1️⃣ You lose muscle after a 3 week training break.
2️⃣ Muscle memory helps you regain muscle quickly when retraining.
3️⃣ Limit muscle loss by eating a high protein diet and being physically active.
How Long Does It Take to Lose Muscle?
If you take a training break, you start losing muscle after 3 weeks.
How quickly you lose muscle depends on how active or inactive you are. Bed rest and limb immobilization drastically speed up muscle loss. Your diet also affects this process.
Most importantly, not all “size loss” is muscle atrophy.
In this article, you will learn all you need to know about detraining and muscle loss.
How muscle loss works
Muscle loss is also known as atrophy, the opposite of hypertrophy. Muscle and strength loss happen due to a lack of exercise (detraining)  . But what happens under the hood is a bit more fascinating.
Strength training stimulates muscle protein synthesis, which leads to muscle gains . When you stop exercising, the benefits are reversed, decreasing muscle size . In other words, you need to use it or lose it.
According to multiple studies, muscle protein synthesis can decrease by an alarming 31% to 53% during periods of inactivity or immobilization  . Five days of muscle disuse can considerably lower muscle protein synthesis rates  . Your muscles stop responding efficiently to protein.
During periods of disuse or inactivity, our bodies enter a state of anabolic resistance. Meaning, our muscles become less responsive to the effects of protein   . This blunts anabolism and is the reason for muscle loss during disuse .
What's the reason for this anabolic resistance?
Scientists are still working on the exact mechanisms. Still, it seems that our bodies are wired to maintain as little muscle as possible during periods of disuse. By reducing muscle mass, the body can conserve energy and resources.
The article continues below.
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Losing size doesn’t mean you lost muscle
During the first week of your break, your muscles may look and feel smaller.
Naturally, you think this shrinkage happens because you’re losing muscle. But that’s not the case. Here’s why:
Your muscles are made of dry mass (such as protein) and wet mass (~76% water) . They also contain glycogen, a type of stored glucose. Your body uses glycogen as a source of energy.
When you're actively training and working out, your muscles store a significant amount of glycogen. According to the research, the storage capacity is a maximum of 4 grams of glycogen per 100 grams of wet muscle. Each gram of glycogen can bind with 3 grams of water.
Now imagine glycogen as a sponge soaking up water. When your glycogen stores are brimming, they're like a full sponge. This is why, during periods of regular training, your muscles look 'full' and defined.
But during detraining your muscle glycogen stores begin to shrink. And as these glycogen stores shrink, the water mass attached to the glycogen also reduces .
Hence, during the first 1-2 weeks of detraining, you may notice that your muscles look smaller. It's not because you're burning muscle, but because you’re losing “wet” mass .
Luckily, this effect is temporary and can be reversed.
When you return to your training routine, your body refills glycogen stores in your muscles. As these stores fill up, they again start retaining water.
These changes are part of the body's natural response to detraining. The changes can be quickly reversed once you resume your training.
The article continues below.
The Science of Detraining: How Long You Can Take a Break from the Gym Before You Lose Muscle Mass & Strength
How long does it take us to detrain? Which strategies we can use to prevent detraining?
You lose muscle after 3 weeks
Multiple studies show that it takes 3 weeks to lose muscle mass.
A study by Fisher and colleagues suggests that a gym break of 3 weeks does not cause muscle atrophy. It could even promote greater muscle growth (hypertrophy) upon return to training .
This benefit is echoed in another study by Ogasawara in 2013. This research explored a training program where gym-goers trained for six weeks, then detrained for three weeks. They repeated this for 24 weeks .
The results showed that muscle hypertrophy was similar to those who trained continuously. The detraining group had 25% fewer training sessions. They got the same gains spending less time in the gym.
How muscle hypertrophy (CSA) changes over the course of 24 weeks of training. The black lines trained for 6 weeks, then took a break for 3 weeks. The white line trained continuously .
Symptoms of muscle loss
There are 3 practical ways to check for muscle loss:
- Are you weaker? Muscle and strength are correlated.
- Size measurements: You can use measuring tape to check for size differences. But this is imperfect, because “muscle size” is also determined by water mass.
- Clothing size: If your gym clothes start fitting more loosely, it's a hint that you could be losing muscle. But it could also be a sign of fat loss.
Body analysis methods:
First of all, it’s hard to measure muscle loss objectively.
There are body analysis methods such as DEXA or BIA (InBody). But you should know that they are mostly used for research. They are accurate when comparing groups of people. They are not accurate at an individual level.
3 Things That Speed Up Muscle Loss
#1 - Inactivity in lifestyle
Being active, in general, is an excellent way to maintain muscle without training.
Let's say you're taking a break from the gym for the summer. Or maybe you're recovering from an illness or rehabbing after an injury.
