Overtraining

During periods of intense overload or overreaching training, athletes may experience an unexplained decline in performance and physiological function that extends over weeks, months or years. This condition is termed overtraining and has been attributed to both physiologica and psychological causes. The precise cause or causes for this breakdown in performance and physiological function are not fully understood. Further, overtraining occurs with each of three major forms of training – so it is likely that the cause or causes and symptoms will vary by the type of training.

Athletes experience various levels of fatique during repeated days and weeks of training, so not all fatique-producing situations can be classified as overtraining(as it is noted previously with overreaching). Fatique that follows one or more exhaustive training sessions usually is relieved by a few days of reduced training or rest and a carbohydrate-rich diet. Overtraining, on the other hand, is characterized by a sudden decline in performance and physiological function that cannot be remedied by a few days of reduced training, rest, or dietary manipulation.

Effects of overtraining: the overtraining syndrome

Most of the symptoms that result from overtraining, collectively reffered to as the overtraining syndrome, are subjective and identifiable only after the individual’s performance and physiological function have suffered. Unfortunately, these symptoms can be highly individualized, making it very difficult for athletes, trainers, and coaches to recognize that performance decrements are brought on by overtraining. A decline in physical performance with continued training is usually the first indication of the overtraining syndrome(see figure below). The athlete senses a loss of muscular strength, coordination, and working capacity and generally feels fatiqued. Other primary signs and symptoms of the overtraining syndrome include:

• Change in appetite;
• Body weight loss;
• Sleep disturbances;
• Irritability, restlessness, excitability, anxiousness;
• Lack of mental concentration;
• Feelings of depression;
• Lack of appreciation for things that normally are enjoyable.

Physiological changes also indicate the presence of the overtraining syndrome.

The underlying causes of overtraining syndrome are often a complex combination of emotional and physiological factors. Hans Selye noted that a person’s stress tolerance can break down as often from a sudden increase in anxiety as from an increase in physical distress. The emotional demands of competition, the desire to win, the fear of failure, unrealistically high goals, and others’ expectations can be sources of intolerable emotional stress. Because of this, overtraining is typically accompanied by a loss of competitive desire and a loss of enthusiasm for training. Furthermore, Armstrong and VanHeest make the important observation that the overtraining syndrome and clinical depression involve remarkably similar signs and symptoms, brain structures, neurotransmitters, endocrine pathways, and immune responses, suggesting that they have similar etiologies.

The physiological factors responsible for the detrimental effects of overtraining are not fully understood. However, abnormal responses have been reported that suggest that overtraining is associated with alterations in the nervous, endocrine, and immune systems. Although a cause-and-effect relationship between these changes and the symptoms of overtraining has not been clearly established, these symptoms can help determine whether an individual is overtrained. In the following discussion, we focus on some of the observed changes associated with overtraining and on potential causes of the overtraining syndrome.

Autonomic nervous system responses to overtraining

Some studies suggest that overtraining is associated with abnormal responses in the autonomic nervous system. Physiological symptoms accompanying the decline in performance often reflect changes in those organs or systems that are controlled by either the sympathetic or parasympathetic branches of the autonomic nervous system. Sympathetic overtraining lead to:

• Increased resting heart rate;
• Increased blood pressure;
• Loss of appetite;
• Decreased body mass;
• Sleep disturbances;
• Emotional instability;
• Elevated basal metabolic rate.

This form of overtraining occurs predominantly among athletes who emphasize highly intense or resistance training methods.

Other studies suggest that the parasympathetic nervous system might be dominant in some cases of overtraining, usually in endurance athletes. In these cases, the performance decrements markedly differ from those associated with sympathetic overtraining. Signs of parasympathetic overtraining, assumed to be the result of volume overload, include:

• Early onset of fatique;
• Decreased resting heart rate;
• Rapid heart rate recovery after exercise;
• Decreased resting blood pressure.

Thus, it appears that athletes in different sports or events will likely exhibit unique signs and symptoms of overtraining syndrome that are related to their training regimens. In fact, some authorities have named these forms of overtraining “intensity related” and “volume related”, recognizing that specific training stressors result in unique signs and symptoms when applied excessively.

Some of the symptoms associated with autonomic nervous system overtraining are also seen in people who are not overtrained. For this reason, we cannot always assume that the presence of these symptoms confirms overtraining. Of the two conditions, symptoms of sympathetic overtraining are the most frequently observed. Although there is not strong scientific evidence to support the autonomic nervous system overtraining theory, the autonomic nervous system definitely is affected by overtraining.

