So long as you train, you can maintain and (hopefully) build your fitness levels. However, stop training and your fitness levels will steadily decline.

The obvious question that you might therefore ask is, “How much fitness will I lose if I decide to take a break, or if I’m forced to stop training because of injury or illness? And how rapidly will this fitness loss occur?” To answer this, it’s important to understand that there are several different components of fitness, including muscular strength, muscular endurance and cardiovascular – heart, lung and circulatory – endurance.

Stop training and the performance decline in each of these components will take place at different rates. So let’s take an imaginary well-trained cyclist and observe what happens to their body over a period of six months following the complete cessation of training.

Day 0 
This is your last training day for the next six months. After today’s ride, you store your bike away, hang up your cycling shoes and join the bulk of Britons who do no regular vigorous exercise whatsoever!

Day 3
After three days of inactivity, you might expect that your fitness has already begun to decline. In reality, however, the losses at this stage are very small. If you had been training hard prior to day 0, after three days of rest, your cycling fitness is now probably enhanced.

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That’s because in those three days, your muscles have had time to fully recover; muscle carbohydrate stores (glycogen) have been topped up, muscle fibres damaged during hard training have been fully repaired, and favourable metabolic changes in the muscles have had time to occur.

Indeed, this peak in performance after a few days of rest is exactly the reason why tapering works, and why you shouldn’t train right up to the day of a big event.

Day 7 (Week 1)
After a week’s complete inactivity, changes begin to occur in the body that result in fitness losses. For example, after three days, your blood volume can be reduced by five to 12 per cent. This means a decrease in the amount of blood your heart can pump – both in terms of amount of blood pumped per beat and total blood volume per minute.

The result is that your heart has to work slightly harder to maintain a given workload on the bike. There are some metabolic changes too. After six days or so, muscles begin to become less efficient at ‘soaking up’ glucose – the body’s premium fuel for exercise – from the bloodstream. This means that during exercise, you need to place more reliance on your (limited) muscle glycogen stores, and also that you become less efficient at building up those glycogen stores after exercise.

A third change is that your muscles start to become less efficient at coping with lactate accumulation during sustained efforts of hard cycling. The upshot is that you won’t be able to sustain quite the same exercise intensity before having to back off, because of the burning sensation in the legs and laboured breathing.

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Weeks 2-3
At this point, your maximal oxygen uptake 
(VO2 max, the prime measure of your aerobic fitness) will have declined by anything from four to 20 per cent. Part of this stems from reduced cardiac output, not helped by the fact that the muscle mass in the pumping chambers of the heart can decrease by almost 20 per cent after three weeks of inactivity.

It also arises because of changes in muscle physiology and biochemistry that are beginning to take place. For example, the fine network of muscle capillaries built up with endurance training begins to decline. As a result, oxygen uptake in the muscles can decline by up to eight per cent.

One month
All of the detraining changes outlined above continue to progress, but fundamental muscle changes are now becoming prominent. By now, your muscle capillarisation will have returned to your pre-training baseline. However, it is still likely to be higher than that in people who have never trained. Alterations are taking place in your muscle biochemistry.

The biochemical pathways that help your muscles burn fat for energy start to become less efficient, making it harder to burn while you ride, which in turn reduces your endurance capacity. On top of that, not only is your overall muscle mass declining (reducing your maximum power and strength), muscle fibres known as Type IIa (used during high-intensity sustained efforts) start to revert to Type IIx fibres, with greatly reduced endurance capacity.

In short, your ability to maintain a full-out effort, for example, a sprint to the finish line, diminishes dramatically.

2-3 months
After two months of inactivity, your heart is noticeably less muscular, the thickness of the muscle walls that comprise the pumping chambers reduced by as much as 25 per cent. The muscle mitochondria are also becoming less efficient at using oxygen to produce energy in your muscles. This efficiency can decline by 25 to 45 per cent up to 12 weeks after training cessation.

