a good thing could potentially be bad. In the 1980s, the first cases of hyponatremia were reported in endurance athletes. Hyponatremia is clinically defined as a serum sodium concentration below the normal range of 135 to 145 mmol/L. Symptoms of hyponatremia generally appear when serum sodium levels drop below 130 mmol/L. Early signs and symptoms include bloating, puffiness, nausea, vomiting, and headache. As the severity increases, due to increasing cerebral edema(swelling of the brain), the symptoms include confusion, disorientation, agitation, seizures, pulmonary edema, coma, and death. How likely is hyponatremia to occur?

The processes that regulate fluid volumes and electrolyte concentrations are highly effective, so consuming enough water to dilute plasma electrolytes is difficult under normal circumstances. Marathoners who lose 3 to 5 L of sweat and drink 2 to 3 L of water maintain normal plasma concentrations of sodium, chloride, and potassium. And distance runners who run 25 to 40 km(15.5-24.9 mi) per day in warm weather and do not salt their food don’t develop electrolyte deficiencies.

Some research has suggested that during ultramarathon running(more than 42 km, or 26.2 mi), athletes can experience hyponatremia. A case study of two runners who collapsed after an ultramarathon race(160km, or 100 mi) in 1983 revealed that their blood sodium concentrations had decreased from a normal value of 140 mmol/L to values of 123 and 118 mmol/L. One of the runners experienced a grand mal seizure; the other became disoriented and confused. Examining the runners’ fluid intakes and estimating their sodium intakes during the run suggested that they had diluted their sodium contents by consuming too much fluid that contained too little sodium.

The ideal resolution to prevent hyponatremia would be to replace water at the exact rate at which it is being lost or to add sodium to the ingested fluid. The problem with the latter approach is that the most sport drinks contain no more than 25 mmol/L of sodium and are apparently too weak to prevent sodium dilution alone, but very strong concentrations cannot be tolerated. Exercise hyponatremia appears to be the result of a fluid overload due to overconsumption, underreplacement of sodium losses, or both. Only a small number of cases have been reported. Thus, it is probably inappropriate to form conclusions from this information to design a fluid replacement regimen for people who must exercise for long periods in the heat.

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Water balance

For optimal performance, the body’s water content should remain relatively constant.

Water balance at rest

Under normal resting conditions, the body’s water content is relatively constant: water intake equals water output. About 60% of our daily water intake is obtained from the fluids we drink and about 30% is from the food we consume. The remaining 10% is produced in our cells during metabolism. Metabolic water production varies from 150 to 250 ml per day, depending on the rate of energy expenditure: higher metabolic rates produce more water. The total daily water intake from all sources averages about 33ml per kilogram of body weight per day. For a 70kg(154lb) person, average intake is 2.3L per day. Water output, or water loss, occurs from four sources:

• Evaporation from the skin;
• Evaporation from the respiratory tract;
• Excretion from the kidneys;
• Excretion from the large intestine.

Human skin is permeable to water. Water diffuses to the skin’s surface, where it evaporates into the environment. In addition, the gases we breathe are constantly being humidified by water as they pass through the respiratory tract. These two types of water loss(from the skin and respiration) occur without our sensing them. Thus, they are termed insensible water losses. Under cool, resting conditions, these losses account for about 30% of daily water loss.

The majority of our daily water loss – 60% at rest – occurs from our kidneys, which excrete water and waste products as urine. Under resting conditions, the kidneys excrete about 50 to 60ml of water per hour. Another 5% of the water is lost by sweating(although this is often considered along with insensible water loss), and the remaining 5% is excreted from the large intestine in the feces. The sources of water gain and water loss are presented in the picture below.

Water balance during exercise

Water loss accelerates during exercise, as seen in the table below. The ability to lose the heat generated during exercise depends primarily on the formation and evaporation of sweat. As body temperature increases, sweating increases in an effort to prevent overheating. But at the same time, more water is produced during exercise because of increased oxidative metabolism. Unfortunately, the amount produced even during the most intense effort has only a small impact on the dehydration, or water loss, that results from heavy sweating.

