Aerobic Detraining

Whether it is injury, sickness, travel, planned training cessation or insufficient amount of training –reversibility of previous gained performance improvement, Detraining, can happen. The decrease in performance will affect you depending on your current status and the time of your interruption (short term, 3-4 weeks and long term >4 weeks). Bear in mind, that as I discuss the physiological effects of short term aerobic detraining, the word detraining will be used to describe a period without training, compared to tapering which sometimes mistakenly is used interchangeably.  

Cardiovascular effects of aerobic detraining. An increase in blood volume, mainly plasma volume (already after first session), is one of the first endurance adaptations. However, it also decreases to pretraining level within one week (~ -12% decrease for 2.91l -2.56l plasma). In an attempt to maintain cardiac output (Q, circulation) for muscular blood supply, your heart will pump faster (~ 4% increase in HR) to make up for decreased stroke volume (SV, amount of blood pumped out with each beat) and decreased ventricular size.  

Since Q is affected, VO₂max will drop proportional to quantity and therefor have a greater effect on highly trained individuals than moderately trained, example ~8% eg. 62.2-57.3ml/kg/min.

For all Exercise Physiologists...

VO2 max=Q x (a-vO2max)

It’s been shown that VO₂max remains 3% lower even after reestablished SV, indicating importance of other metabolic factors. However, one should keep in mind that recent data has shown that VO₂max might not be the best predictor of performance among high level athletes.

At the Muscular level of aerobic detraining capillary density is reduced, but not enough to return to pretrained state within four weeks of detraining. This will could possibly be another factor affect SV. Additionally, it reduces the gas exchange necessary for aerobic metabolism which in conjunction with reduction of oxidative enzymes leads to an increased reliance on anaerobic metabolism. This shift towards anaerobic reliance has been shown through an increase in RER, respiratory exchange ratio, at set intensity. RER is the difference between carbon dioxide produced and oxygen used, measure in exhaled air as carbon dioxide is a byproduct of anaerobic metabolism. 

RER=(Volume CO2)/(Volume O2)

Another supported factor for an increased glycogen dependency is related to the decrease in muscle GLUT-4 transporter protein. GLUT-4 is responsible for transport glucose (carbohydrates) into the sarcoplasm for glycolysis (glucose breakdown for anaerobic ATP production) or glycogen synthesis (glucose storage). It might sound contradicting that lack of this transporter would increase use of carbohydrates. However, GLUT-4 is stimulated by the combination of insulin and exercise.  Since lack of exercise decreases insulin sensitivity, leaving a lot of glucose in rested state, the uptake will be increased during exercise. Increased insulin sensitivity with training will enable GLUT-4 activity and glucose uptake even in rested state. Therefore, some research states that GLUT-4 activity decreases only as a secondary outcome of decreased insulin sensitivity. Additionally, the fat enzyme muscle lipase has shown to decrease during inactivity meanwhile increasing at the adipose tissue leaving less room for breakdown and more for build up of fat.

It is possible that short-term detraining have a greater effect on peak aerobic performance as submaximal performance has shown to be maintained amongst distance runners while endurance trained athletes decreased their TTE (time to exhaustion) with 4-25%. The increased lactate production might be one of the factors. As lactate is a product of glycolysis, this increased dependency on glucose will increase blood lactate with a greater change in trained individuals.

While muscular glycogen has shown to decrease with as much as 20% in one week, other anaerobic substrate has not been studied as much in short term detraining and/or not shown to a change indicating that it needs longer time for detraining and would relate more to detraining in relation to resistance training. The same is true for hormonal changes.

So does this mean that days and periods off are bad? Absolutely not. The importance of recovery days and off season is well knows. However, it’s importance to differentiate between detraining, off season and other light periods following an athletes periodization. 

This article is based on content and sources in the review by:

Review by:
Mujika, I., Padilla, S., Detraining: Loss of Training-Induced Physiological and Performance adaptations. Part I. Sports Med, 30 (3): 145-154, 2000.

Additional sources

McArdle, William D., et al. Exercise Physiology: Nutrition, Energy, and Human Performance. 7th ed., Wolters Kluwer Health/Lippincott Williams & Wilkins, 2015.