by NSCA's Essentials of Personal Training, Second Edition
Kinetic Select May 2017


Understanding how the body adapts to the overload of aerobic exercise is critical to designing effective exercise training programs, monitoring exercise responses and progress, and assessing training outcomes.

The following is an exclusive excerpt from the book NSCA's Essentials of Personal Training, Second Edition, published by Human Kinetics. All text and images provided by Human Kinetics 

When intensity, duration, frequency of training, or any combination of these factors exceeds an individual’s capacity for adaptation, overreaching and overtraining may occur. Exceeding adaptation capacity without sufficient recovery normally leads to decrements in physical performance that are based on complex interactions among several biological systems and psychological influences (3,20,44,46,68).

Overreaching refers to short-term training, without sufficient recuperation, that exceeds an individual’s capacity. Successful recovery from overreaching can occur within a few days or up to two weeks with an adequate recovery intervention (3,46). While some authorities view overreaching as an unplanned and undesirable consequence of strenuous training, others view it as a training technique to enhance performance (3,71).

Although short-term overreaching results in a decrement in performance, when it is followed with appropriate recovery periods the result may be an enhanced performance as compared to baseline. Overtraining syndrome is more serious and results from untreated overreaching that produces long-term impairments in performance and other conditions that may require medical intervention.

Two types of overtraining have been theorized to exist; the difference is in the predominance of either the sympathetic or the parasympathetic nervous system (3,44,71). Apparently, aerobic endurance overtraining results predominantly from an excessive volume overload (parasympathetic dominant), whereas anaerobic or resistance overtraining (sympathetic dominant) primarily results from excessive high-intensity overload.

These different types of overtraining have been reported to have different signs and symptoms, although performance decrements are a key common aspect of both. Discussion of the many complicated and not fully understood aspects of overtraining is beyond the scope of this chapter, and the reader is referred to other sources for more complete information on these concepts (3,20,44,68).

A large number of markers for overreaching or overtraining have been identified by researchers. See the following sidebar for a list of some of the most common markers (i.e., signs and symptoms) of aerobic endurance overreaching, overtraining, or both (3,20,44,46,68).

There is a high degree of variability between individuals with regard to developing overtraining. Training practices that cause some individuals to thrive lead to overtraining in others. Unfortunately, there are also highly individualized responses and symptoms for overreaching and overtraining that make it difficult for clients and personal trainers to recognize (3,46,71). Besides a decrement in performance, people generally exhibit only a few, if any, other signs and symptoms of overreaching or overtraining. Being familiar with each client’s progression through training is essential for preventing overtraining.

A decline in performance coupled with one or more of the easily recognizable markers (i.e., fatigue, malaise, loss of enthusiasm for training, increased soreness) should lead to suspicion of overtraining. Checking the heart rate response to a standardized submaximal exercise load would be another appropriate method to monitor clients undergoing strenuous aerobic endurance training. For prevention of overtraining, an important component would be a properly planned periodization program.

It is critical for the client to have sufficient rest between training days to facilitate the recovery process. The amount of rest, however, depends on the duration and intensity of the training program and should be individualized for each client. Periods of high-volume or high-intensity training especially require sufficient recovery. Keep in mind that individuals undergoing strenuous and frequent endurance training also need sufficient carbohydrate intake to maintain muscle glycogen stores. Successive days of training can gradually reduce glycogen levels and impair performance (46,71).


The ways in which the body responds to detraining are analogous to the ways in which it responds to training. Once training is stopped, muscular endurance decreases after only two weeks. After four weeks, one study showed reductions in the trained muscles’ respiratory ability, decreases in glycogen levels, and increases in lactate production that demonstrate obvious changes in the muscle metabolism (10). Another study showed a 7% decline in both V. O2max and maximal cardiac output, as well as 17% to 19% decreases in aerobic enzyme levels, after training was stopped for only 12 days (11). Another investigation demonstrated that when aerobically trained rats stopped training, there was a site-specific decrease in the BMD of the tibia (66).


Our interest in and understanding of the physiology of aerobic endurance training has greatly expanded in the last few decades. Aerobic endurance training is a potent stimulus to physiological changes in the cardiovascular, respiratory, skeletal muscle, metabolic, endocrine, and skeletal systems and has substantial effects on body composition and performance. To be a highly effective personal trainer, it is crucial to have a clear understanding of both the acute responses and chronic adaptations of the many physiological systems of the human body.

Understanding how the body adapts to the overload of aerobic exercise is critical to designing effective exercise training programs, monitoring exercise responses and progress, and assessing training outcomes. The personal trainer must also recognize the effects that genetics, sex, age, specificity, overtraining, and detraining have on physiologic responses and adaptations.

From the National Strength and Conditioning Association (NSCA) comes this resource packed with more than 100 drills to help in the development of agility and quickness training programs. Applicable to almost every sport, Developing Agility and Quickness focuses on improving athletes’ fleetness of foot, change-of-direction speed, and reaction time. The book is available in bookstores everywhere, as well as online in the NSCA Store.

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