Do you spend your time on the couch, or are you going on hikes and being active in other ways?
As I covered above, you probably won't lose muscle in the first 3 weeks. But it does depend on how active you are outside of the gym.
#2 - Limb immobilization (casting and bracing)
“We conclude that even short periods of muscle disuse can cause substantial loss of skeletal muscle mass and strength (...)”
- Wall and colleagues 
A review of 86 studies found that quadriceps muscle size went down by −5% after one week of immobilization . Immobilization for more than 7 days leads to further decreases in muscle strength and size.
The effect is the same whether you're immobilizing your limb with a brace or a cast. It's not only the size but also the strength of your muscles that takes a hit, with knee extensor strength declining by up to 36% after 14 days of single-leg immobilization.
The takeaway is that in some situations, muscle loss is inevitable.
I share advice on how to limit atrophy it in the section: Prevent Muscle Loss During a Training Break
#3 - Bed rest
Similar to limb immobilization, bed rest speeds up muscle atrophy.
After one week of bed rest, study participants lost 1.4 kg of lean mass, and 3.2% of their quadriceps muscle size .
But there is good news: it’s easy to regain lost muscle. Read on to learn why.
Figure showing muscle loss during bed rest .
You Regain Muscle Quickly When Retraining
Muscle memory - muscles remember their former glory
One reason behind this rapid growth might be the build up of myonuclei. These nuclei are part of muscle cells, which exist within our muscles. They are responsible for muscle fiber growth .
Just as the brain is the control center of the human body, a nucleus is the control center of a cell. It contains most of the cell's genetic material and regulates its activities, such as growth and reproduction.
When you train, you add more myonuclei to your muscle cells.
More myonuclei means more potential muscle growth.
|Technical details: the science of myonuclear permanence
The scientific community has an ongoing debate about the causes of muscle memory. Some studies say the myonuclei theory is correct, while others suggest alternative theories. Epigenetic memory is proposed as an alternative theory.  .
How long it takes to regain lost muscle
How long you need to retrain to return to your previous form, depends on:
- How long you’ve been on a break
- How active or inactive you were during your break
- How much protein and how many calories you ate
Let’s say you detrained for 5 weeks, and ate plenty of protein while being active on a daily basis. In this situation, you didn’t lose too much muscle mass, and you could probably regain what you lost in 2-4 weeks.
But what if you were detraining for 6 months, while being inactive and eating too little protein? In this case, it will take you longer to regain the muscle. It’s hard to give an exact number on how many months you need to retrain.
As a rough estimate, it could take anywhere from 3 to 6 months.
The speed of regain depends on:
- How hard you train
- How often you train
- Calorie intake
- Protein intake
Prevent Muscle Loss During a Training Break
Eat a high protein diet
Protein plays a key role in maintaining and building muscle mass. When you're on a training break, you may eat less. This could result in a lack of protein.
A study by Wall and colleagues suggests that protein can mitigate the effects of muscle atrophy from detraining .
Aim for 1.6 to 2.2 grams of protein per kilogram of bodyweight.
Get enough calories
Calories are anabolic. You generally have to eat more calories to build muscle.
When you don't eat enough calories, your body uses muscle tissue for energy. This can speed up muscle loss, especially during a training break when your muscles aren't being actively stimulated.
How many calories to eat
A good rule of thumb is to consume your maintenance calories. These are the number of calories you'd need to maintain your current weight.
The graph below shows how lean mass correlates with calories. Every dot and triangle represents one study. The further you move to the right on the graph, the fewer calories the participants were eating .
Takeaway: Eat enough calories during detraining.
Do strength training once a week
Instead of taking a complete break from training, you could consider a deload. In other words, train less.
If you’re on a vacation, you can do bodyweight exercises such as push-ups and pull-ups. Just a little bit of training goes a long way in maintaining muscle.
Be active in other ways
Everyday activities help maintain muscle mass during periods of reduced training. Go on walks, hikes, or even light cardio. This applies whether you’re on a vacation or simply taking a deload.
You could also try neuromuscular electrical stimulation (NMES). This technique uses electrical impulses to stimulate muscle contraction. Dirks and colleagues say it can prevent muscle loss during immobilization .
Frequently Asked Questions
Does muscle turn into fat when someone stops working out?
No. Muscle and fat are gained and lost by different mechanisms. Muscle cells are made up of protein and amino acids. Fat cells are made up of triglycerides (lipids).
But, if you stop exercising, you will start losing muscle after a while. And, you will probably gain fat.
You gain muscle if you do strength training while eating enough protein and calories.
You lose body fat by eating fewer calories than you burn. Fat is also lost through exercise.
Any of these combinations are possible:
- Losing muscle and losing fat
- Losing muscle and gaining fat
- Gaining muscle and losing fat
- Gaining muscle and gaining fat
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