Hormonal responses to overtraining

Measurements of various blood hormone concentrations during periods of overreaching suggest that marked disturbances in endocrine function accompany excessive stress. As shown in the figure below, when swimmers increase their training 1.5-to-2-fold, blood concentrations of thyroxine and testosterone usually decrease and blood concentrations of cortisol increase. The ratio of testosterone to cortisol is thought to regulate anabolic processes in recovery, so a change in this ratio is considered an important indicator, and perhaps a cause, of the overtraining syndrome. Further, most overtraining studies have been conducted on aerobically trained endurance athletes. Fewer studies exist on anaerobically trained and resistance-trained athletes. Using the terminology introduced in the last section, intensity-related overtraining(anaerobic and resistance training) does not appear to alter resting hormonal concentrations.

Overtrained athletes often have higher blood concentrations of urea, and because urea is produced by the breakdown of protein, this indicates increased protein catabolism. This mechanism is thought to be responsible for the loss in body mass seen in overtrained athletes.

Resting blood concentrations of epinephrine and norepinephrine have also been reported to be elevated during periods of intensified aerobic or volume training. These two hormones elevate heart rate and blood pressure. This has led some exercise physiologists to suggest that the blood concentrations of these catecholamines should be measured to confirm overtraining. However, other studies have found no change in these catecholamines during intensified training, and some have even found decreased resting values.

Acute overload training and overreaching often produce most of the same hormonal changes reported in overtrained athletes. For this reason, measuring these and other hormones might not provide valid confirmation of overtraining. Athletes whose hormone concentrations appear abnormal may simply be experiencing the normal effects of hard training. Further, the time interval between the last training bout and the resting blood sample is very important. Some potential markers remain elevated for more than 24h and might not reflect a true resting state. These hormonal changes simply might reflect the stress of training rather than a breakdown in the adaptative process. Consequently, many experts have now concluded that no blood marker conclusively defines the overtraining syndrome.

Armstrong and VanHeest proposed that the various stressors associated with the overtraining syndrome act primarily through the hypothalamus. They postulated that these stressors activate the following two predominant hormonal axes involved in the body’s response to stressors:

• The sympathetic-adrenal medullary axis(SAM), involving the sympathetic branch of the autonomic nervous system;
• The hypothalamic-pituitary-adrenocortical axis(HPA).

This is illustrated in the picture a below. Figure b illustrates the brain and immune system interactions with these two axes. These two figures are quite complex and go well beyond the scope of an introductory-level exercise physiology text. However, a cursory study of the interactions depicted in these figures will give an appreciation of the complexity of this syndrome. Importantly, note that the stressors have their initial effect on the brain(hypothalamus). Thus, it is highly likely that brain neurotransmitters play an important role in the overtraining syndrome. Serotonin is a major neurotransmitter that is suspected to play a significant role in the overtraining syndrome. Unfortunately, plasma concentrations of this important neurotransmitter do not accurately reflect those concentrations in brain. Advances in techonology should provide the necessary tools to help us better understand what is going on inside the brain.

A major role for cytokines in the overtraining syndrome, recently has been proposed, providing support for the Armstrong and VanHeest model in figure b. Elevated circulating cytokines result from infection as well as from skeletal muscle, bone, and joint trauma associated with overtraining. They appear to be a normal part of the body’s inflammatory response to infection and injury. It is theorized that excessive musculoskeletal stress, coupled with insufficient rest and recovery, sets up a cascade of events whereby a local acute inflammatory response evolves into chronic inflammation and eventually into systemic inflammation. Systemic inflammation activates circulating monocytes, which can then synthetize large quantities of cytokines. Cytokines then act on most of the brain and body functions in a manner consistent with symptoms expressed in the overtraining syndrome.

Immunity and overtraining

The immune system provides a line of defense against invading bacteria, parasites, viruses, and tumor cells. This system depends on the actions of specialized cells(such as lymphocytes, granulocytes, and macrophages) and antibodies. These primarily eliminate or neutralize foreign invaders that might cause illness(pathogens). Unfortunately, one of the most serious consequences of overtraining is the negative effect it has on the body’s immune system. In fact, from the model proposed in the figure above, compromised immune function is potentially a major factor in the initiation of the overtraining syndrome.

