I live in Tempe, Arizona. In Tempe, although you can wear shorts and a t-shirt year-round, come August and September, you end up regretting your decision to live in the Southwest. Oh yea, summers down here are like reverse-winters. You do everything in your power to stay indoors. Only the most rugged die-hards and well-acclimated desert ruffians participate in voluntary outdoor activities during our hot season.
So when JB came to town for a quick 2 and-a-half-day business trip (during the hottest season of the year) and suggested taking me for a mountain hike, I was somewhat reluctant. But JB can be persuasive so off we went, JB laughing at my face-covering sunglasses and SPF 2000 sunscreen.
A few hours later, as we sat atop a peak on South Mountain overlooking Phoenix, it felt awesome for me to have a reprieve from my research and to have another one of my many enlightening discussions with John. As we sat, feeling the sun on our (well-protected) faces and enjoying the view of the city, we discussed a number of everyday subjects; you know, potential pathways that can shut down fat oxidation, blood type antigens and nutrition, stuff like that.
All this while doing our best to find the perfect rock of ideal size and weight to peg the saguaro cactus sitting 10 yards away.
We made our way down the mountain, joking about obtaining some veterinarian-grade glucosamine-chondroitin for our aching joints and trying to avoid the boulders that would cause the most impact on our knees upon landing. It was it this point that our conversation turned to the topic of this article.
On our decent, I admitted to John that just prior to his visit, I had been experiencing a sense of heightened vigilance and over stimulation, kind of like a feeling of chronic sympathetic nervous system activation. After taking a bit of ribbing from the sarcastic side of JB, he went ahead and admitted that he too had been feeling the same thing for a few weeks prior and was, in fact, feeling this same thing the weekend he was here.
He told me that after at least 8 weeks of hard power training, the CNS demands were starting to wear on him. Add on several plane rides in just a few weeks and an impending dissertation deadline and he was straight up stressed.
Now, since we've remained friends for almost 5 years, John and I used to laugh about being "hypercortisolemic" in the past, which was our very nerdy term we used to describe being completely stressed out (thus the cortisol release) in our university programs and overwhelmed with unreal amounts of work.
But this feeling we shared wasn't cortisol. I was baffled; especially considering that experts tell us exercise is the best remedy for what we were feeling. We had both been training very intensely for the prior few months and had absolutely no relief.
So when John started kicking around terms like parasympathetic and sympathetic overtraining, I knew he was onto something. Perhaps stress, over training, feeling "on edge", and having a "tuned up" sympathetic nervous system were all related.
Overtraining - The Basics
The general definition of overtraining is this - a syndrome occurring in athletes who train too frequently/in excess OR who may not allow for adequate recovery from intensive exercise. As a result of this inadequate recovery, performance is impaired (1).
While poor sport performance is the main symptom of overtraining, other non-specific symptoms may include other oldies but goodies such as: decreased motor coordination, decreased force production, altered immune, hormonal, and autonomic activity, and emotional, mood, and sleep disturbances.
While overtraining may seem scary, you don't just feel great one day and then wake up the next overtrained. There is a phase that precedes the overtraining syndrome called "overreaching" (or "short-term" overtraining). Overreaching is characterized by training fatigue and a reduction in maximal performance capacity that that can last a few days up to 2 weeks.
Typically, overreaching is inevitable during competitive training and therefore it is usually built into the program as a planned overreaching phase (and followed by a planned recovery phase). Without the recovery plan, overreaching can, however, progress into full blown overtraining.
Again, with overtraining, you may experience the symptoms listed above. While researchers aren't completely clear on why it occurs, many suggest that overtraining results from the accumulation of exercise and non-exercise fatigue/stress. These stressors build up leading to exhaustion and mood disturbances that can last months (2).
If left unchecked, overtraining can necessitate recovery periods lasting up to a year (3). No, that's not a typo. If you fall into full-blown overtraining, it could be a full year before you feel good again. Imagine how that might impact a pro athlete.
*As a side note, I think it's also important to note that overtraining stress equals the sum of the training and the non-training stress factors. Although training is the major contributor to overtraining syndrome, occupational, educational, and social stressors are accumulative and play a significant role (2). That's why someone who's only training 3 days per week can indeed be overtrained.
Most gym devotees would laugh if someone suggested that they could be overtrained while on a 3 day per week maintenance program but it's true. If the 3 days of training adds to some serious extracurricular stress, that's all it might take.
Two Types Of Overtraining
While researchers have typically considered overtraining one phenomenon, new data indicate that overtraining syndrome may affect the nervous systems (autonomic) of aerobic and anaerobic athletes differently. These differences are so pronounced that two types of overtraining have been described.
Since this area of research is new, the two types are not well classified just yet. However enough evidence is available to influence our perception of the demands of the two different types of activities (1,2,5,6,7).
Scientists have described the two types of overtraining as "Basedowoid" and "Addisinoid", the first type expressing itself as a sympathetic nervous system dominance (hyper, irritable, etc.) and the second type expressing itself as a parasympathetic nervous system dominance (fatigue, depression, etc; 8).
Basedowoid overtraining has been dubbed "classical" or sympathetic overtraining and results in performance incompetence, hyperexcitability, restlessness, and increased sympathetic activity at rest (6). Whereas, Addisinoid overtraining has been called "modern" or vagal/parasympathetic overtraining and is characterized by decreased sympathetic activity with predominating parasympathetic activity at rest and during exercise.
While it also results in persistent performance decrements, high fatigue, apathy, altered mood state, altered immune and reproductive functions are the consequences (4). Don't know what sympathetic and parasympathetic mean? Don't worry â€¦quot; we'll get to that in a minute. But first, here's a chart illustrating the differences.