In general, the amount of sweat produced during exercise is determined by:

·         Environmental temperature, radiant heat load, humidity and air velocity;

·         Body size;

·         Metabolic rate.

These factors influence the body’s heat storage and temperature. Heat is transferred from warmer areas to cooler ones, so heat loss from the body is impaired by high environmental temperatures, radiation, high humidity, and still air. Body size is important because large individuals generally expend more energy to do a given task, so they typically have higher metabolic rates and produce more heat. But they also have more surface area(skin), which allows more sweat formation and evaporation. As exercise intensity increases, so does the metabolic rate. This increases body heat production, which in turn increases sweating. To conserve water during exercise, blood flow to the kidneys decreases in an attempt to prevent dehydration; but like the increase in metabolic water production, this too may be insufficient. During high- intensity exercise under environmental heat stress, sweating and respiratory evaporation can cause losses of as much as 2 to 3L of water per hour.

Dehydration and exercise performance

Even minimal changes in the body’s water content can impair endurance performance. Without adequate fluid replacement, an athlete’s exercise tolerance shows a pronounced decrease during long-term activity because of water loss through sweating. The impact of dehydration on the cardiovascular and thermoregulatory systems is quite predictable. Fluid loss decreases plasma volume. This decreases blood pressure, which in turn reduces blood flow to the muscles and skin. In an effort to overcome this,heart rate increases. Because less blood reaches the skin, heat dissipation is hindered, and the body retains more heat. Thus, when a person is dehydrated by 2% of body weight or more, both heart rate and body temperature are elevated during exercise above values observed when normally hydrated.

As one might expect, these physiological changes will decrease exercise performance. Figure below illustrates the effects of an approximate 2% decrease in body weight attributable to dehydration from the use of a diuretic on distance runners’ performance in 1500m, 5000m, and 10,000m time trials on outdoor track. The dehydration condition resulted in plasma volume decreases between 10% and 12%. Although the average VO₂max did not differ between the normally hydrated and dehydrated trials, mean running velocity decreased by 3% in the 1500m run and by more than 6% in the 5000m and 10,000m runs. The greater the duration of the performance, the greater is the expected decline in performance for the same degree of dehydration. These trials were conducted in relatively cool weather. The higher the temperature, humidity, and radiation, the greater the expected decrement in performance for the same degree of dehydration. The decrement in performance would be progressively greater with greater levels of dehydration.

The effect of dehydration on performance in muscular strength, muscular endurance, and anaerobic types of activities is not as clear. Decrements have been seen in some studies, whereas other studies have shown no change in performance. In one of the best-controlled studies, researchers at Penn State University reported that 2% dehydration resulted in significant deterioration of basketball skills in 12- to 15-year-old boys who were skilled basketball players.

Wrestlers and other weight-category athletes commonly dehydrate to get a weight advantage during the weigh-in for a competition. Most rehydrate after the weigh-in before the competition and experience only small decrements in performance.

Detraining

Tapering, by reducing the training stimulus, can facilitate performance. What happens to highly conditioned athletes who have fine-tuned their performance abilities to peak level but then come to the end of their competitive season? Many athletes in team sports go into physical hibernation at this time. Many have been working 2 to 5h each day to perfect their skills and improve their physical condition, and they welcome the opportunity to completely relax, purposely avoiding any strenuous physical activity. But how does physical inactivity affect highly trained athletes?

Detraining is defined as the partial or complete loss of training-induced adaptations in response to either the cessation of training or a substantial decrement in the training load – in contrast to tapering, which is a gradual reduction of the peak training load over only a few days to a few weeks. Some of our knowledge about physical detraining comes from clinical research with patients who have been forced into inactivity because of injury or surgery. Athletes generally agree that suffering the pain of an injury is bad enough, but the situation is even worse when it forces them to stop training. Most fear that all they have gained through hard training will be lost during a period of inactivity. But recent studies reveal that a few days of rest or reduced training will not impair and might even enhance performance, similar to what we see with tapering. Yet at some point, training reduction or complete inactivity will decrease physiological function and performance.