Many studies have shown that excessive training suppresses normal immune function, increasing the overtrained athlete’s susceptibility to infections. This is illustrated in the picture below. Studies also show that short bouts of intense exercise can temporarily impair the immune response, and successive days of heavy training can amplify this suppression. Several investigators have reported an increased incidence of illness following a single, exhaustive exercise bout, such as running a full 42km(26.2 mi) marathon. This immune suppression is characterized by abnormally low concentrations of both lymphocytes and antibodies. Invading organisms or substances are more likely to cause illness when these concentrations are low. Also, intense exercise during illness might decrease one’s ability to fight off the infection and increase the risk of even greater complications.

Predicting the overtraining syndrome

We must remember that the underlying cause or causes of the overtraining syndrome are not fully known, although it is likely that physical or emotional overload, or a combination of the two, might trigger this condition. Trying not to exceed an athlete’s stress tolerance by regulating the amount of physiological and psychological stress experienced during training is difficult. Most coaches and athletes use intuition to determine training volume and intensity, but few can accurately assess the true impact of a workout on the athlete. No preliminary symptoms warn athletes that they are on the verge of becoming overtrained. By the time coaches realize that they have pushed an athlete too hard, it is often too late. The damage done by repeated days of excessive training or stress can be repaired only by days, and in some cases weeks or months, of reduced training or complete rest.

Numerous investigators have tried to identify markers of the overtraining syndrome in its early stages by using assorted physiological and psychological measurements. A list of potential markers is provided in the table below. Unfortunately, none has proven totally effective. It is often difficult to determine whether the measurements obtained are related to overtraining or whether they simply reflect normal responses to overload or overreaching training.

Potential markers of OR, OT and OTS
Marker	Response	Potential marker for
Physiological and psychological		OR	OT	OTS
HRrest and HRmax	Decreased		X	X
HRsubmax and VO2submax	Increased	X		X
VO2max	Decreased			X
Anaerobic metabolism	Impaired		X
Basal metabolic rate	Increased			X
RERsubmax,max	Decreased		X	X
Nitrogen balance	Negative			X
Nerve excitability	Increased			X
Sympathetic nervous response	Increased			X
Psychological mood states	Altered	X
Risk of infection	Increased	X
Hematocrit and hemoglobin	Decreased		X
Leukocytes and immunophenotypes	Decreased		X
Serum iron and ferritin	Decreased		X
Serum electrolyte levels	Decreased			X
Serum glucose and free fatty acids	Decreased		X
Plasma lactate concentration, submax, max	Decreased		X	X
Ammonia	Increased		X	X
Serum testosterone and cortisol	Decreased	X
ACTH, growth hormone, prolactin	Decreased			X
Catecholamines, rest, night	Decreased			X
Creatine kinase	Increased			X

OR = overreaching

OT = overtraining

OTS = overtraining syndrome

HR = heart rate

RER = respiratory exchange ratio

ACTH =  adrenocorticotropic hormone

Possibly the best method to identify the overtraining syndrome is to monitor the athlete’s heart rate during a standardized workout, such as a fixed-paced run or swim, using a digital heart rate monitor. The data presented in the figure below illustrate a runner’s heart rate response during a 1 mi(1.6km) run performed at a fixed pace of 6 min/mi(3.7 min/km), or 10mph(16km/h). This response was monitored when the runner was untrained(UT), after the runner had trained(TR), and during a period when the runner demonstrated symptoms of overtraining syndrome(OT). This figure allows that heart rate was higher when the runner was in the overtrained state than when the runner was responding well to training. Similar findings have been reported for swimmers. Such a test provides a simple and objective way to monitor training and can possibly provide a warning sign of the onset of the overtraining syndrome.

Reducing the risk and treating the overtraining syndrome

Recovery from the overtraining syndrome is possible with a marked reduction in training intensity or complete rest. Although most coaches recommend a few days of easy training, overtrained athletes require considerably more time for full recovery. This might necessitate the total cessation of training for a period of weeks or months. In some cases, counseling might be needed to help the athletes cope with other stress in their lives that might contribute to this condition.

The best way to minimize the risk of overtraining is to follow periodization training procedures, alternating easy, moderate, and hard periods of training. Although individual tolerance varies tremendously, even the strongest athletes have periods when they are susceptible to the overtraining syndrome. As a rule, one or two days of intense training should be followed by an equal number of easy training days. Likewise, a week or two of hard training should be followed by a week of reduced effort with little or no emphasis on anaerobic exercise.

Endurance athletes(such as swimmers, cyclists and runners) must pay particular attention to their carbohydrate intake. Repeated days of hard training gradually reduce muscle glycogen. Unless these athletes consume extra carbohydrate during these periods, their muscle and liver glycogen reserves can be depleted. As a consequence, the most heavily recruited muscle fibers are not able to generate the energy needed for exercise.