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The Science behind detraining and reversibility

Every athlete or individual has experienced time when they have been unable to train, this can be for numerous reasons but most commonly due to injury. The first concern that comes to mind is how much fitness they will lose, will they be able to get it back. This article investigates the adaptations the body makes to training, both aerobically and anaerobically, and what happens when training stops or changes using a real life example. The follow up article to this will look at how long it takes for detraining to occur and what we can do to prevent it or slow it down. the second part of this mini series will also look at perfecting your training and training zones for optimal performance.

The data used in this article is purely my own, between 2011 and 2018 I was training as an endurance road cyclist, I had 2 years off where I ran a little and started weight training more than I did when I was cycling. I will focus on this data and specifically cycling however the theory is still sound and correct and can be applied to anyone. The data is also personal and a sample size of just one so of course they will be variations but again the theory and general trends are correct.

Physiological adaptations to training

To understand what happens during periods of detraining (different to periods of rest) we must first understand what adaptations are made in the body during training. This is where we explore the idea of aerobic and anaerobic training. Put simply, aerobic exercise is when the body uses oxygen to allow for a sustained effort of over of more than a few minutes, whereas anaerobic exercise is performed over a short period of time with energy coming purely from glucose already stored within the body, oxygen is not used during anaerobic activity. This is heavily linked to how Adenosine Triphosphate (ATP) is resynthesized in the body by the energy systems, to find out more about this check out our blog on The Energy Systems for Endurance Athletes. Anaerobic metabolic adaptations see an increased level of anaerobic substrates that can be stored within the skeletal muscle, ATP, Phosphocreatine (PCr), Creatine and glycogen, all of which are required to provide the body with instant energy. Key enzymes that are utilised during glycolysis (instant energy release) increase in quantity and activity meaning more energy can be produced faster.

Fig 1. Racing in the summer of 2017

Aerobic metabolic adaptations are a lot more in depth and cover a lot more anatomy that purely what we can store in skeletal muscles. For this we can use the bigger picture and well used formula of VO2 max as an indicator of Aerobic fitness, this is the maximum rate at which the body can utilise oxygen during exercise. The fundamental key to improved aerobic performance is how well the body can use oxygen throughout the body, and therefore the most substantial adaptation is the increase in mitochondria within skeletal muscle fibre. Mitochondria’s primary role within the cell is to produce energy through respiration. Aerobic training increases the oxidisation of fatty acids for energy during rest and submaximal exercise, fat metabolism or lipolysis also becomes more apparent at submaximal exercise irrelevant of fuel input; simply meaning our bodies can use fat as energy more often. Lipolysis creates less waste products than glycolysis allowing the body to work harder for longer.

Further adaptations to enhance the aerobic performance of the body include hypertrophy of the slow-twitch muscle fibres (rich in Mitochondria and Myoglobin). Cardiac hypertrophy occurs, mostly in the left ventricle, meaning the heart can contract harder and pump more blood around the body with each beat (stroke volume). Red blood cells (rich in haemoglobin) and blood plasma (rich in electrolytes and proteins) both increase in response to aerobic training allowing more efficient delivery of oxygen and essential proteins to working muscles.

Fig 2. The cardiovasucalr system plays a huge role in resynthsizing ATP for sustained exercise

The reality of reversibility

So, let’s look at some of my data, take this with a pinch of salt as it is one sample size and my training changed rather than stopped completely. I last raced in Spring of 2018 and have not ridden my bike properly until a few weeks prior to writing this blog and doing the testing. I have however been riding to work most days (15-minute ride each way) and have been doing a lot more weight training including the legs and plyometric work.

The first 2 results below show my aerobic efforts, the top result (Fig 3) is from 6th April 2020. It shows a constant threshold effort at an average of 152bpm, this was the same perceived effort as the data from below. The second set of data (Fig 4) is from a race in June 2016, the second half of the race shows a very similar perceived effort with an average heart rate of 171bpm.

