Physiology of Aerobic Detraining
It is thanksgiving week and I have a bit of time off (aside from the mountain of school work I will be neglecting until week’s end) to delve into a bit more detail on a subject that might interest those of you that run, cycle, or what have you as a serious hobby, or even competitively. If you are familiar, you might have heard that the cessation of aerobic, cardiovascular exercise leads to a rapid decrease in ability; well, in this article, we will examine how quickly performance decreases from cessation of exercise, and what adaptations occur that impact said performance. If you find this interesting, then stay tuned!
Rate of Aerobic Detraining
The rate at which we see decreases in aerobic function from ceasing all exercise are just about immediate, sadly . These changes cannot necessarily be quantified as one stable number, because there are many adaptations that change over time – some affected almost immediately and others not for weeks or more . So, some decline within the first week and some not until 2-3 weeks later .
Understanding the Physiology
This will be, without a doubt, the most in-depth part of the article (shocker!), so we will break this up into subsections so as to make for an easier read.
If we use VO2 as our overall marker for performance changes, then we can better understand the impact of the following adaptations on VO2. Quickly, VO2max is the amount of oxygen used during, typically, exercise – the higher it is, the better (for health, performance, etc.). Again, typically, people who have been endurance training for some time (and seriously) see a rise in their VO2max, so if these people stop training, as we are trying to find out, VO2max seems to decline in the first 1.5 weeks . This decline can range between 4 and 20%, with more trained individuals experiencing more on the upper side of the scale, especially 8 weeks or longer .
Interestingly, this decline is nearly immediate, but seems to plateau after the initial week to week and a half for about 2 weeks, and then continues to decline .
So, what is the reason for this sudden and rapid decline in VO2max?
The answer is, at least partially, due to cardiac output. The amount of blood the heart is able to pump in a given amount of time (typically 1 minute) has a significant impact on all facets of performance, but certainly in our example of VO2 as a decrease (9%) in cardiac output inevitably leads to less nutrients, less oxygen delivery throughout the system . That said, why might cardiac output decline?
Cardiac output seems to decline due to a 10-14% drop in stroke volume (aka, the amount of blood released per beat) within the first week and a half . This decrease is most represented in the decrease in blood volume (5-12% decrease) as it diminishes due to less plasma and fewer red blood cells; this effect occurs within, even, 2 days’ time . The other component of cardiac output is heart rate, and as expected, if stroke volume decreases, heart rate compensates by increasing (5-10%) to continue to supply blood to the body .
With exercise, as with many physiological conditions, there is an increase in the number and kinetics of enzymes that help propagate reactions – in this case, the synthesis of adenosine tri-phosphate (ATP) through the citric acid cycle and the electron transport chain (aerobic metabolism). So, among measured (this is key, there are far more adaptations than just what is measured) enzymes, succinate dehydrogenase and citrate synthase, we see a rapid (within first 1.5 weeks) and substantial decrease (39% after 8 weeks) in activity consequently leading, presumably, to a decrease in ATP production .
Interestingly, for those who have been training seriously for years, although we see a dramatic decrease in enzymatic activity, and that activity stabilizes after about 8 weeks; if that stabilization holds true beyond what we have studied, then enzymatic activity is still higher than sedentary individuals .
Muscle Fibers & Capillarization
There is little to no change when examining muscle as we might see if we examined strength trained individuals where a shift from fast twitch to slow twitch fibers may be possible – in endurance athletes, we little to no change after 12 weeks . However, there is some debate if capillarization of the muscle decreases over time with most evidence pointing to little to no change with capillary density still remaining 50% higher in detrained regular trainees in comparison to sedentary individuals .
It is completely unaffected by exercise or lack of exercise in healthy individuals .
In terms of the heart muscle size, there seems to be, again, conflicting information on if the ventricles decrease in thickness . It would make sense to assume that if the ventricles increase in size with exercise, then a lack of exercise would see atrophy, so my best guess would be to see a decrease in left ventricle size (25% decrease), but likely after a 2-3 weeks .
As we know, exercise promotes insulin sensitivity to a great degree . So, what happens when we cease exercise, especially cardiovascular exercise? As expected, we see a decrease in insulin sensitivity. This likely occurs due to a decrease in the GLUT-4 transporter which reduces by 17-33% over the first week or so of detraining . This transporter is necessary for the cell to take in glucose from the blood, so less of the protein (the transporter is a protein), the less shuttling of glucose from the blood into the cell for use – aka, insulin resistance (or, otherwise stated, insulin insensitivity).
In total, we are left with a variety of different answers, but from a broad perspective, we know that detraining, no matter how aerobically fit you are, happens immediately. Within days, your body slowly decreases in many positive bodily adaptations (cardiac output decreases, enzymatic kinetics decrease). Other adaptations take place at a slower rate, but still inevitable with decreases in heart muscle strength, insulin sensitivity, and possible, eventual, muscle fiber adaptations. Interestingly, certain adaptations (enzymatic activity, for example) remain higher than sedentary values after several months of relative inactivity (no exercise).
Writer: Nicolas Verhoeven
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