Endurance Training ↔ Overtraining & Muscle Loss? Run to Exhaustion & Sympathetic, Medium Intensity Steady State & Parasympathetic, HIIT-Like Training & No Overtraining

HIIT-like 400m sprinting is exhausting, but unlike running to exhaustion and medium intensity steady state cardio it's not going to mess up your nervous system.
Not one but two recent studies confirm what many of us have experienced first hand: Endurance training - specifically during a cut - is a double-edged sword. On the one hand, it's a neat way to augment the energy deficit, when you're dieting and maintain in a eucaloric state, when you're not. On the other hand, however, even moderate endurance training can alter the sympathetic and parasympathetic balance and thus create an imbalance that is characteristic of any form of overtraining.

Speaking of overtraining: As a SuppVersity reader you should actually be aware of the fact that scientists distinguish two different types of overtraining: Sympathetic and parasympathetic overtraining
You can learn more about HIIT, which appears to be less overtraining prone than MISS.

Never Train To Burn Calories!

Tabata = 14.2kcal /min ≠ Fat Loss

30s Intervals + 2:1 Work/Rec.

Making HIIT a Hit Part I/II

Making HIIT a Hit Part II/II

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Due to the fact that the symptoms (see Figure 1) closely resemble those Morbus Basedow (engl. Grave's Diseases) and Addison's Disease, respectively, sympathetic and parasympathetic overtraining are also called Basedowoid and Addisinoid overtraining.
Figure 1: Overview of the symptoms of the two major forms of overtraining.
You can see that the symptoms partly overlap. That's yet not the only problem you have if you want to diagnose the type of overtraining. In many resistance trainees, for example, you find either mixed forms or see a transition from classic sympathetic to parasympathetic overtraining over time (assuming the athlete doesn't do anything to normalize his / her sympathetic nervous system function).
There is no formula to calculate how much exercise you can sustain, but I'd suggest you take a look at my previous articles on heart rate variability and overtraining ("Are You Overtraining? Two Scientifically Proven Methods to Test Yourself - Method 1: Heart Rate Variability Analyses" | read more). They will help you to check, where you're at, if you have a baseline reading that was taken, when you've been completely rested  | learn more.
For the average study participant in a recent experiment that was conducted by scientists from the , The 42nd Hospital of PLA, the Xinqiao Hospital and the Chongqing Normal University in China, the duration and intensity of their cardio workouts (running) determined, whether the prescribed workout routines that consisted of ...
  • There is such a thing as overtraining, folks | read more
    4 times a week running at 100% of their maximal heart rate until they were exhausted (utmost intensity group)
  • 30 minutes of running four times per week (moderate intensity group)

  • 3 - 5x 1200 m runs per day with a  5-min break every 400 m four times per week (high intensity group)
made them overtrain or not, and whether their para- or sympathetic nervous system was overreacting.
Table 1: Characteristics of study groups at pre and post | Data are means XS± . Pre, pretraining; post, at the end of 8-week training; mid, at the end of 4-week training. Utmost, utmost intensity endurance training; moderate, moderate intensity endurance training; high, high intensity endurance training (Tian. 2014)
The subjects, 72 nonsmoking male students whose characteristics are summarized in Table 1, followed the routine they had been randomized to for 8 weeks. As you can see, there were no statistical significant changes in body composition over the course of the 8-week study. Although, it sould seem that the body fat percentage (I assume BFR is body fat) declined a tad bit more in the high intensity group.
Greater fat loss with HIIT, this wouldn't be a surprise - That's no news for you as a SuppVersity reader. I've repeatedly pointed out that the short intense workouts are more suitable for fat loss; and that not in spite of, but rather because they may burn less body fat during exercise.

If you have no idea what I am talking about, I suggest you take another look at my June 2012 article "Are You Still Burning Calories or Already Losing Fat? Study Shows: 5x15 Min HIIT Reduce Body Fat & Improve Fitness Twice as Effectively as 5x40min of Classic Cardio" (learn more) after you've finished this article.
Where the subjects differed, however, was in their response to the specific aerobic exercise programs they've been assigned to (I will directly quote the results from Tian et al (2014) and briefly comment on each of them):
  • Heart rate variability (HRV): No significant changes in HRV parameters were found in all groups at pre and mid. But at post, the moderate intensity group showed more significant increases in RMSSD, PNN50, HF, LF and SDNN (P < 0.05 or 0.01) and much greater reduction in LF/HF than the other two groups (P was 0.033, 0.037 respectively). HFn of the moderate intensity group was significantly higher than that of the utmost intensity group (P = 0.012), while the opposite pattern occurred in LFn and LF/HF of the two groups (P was 0.025, 0.015 respectively).