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After three months, you also begin to undergo ‘hormonal detraining’. Hormones are chemical messengers that regulate the body’s biochemistry; as you become detrained, more stress hormones are released during exercise, which basically means that the same exercise intensity becomes more stressful for the body, which in turn increases recovery times.

6 months

By six months, your fitness declines have mostly stabilised. However, there are still undesirable changes taking place. For example, the actual volume of mitochondria per unit volume of muscle is declining, further reducing your ability to utilise oxygen during exercise.

You’ve almost certainly gained body fat due to a lower daily calorie burn and a loss of muscle mass. So while you may not have gained weight on the scales, you will have almost certainly become ‘fatter’, with all the health risks that involves.

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The science

The decline of fitness 
components during detraining
There are a number of components of fitness that become ‘detrained’ when you cease training. These include:

Cardiovascular Stop training and your blood volume falls, which means there is a decrease in the total amount of blood your heart can pump during exercise. The dimensions of the heart muscle also decrease, as does respiratory (lung) function, caused by weakening of the muscles in the ribcage. All of this reduces the amount of oxygen you can transport to your working muscles, which means your maximal performance will start to decline.

Metabolic Over time, your muscles will find it harder to produce energy from your fat stores. This effectively decreases your endurance since carbohydrate stores – the other major fuel source during exercise – are quite limited, whereas energy from fat stores is virtually unlimited. Something called ‘insulin sensitivity’ is also reduced, which means your muscles find it harder to take up glucose (muscle fuel).

A less efficient oxygen metabolism means that for a given effort level, higher levels of fatiguing blood lactate are produced in your muscles, making it 
harder for you to maintain 
high training intensities.

Muscular detraining In your cycling muscles, the density of capillaries – the tiny blood vessels that carry oxygen to your muscles – decreases, and the concentration of enzymes in muscle mitochondria (the ‘aerobic energy factories’ within cells) used to release energy from oxygen, also decreases. 
As well as this, muscle fibres shrink in cross section and 
cell-signalling hormones involved with gaining and maintaining strength decline, leading to strength losses.

Fitness Tip

Keep your fitness in 12 minutes a week!
If you’re a seasoned cyclist, don’t worry about taking a training break of up to two weeks as your fitness losses will not be too large. However, fitness losses for those new to cycling, or those or with a low fitness base will be proportionately greater.

For breaks of more than two weeks, you’ll need some kind of training stimulus to avoid a bigger decline in your fitness. For example, interval sessions consisting of just 8×30-second efforts, two or three times a week – just 12 minutes a week – can be very effective at preserving aerobic fitness.

During any period where you drastically cut down on training volume, remember you will also need to curtail your calorie intake in order to prevent the performance-blunting effects of increased body fat.

If you haven’t already done so, think about investing in a turbo-trainer or other training device for the long, dark winter nights and inclement weather.

Bad and good news

Stop training and your fitness begins to decline quite soon after, continuing to fall away the longer you stay off the bike, until after six months of inactivity, you’ll be a long way behind in the fitness stakes.

There is good news, however; although you’ll lose plenty of fitness, you’ll likely maintain a higher fitness baseline than someone who’s never trained. Studies have shown that the muscles of previously trained people are better at coping with lactate accumulation, and have better capillarisation and mitochondrial density than lifelong couch potatoes.

If you need to take a short training break, there’s more good news as research shows that a relatively short break (two weeks or so), is not considered too 
disruptive, providing the previous training adaptations have been achieved over a period of at least 12 weeks or more.

And if the winter months are a real struggle for you, there’s 
evidence that even relatively small volumes of the right kind of training can reduce detraining to a minimum.

For example, research shows that reducing your training volume by two thirds won’t harm your fitness, provided you include some 
very intense work, such as 
intervals, in the remaining one third.

Indeed, just by doing a 
little bit of high-intensity interval work, such as 7 x30-second 
‘all-out’ bouts performed three times per week, may be enough to preserve the bulk of your hard-earned cycling fitness.

This post was updated on 14 December 2017