Endurance Athletes And Overtraining
Given the high volume of training that endurance athletes perform, the idea of these athletes often showing signs reflecting an "exhaustion of the autonomic nervous system" (i.e. addisinoid or parasympathetic overtraining) seems understandable (6,9). In other words, sympathetic overtraining occurs when the body is reacting to stress inappropriately by increasing the activity of the stress system (sympathetic).
However, once this system "burns out", the sympathetic nervous system decreases in activity and the parasympathetic system dominates. As a result, overtrained endurance athletes can expect the outcomes listed by Kraemer and Fry.
Decreased total testosterone/cortisol ratio
Decreased free testosterone/cortisol ratio
Decreased total testosterone/SHBG (sex-hormone binding globulin) ratio
Altered cortisol levels
Decreased sympathetic tone
Decreased nocturnal and resting catecholamines
Increased sympathetic stress response
Increased creatine kinase
Decreased exercise-induced lactate
Consistent with sympathetic nervous system depression, endurance athletes may also experience a low resting heart rate, rapid heart rate recovery post exercise, metabolic alterations such as hypoglycemia and decreased maximal plasma lactate during exercise, and an increase in oxygen requirement resulting from excessive recruitment of motor units and incomplete recovery from this increased nerve stimulation (2, 4).
Anaerobic Athletes And Overtraining
Sympathetic type overtraining is a bit more difficult to characterize than parasympathetic. Increased neural demand may bring this type of overtraining to light however there are many other factors that can contribute to sympathetic overtraining including psycho-emotional stress, too many competitions, social, educational, occupational, economical and scheduling stressors, nutritional deficiencies, and travel schedules (6).
Until recently, much of the attention over OT has been directed towards those in endurance activities; however, in a survey of overtrained athletes it was found that 77% were also involved in sports requiring high levels of strength, speed, or coordination (4). Obviously, this type of overtraining affects a whole lot of athletes and needs to be better studied and characterized.
From what we do know, unlike the overtraining seen with aerobic/endurance activities (parasympathetic), resistance training/anaerobic overtraining (sympathetic) does not exhibit the same hormonal effects.
In contrast to the increased cortisol concentrations (eventually leading to a blunted stress response) and the blunting of GH and reproductive axes seen in endurance athletes, heavy resistance exercise has shown to result in undisturbed cortisol concentrations and consistent acute elevations in testosterone concentrations, along with elevated resting testosterone concentrations after training programs lasting several months in duration (1,4).
Confusingly, while aerobic and anaerobic overtraining often manifest differently, depending on certain training variables (the choice of exercise, the order of exercise, the volume, the rest intervals, and the load), the two types of overtraining may begin to look more similar.
Simply by increasing just the volume of training, an anaerobic overtraining profile may begin to resemble that of aerobic overtraining. In such cases, both long-term heavy resistance exercise and aerobic training will result in similar exercise-induced hormonal and sympathetic responses (4).
Exercise And The Parasympathetic Nervous System
Regular exercise can result in a series of positive parasympathetic adaptations including:
Increased Vagal Tone
Impulses from the vagus nerve produce inhibition of heart rate. Vagal tone (and heart rate) represents the relationship between parasympathetic activity and sympathetic activity. With regular exercise, the parasympathetic nervous system (the brakes on heart rate, if you will), increases in activity and therefore heart rate remains lower at rest.
Dynamic Heart Rate Variability
A healthy beat-to-beat variation in heart rhythm normally results from an exercise program. This variability represents the sympathetic and parasympathetic balance in the regulation of the heart by the nervous system (11). Think of it this way, when you're healthy the sympathetic nervous system and parasympathetic nervous system are dynamic and are always playing tug of war with your heart rate. Again, this is a good thing.
The reflex is involved in the regulation of blood pressure and heart rate. Changes in sensitivity of the baroreflex could be the result of increases in blood volume, increased vagal tone, cardiac muscle growth, and changes in baroreceptor (neural receptors sensitive to changes in blood pressure) density. Endurance training increases baroreflex function, allowing for rapid changes to regulate blood pressure and heart rate (10).
As you can see, the parasympathetic nervous system provides a host of benefits. A healthy autonomic system is characterized by all 3 of these parasympathetic adaptations. Furthermore, any activity or behavior that chronically activates the sympathetic nervous system and/or diminishes parasympathetic (vagal) tone will increase the risk of cardiovascular events.
In contrast, therapies that favor parasympathetic dominance, and decrease sympathetic tone, will improve prognosis (12). With all this benefit associated with parasympathetic dominance, the next logical question would be this: Why is the parasympathetic overtraining response considered a problem?
In parasympathetic overtraining there may be a failure to mount a stress response due to chronic overactivation of stress. It is evident that in our society we have more technology to make our lives easier but we're more stressed out than ever in our school programs, jobs and careers, relationships, and our daily lives in general.
Understanding that exercise, itself, is a stressor and imposes demands on the various systems of the body, you can see how overload can occur, causing a virtual shutdown of the sympathetic system and forced parasympathetic dominance.
This week, I'll leave you with a characterization of the different types of overtraining and how the effect the nervous system. Next time, we'll talk more about stress, disuss the difference between exercise and non-exercise stress and talk about some very interesting connections between mood and training status.
Click here for part two!
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About The Author
Tammy Thomas is a registered dietitian and a certified strength and conditioning specialist (CSCS) who holds a Master's degree in Exercise Science focusing on Nutritional and Exercise Biochemistry. Currently she does training and nutrition writing and consulting for individuals with rheumatoid arthritis and other autoimmune diseases at www.proactivitysupport.com. She can be reached at firstname.lastname@example.org.
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