Fig 3. Heart rate data from a threshold effort in April 2020

Fig 4. Heart rate data from a threshold effort in June 2016

So why has my lactate threshold heart rate drop by 19bpm? This is because my metabolic processes are now less efficient, each cell contains less mitochondria, my blood volume and blood plasma have decreased, the strength of my left ventricle is less and therefore unable to contract as strongly as it did when I was training properly. The combination of my heart being able to pump less blood, my metabolism and working muscles becoming less efficient at using oxygen being delivered by the blood, and the way in which my body now uses fuel for activity all contribute to this decline. The aerobic test I did was effectively a threshold test to see how my body is creating energy, and how hard I can work whilst still clearing byproducts from energy production within the body. This is where we come onto thresholds, Aerobic and anaerobic, and how fuel is used during exercise.

Aerobic and Anaerobic thresholds

Possibly the most talked about but misunderstood area of training, thresholds are when the body begins to produce energy in a different way and from different fuels (Fatty Acids, Carbohydrates and sugars). The aerobic threshold is the point where the body is less able to rely on lipolysis (burning fats for fuel) anything below this threshold in theory means the body can resynthesise ATP indefinitely. As soon as you go above your aerobic threshold the body begins to use carbohydrates (Glycolysis) as well as fats for fuel, this process begins to accumulate Lactate (a byproduct of glycolysis) but at a level in which the body can get rid of lactate faster than it is being produced. As work intensity increases you will then get to your anaerobic threshold. This is the threshold where I am working at in the graph above (Fig 3 and Fig 4) and a very important factor to understand when it comes to aerobic or endurance performance.

When you work above your anaerobic threshold blood lactate levels begin to rise as the body is no longer able to get rid of lactate faster than it is being produced. Exercise at intensity above your lactate threshold effectively means the body cannot burn fats at all for fuel and is solely reliable on glycolysis (carbohydrates and sugars). Blood lactate rises faster and eventually the body must either stop or drop the intensity. As my data shows my anaerobic threshold dropped by 19bpm (from 171 to 152) over 4 years of non-specific aerobic training. Interesting when testing for maximum heart rate (a less reliable indicator of aerobic performance) I found that this had also dropped from my 2016 data, from 185bpm in 2016, to 171bpm now (shown in Fig 5). So, my max heart rate now is what my anaerobic threshold was back in 2016.

Fig 5. Maximum recorded heart rate from 2016 (top graph) and 2020 (bottom graph)

So why did my aerobic performance drop and anaerobic performance improve?

This is where training specificity is so important and how you can change what the body excels at. For me I spent years training my aerobic performance, making my body work as hard as it possibly could without increasing blood lactate levels. Becoming very efficient at utilising oxygen for sustained efforts and racing over a period of around 2 hours. When I stopped training on my bike in 2018, I began to train anaerobically a lot more in the form of strength and plyometric training. If we think back to the physical adaptations the body makes in response the anaerobic training, my muscles are now able to store more ATP, Phosphocreatine (PCr), Creatine and glycogen. Key enzymes that are utilised for rapid ATP synthesis (The Lactic Acid energy system) increase in quantity and activity meaning more energy can be produced faster.

So, my body is now able to produce more energy very rapidly through readily available ATP in the muscles and a faster breakdown of Glucose into energy via glycolysis. When I am producing efforts of around 5 minutes like the hill climbs I did during my recent testing, my body does not require any oxygen to recharge its energy sources as I already have the energy available. Previously my body would not have been able to work anaerobically for this long so I would have had to lower intensity to allow my body to resynthesise ATP by using oxygen. However, my aerobic effort, the 60-minute threshold test, showed big declines as I am now less able to utilise oxygen like I once could. My heart rate was lower, fuel for energy was being used more from Carbohydrates and glucose, which eventually ran out, as opposed to from fats and less oxygen was being absorbed by my body because i have less mitochondria to be able to make use of it.

Part 2 of this blog takes a closer look at how to find your threshold levels, how to train them for improved performance and how to use this knowledge to prevent declines during periods of enforced rest.

We hope you enjoyed this blog and found it useful. The energy systems and threshold training are very complex mechanisms so if you would like to know more or have any question please do ask. Either comment on the blog or get in contact

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