Muscle Atrophy: Why And How Fast Do You Lose Muscle

Some Background

Taking time off feels terrible. Just as you’re getting into the swing of things, you get injured, your work schedule gets busy, and life takes over.

However, inactivity can be voluntary. Feeling counterproductive and guilty, it’s easy to forget that rest is crucial to the fitness process. After all, growth happens during recovery.

But how much time off is too much? Whether resting or injured, understanding inactivity and its effects on the progress you worked so hard for leads to better fitness decisions.

The Relevant Factors

Just as with other aspects of fitness and nutrition, everyone reacts differently. Hence, numerous factors play a role when considering how fast your precious progress disappears.

Prior fitness level, diet, age, gender, and length of time off are just a few variables determining how fast muscular atrophy sets in.


How long and how inactive you are during your time away from exercise is a huge factor. A 2015 University of Copenhagen study1 found that it only takes two weeks of high leg inactivity to “lose a third of the muscular strength.”

In the study, the researcher fully immobilized one leg through a leg cast.

To put the findings in perspective, after two weeks in a leg cast, a young person loses enough leg muscle to “leave them on par with a person who is 40-50 years their senior.”

The researchers also found that subjects with higher mass stand to lose more than those with lower amounts. So “if you’re fit and become injured, you’ll most likely lose more muscle mass than someone who is unfit, over the same period of time.”

Five Days Too Many

When considering how long you can stay away from exercise, two weeks is relatively long. That’s plenty of time for recovery, work, or vacation.

Unfortunately, a 2014 University of Netherlands study2 shortens that period. After noting that most studies investigated periods of disuse around 2 weeks (as above), the researchers looked at shorter times.

Specifically, the study focused on the “impact of 5 days of disuse on skeletal muscle mass, strength, and associated intramuscular molecular signaling responses.” Similar to the University of Copenhagen study, subjects couldn’t use one of their legs for the duration of the experiment.

Alarmingly, the scientists concluded that even 5 days of total immobilization, “can cause substantial loss of skeletal muscle mass and strength and are accompanied by an early catabolic molecular signaling response.”

The Difference

The aforementioned research sheds a light on the detraining process. It gives a better idea as to how long we can rest and recover. However, the test subjects experienced total immobilization!

Both studies used a leg cast, which is the equivalent of being bedridden. While illuminating, the method is not reflective of how most people spend their time away from exercise.

Unlike the test subjects, you’re not likely to be totally immobile on your vacation or time off. Hence, your activity level and results will be different.

Finally, even though both studies concentrated on leg muscle loss specifically, different muscle groups experience atrophy and strength loss at different rates.

Detraining and Diet 

A 2016 study3 focusing on the effects of leucine – a branched-chain amino acid (BCAA) which assists muscular health – has shed light on its effect of diet during periods of detraining.

While highlighting the effects of physical inactivity, the researchers concluded that “leucine supplementation may partially protect muscle health during relatively brief periods of physical inactivity.”

Hence, there is some proof that a proper diet slows down the rate of muscular atrophy during physical inactivity.

A University of Texas study4  mirrored the findings. Noting the short-term effects (“acceleration of muscle loss, function, and glucose tolerance”), the researchers found that “optimizing nutritional practices and protein intake may reduce the consequences of disuse by preserving metabolic homeostasis and muscle mass and function.”

Specifically, since “mammalian target of rapamycin complex 1 (mTORC1) signaling and muscle protein synthesis are negatively affected by disuse,” the study suggests that leucine supplementation may partially protect skeletal muscle during physical disuse.

The researchers stressed however that “simply encouraging increased protein and/or energy consumption is a well-intentioned, but often impractical strategy to protect muscle health.”

Getting Specific

Rest is well-intentioned, and injury can be inescapable. However, time off doesn’t have to mean guaranteed losses. Specifically, as the aforementioned studies suggest, retention is possible with the right diet.

If you’re looking for dietary guidelines and specifics, an older study may pave the way.

The 2004 study5 examined whether “essential amino acid and carbohydrate supplementation could offset the catabolic response to prolonged inactivity.” The findings were encouraging.

A group of bedridden subjects consumed 18.5 grams of essential amino acids and 30 grams of carbohydrates three times a day over 28 days. At the study’s conclusion, the researchers noted that compared to the control group, the former maintained lean leg muscle mass, and that “strength loss was more pronounced” in the non-essential amino acid consuming subjects.

Hence, “essential amino acid and carbohydrate supplementation may represent a viable intervention for individuals or prolonged bedrest.”

Muscle Atrophy and Sarcopenia

Common to the aging process, sarcopenia naturally increases muscle loss in older people. The process effectuates loss of skeletal muscle mass at 0.5–1% per year and sets in around the age of 30. That figure is much higher for physically inactive people, coming in around 3-5%.6

A 2013 study7 found that short term detraining “is of particular relevance in the development of sarcopenia as it has been suggested that successive short periods of muscle disuse accumulate throughout an individual’s lifespan and contribute considerably to the net muscle loss observed with aging.”

Hence, detraining in older age, combined with sarcopenia, may cause quicker or greater muscle loss.

Cardiovascular Effect

Inactivity doesn’t affect only muscle mass. Although not visible, detraining also impacts internal organs. Often, diminished cardiovascular fitness accompanies losses in strength and size.

Exercise – weightlifting in particular – contracts muscle. In turn, the blood vessels within your contracted muscle become constricted. As a response to increased exercise, your heart pumps more blood through the constricted blood vessels to facilitate more exercise.

Consequently, one of the heart’s chambers – the left ventricle – becomes thicker and stronger as you continue your exercise routine.

Detraining reverses this process. Specifically, periods of inactivity adversely affect your cardiovascular health by reversing the aforementioned process. As the demand decreases, so does the need for a thicker and stronger left ventricle.


Even though it feels counterproductive, planning a week off is crucial for proper recovery. A lengthy injury is a different matter altogether, so preventive rest is imperative.

As the studies indicate, as little as 5 days away is all it takes for atrophy to set in. However, not allowing proper recovery may lay you off for much longer periods. When it comes to time off and muscle loss, an ounce of prevention really is worth a pound of cure.



1 Comment

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