Muscular strength and power

When a broken limb is immobilized in a rigid cast, changes begin almost immediately in both the bone and the surrounding muscles. Within only a few days, the cast that was applied tightly around the injured limb is loose. After several weeks, a large space separates the cast and the limb. Skeletal muscles undergo a substantial decrease in size, known as atrophy, when they remain inactive. This is accompanied by considerable loss of muscular strength and power. Total inactivity leads to rapid losses, but even prolonged periods of reduced activity lead to gradual losses that eventually can become quite significant.

Similarly, research confirms that muscular strength and power both are reduced once athletes stop training. The rate and magnitude of loss appear to vary by the level of training. Highly skilled accomplished weightlifters appear to have a rather rapid decline in strength within a few weeks of discontinuing intense training. With previously untrained people, strength gains can be maintained from several weeks up to over seven months. In a study of young(20-30 years) and older(65-75 years) men and women who trained for nine weeks, the increase in strength(1-repetition maximum) averaged 34% for the younger subjects and 28% for the older subjects, with no differences between men and women. After 12 weeks of detraining, none of the four groups had significant losses in strength from the end of the nine-week training program values. After 31 weeks of detraining, there was only an 8% loss in the younger subjects and a 13% decrease in the older subjects.

A study with collegiate swimmers revealed that even with up to four weeks of inactivity, terminating training did not affect arm or shoulder strength. No strength changes were seen in these swimmers, whether they spent four weeks at complete rest or whether they reduced their training frequency to one or three sessions per week. But swimming power was reduced by 8% to 13.5% during the four weeks of reduced activity, whether the swimmers underwent complete rest or merely reduced training frequency. Although muscular strength might not have  diminished during the four weeks of rest or reduced training, the swimmers might have lost their ability to apply force during swimming, possibly attributable to a loss of skill.

The physiological mechanisms responsible for the loss of muscular strength as a consequence of either immobilization or inactivity are not clearly understood. Muscle atrophy causes a noticeable decrease in muscle mass and water content, which could party account for a loss in the development of maximal muscle fiber tension. Changes occur in the rates of protein synthesis and degradation as well as in specific fiber type characteristics. When muscles aren’t used, the frequency of their neurological stimulation is reduced, and normal fiber recruitment  is disrupted. Thus, part of the strength loss associated with detraining could result from an inability to activate some muscle fibers.

The retention of muscular strength, power, and size is extremely important for the injured athlete. The athlete can save much time and effort during rehabilitation by performing even a low level of exercise with the injured limb, starting in the first few days of recovery. Simple isometric contractions are very effective for rehabilitation because their intensity can be graded and they do not require joint movement. Any program of rehabilitation, however, must be designed in cooperation with the supervising physician and physical therapist.

Muscular endurance

Muscular endurance performance decreases alter only two weeks of inactivity. At this time, not enough evidence is available to determine whether this performance decrement results from changes in cardiovascular capacity. In this section, we examine muscle changes that are known to accompany detraining and that could decrease muscular endurance.

The localized muscle adaptations that occur during periods of inactivity are well documented, but knowledge of the exact role that these changes play in the loss of muscular endurance still eludes us. We know that from postsurgery cases that after a week or two of cast immobilization, the activities of oxidative enzymes such as succinate dehydrogenase(SDH) and cytochrome oxidase decrease by 40% to 60%. Data collected from swimmers, shown in the figure below, indicate that the muscles’ oxidative potential decreases much more rapidly than the subjects’ maximal oxygen uptake with detraining. Reduced oxidative enzyme activity would be expected to impair muscular endurance, and this most likely relates to submaximal endurance capacity rather than to maximal aerobic capacity, or VO₂max.

In contrast, when athletes stop training, the activities of their muscles’ glycolitic enzymes, such as phosphorylase and phosphofructokinase, change little, if at all, for at least four weeks. In fact, Coyle and colleagues’ observed no change in glycolytic enzyme activities with up to 84 days of detraining compared with a nearly 60% decrease in the activities of various oxidative enzymes. This might at least partly explain why performance times in sprint events are unaffected by a month or more of inactivity, but the ability to perform longer endurance events may decrease significantly with as little as two weeks of detraining.