    As you would expect the changes in HRV in the moderate and utmost intensity group reflect increases in parasympathetic and sympathetic nervous system activity, respectively.
  • Circadian Changes in Cold Pressor Test (CPT): From pre to post marked differences were not found in SBP and DBP of all groups and their increases. At post HR was much less increased in utmost intensity group during CPT than the other two groups (average P < 0.05).

    Next to a high basal heart rate an inhibited increase in heart rate is another characteristic of later stages of sympathetic overtraining.
  • Plasma catecholamine (NE & EPI): Norepinephrine (NE) concentration was considerably lower in utmost intensity group than the other two groups (P was 0.001, 0.00 respectively). At post marked inter-group differences were still not found in plasma PEI concentration.

    A reduced catecholamine release is a classic characteristic of long(-er) term sympathetic overtraining - a phenomenon, some people may call "adrenal fatique" that occurs after an initial phase of catecholamine overproduction in sympathetic overtraining.
Overall, the results of the study at hand confirm previous research that found associations between classic "moderate intensity" endurance training and parasympathetic dominance (Yamamoto. 2001; Pichot. 2002; Myslivecek. 2002).

For the utmost intensity group, on the other hand, the scientists diagnosed an "over-excited SN [sympathetic nervous system]" (Tian. 2014), which is in contrast to the medium intensity and high intensity group, where the head-up tilt test did not indicate an "impairing effect on autonomic regulation" (Tian. 2014).
What about muscle loss? Oh, yes! I almost forgot that scientists from the University of the Witwatersrand (Oost- huyse. 2014) in South Africa have recently been able to show that 3 h of race- simulated cycling on 4 consecutive days may improve the cyclists' ability to tap into their fat stores as an energy reserve. Unfortuna- tely, it will also lead to a 28-46% greater reliance on endogenous protein catabolism during exercise on day 2-4.
Now, every SuppVersity reader knows that protein catabolism doesn't necessarily translate ot "muscle loss", but for the average 10h of cardio + 20% energy deficit "dieter", it could.
Bottom line: A least in the study at hand, the intense, albeit better short bouts of high intensity exercise in the HIIT-like high intensity group of the study at hand turn out to be the least overtraining prone type of aerobic activity.

Even the classic medium-intensity cardio training appears to be more overtraining-prone, due to the comparatively long duration and the subsequent increase in parasympathetic nervous system activity. If you're looking for a "side-effect free" cardio regimen, 3-5x intervals of 3x400m sprints could be a good way to incorporate cardio training into your exercise routine.

One thing we should keep in mind, though, is that someone who is sympathetically overtraining in the gym with all its negative consequences (see Figure 1) would probably be better of with classic "moderate intensity cardio" to bring up the parasympathetic tone and avoid "weight lifting induced" sympathetic dominance | Comment on Facebook!
  • Myslivecek, P.R., Brown, C.A. and Wolfe, L.A. (2002) Effects of Physical Conditioning on Cardiac Autonomic Function in Healthy Middle-Aged Women. Canadian Journal of Applied Physiology, 27, 1-18. 
  • Oosthuyse, T., & Avidon, I. (2014). Changes in substrate utilisation and protein catabolism during multiday cycling in well-trained cyclists. Journal of Sports Sciences, (ahead-of-print), 1-11.
  • Pichot, V., Busso, T., Roche, F., Garet, M., Costes, F., Duverney, D., Lacour, J.R. and Barthélémy, J.C. (2002) Autonomic Adaptations to Intensive and Overload Training Periods: A Laboratory Study. Medicine & Science in Sports & Exercise, 34, 1660-1066. 
  • Tian, Kaixin, et al. "Effect of Endurance Training on the Autonomic Nervous System Function of Young Male." International Journal of Clinical Medicine 5.19 (2014): 1189.
  • Yamamoto, K., Miyachi, M., Saitoh, T., Yoshioka, A. and Onodera, S. (2001) Effects of Endurance Training on Resting and Post-Exercise Cardiac Autonomic Control. Medicine & Science in Sports & Exercise, 33, 1496-1502. 
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