One notable change in the muscle during detraining is a change in its glycogen content. Endurance-trained muscle tends to increase its glycogen storage. But four weeks of detraining has been shown to decrease muscle glycogen by 40%. Figure below illustrates the decrease in muscle glycogen content after four weeks of inactivity, but the swimmers’ values decreased until they were about equal to those of the untrained people. This indicates that the trained swimmers’ improved capacity for muscle glycogen storage was reversed during detraining. 

Measurements of blood lactate and pH after a standard work bout have been used to assess the physiological changes that accompany training and detraining. For example, a group of collegiate swimmers were required to perform a standard-paced, 200 yd(183 m) swim at 90% of their seasonal best following five months of training and then to repeat this test at the same absolute pace once a week for the following four weeks of detraining. The results are shown in the table below. Blood lactate concentrations, taken immediately after this standard swim, increased from week to week during a month of inactivity. At the end of the fourth week of detraining, the swimmers’ acid-base balance was significantly disturbed. This was reflected by a significant increase in blood lactate concentrations and a significant decrease in the concentrations of bicarbonate(a buffer).

Blood lactate, pH, and bicarbonate(HCO3-) values in collegiate swimmers undergoind detraining
	Weeks of detraining
Measurement	0	1	2	4
Lactate(mmol/L)	4.2	6.3	6.8	9.7
pH	7.26	7.24	7.24	7.18
HCO3- (mmol/L)	21.1	19.5	16.1	16.3
Swim time(s)	130.6	130.1	130.5	130.0

Speed, agility, and flexibility

Training produces less improvement in speed and agility than it does in strength, power, muscular endurance, flexibility and cardiorespiratory endurance. Consequently, losses of speed and agility that occur with inactivity are relatively small. Also, peak levels of both can be maintained with a limited amount of training. But this does not imply that the track sprinter can get by with training only a few days a week. Success in actual competition relies on factors other than basic speed and agility, such as correct form, skill, and the ability to generate a strong finishing sprint. Many hours of practice are required to tune performance to its optimal level, but most of this time is spent developing performance qualities other than speed and agility.

Flexibility, on the other hand, is lost rather quickly during inactivity. Stretching exercises should be incorporated into both in-season and off-season training programs. Reduced flexibility has been proposed to increase athletes’ susceptibility to serious injury.

Cardiorespiratory endurance

The heart, like other muscles in the body, is strengthened by endurance training. Inactivity, on the other hand, can substantially decondition the heart and the cardiovascular system. The most dramatic example of this is seen in a study conducted on subjects undergoing long periods of total bed rest; they weren’t allowed to leave their beds, and physical activity was kept to an absolute minimum. Cardiovascular and metabolic function were assessed at a constant submaximal rate of work and at maximal rates of work both before and after the 20-day period of bed rest. The cardiovascular effects that accompanied bed rest included:

• A considerable increase in submaximal heart rate;
• A 25% decrease in submaximal stroke volume;
• A 25% reduction in maximal cardiac output;
• A 27% decrease in maximal oxygen consumption.

The reductions in cardiac output and VO2max appear to result from reduced stroke volume; this appears to be largely attributable to a decreased plasma volume, with reductions in heart volume and ventricular contractility playing a smaller role.

It is interesting that the two most highly conditioned subjects in the study(the two who had the highest    VO₂max values) experienced greater decrements in  VO₂max than the three less fit people, as shown in the figure below. Furthermore, the untrained subjects regained their initial conditioning levels(before bed rest) in the first 10 days of reconditioning, but the well-trained subjects needed about 40 days for full recovery. This suggests that highly trained individuals cannot afford long periods with little or no endurance training. The athlete who totally abstains from endurance training at the completion of the season will experience great difficulty getting back into top aerobic condition when the new season begins.

Inactivity can significantly reduce VO₂max. How much activity is needed to prevent such considerable losses of physical conditioning? Although a decrease in training frequency and duration reduces aerobic capacity, the losses are significant only when frequency and duration reduces aerobic capacity, thee losses are significant only when frequency and duration are reduced by two-thirds of the regular training load. However, training intensity apparently plays a more crucial role in maintaining aerobic power during periods of reduced training. Training at 70% VO₂max appears to be necessary to maintain maximal aerobic capacity.