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Blog by Scientist Cas Fuchs (Researcher at the Maastricht University, Muscle Physiology, Sports Nutrition, Recovery Strategies) What is the effect of inactivity on our muscle mass and how can we avoid losing it?

It is well-known that our muscles have a high level of plasticity, which means that they respond very well to activity but also inactivity. In other words, our muscles will gain in size if we train, whereas they will lose size if we don’t use them. Hence the common term: Use it or Lose it!

Especially during these times of the Corona virus this becomes all of a sudden very relevant:  gyms are closed and in some countries you cannot even leave the house/apartment, which in many cases will lead to inevitable muscle loss. So how much muscle do we actually lose and are there ways how we can prevent this? Well, that’s exactly what I will explain in this blog post.

So first of all, there are broadly 3 ways how we can measure inactivity and the impact on our muscles.

1.     Step reduction (simply being inactive; couch potato)

2.     Immobilization (you broke a limb and are casted)

3.     Bed rest (you are ill or severely injured)

Especially step reduction and bed rest are of major importance in these periods as (unfortunately) many people will be subjected to either of them. People that can’t leave their (small) apartment are subjected to a huge reduction in their daily activities and will be “forced” to become much more inactive. Even worse, if you become ill and are subjected to bed rest (hopefully at home, but perhaps even in the hospital at the ICU).

So, let me first briefly explain you a bit about muscle physiology before we take a look about what we know from the science on strongly reducing our inactivity level and bed rest.

Muscle protein turnover

Our muscles are continuously turning over (so even when you are reading this right now), this means that we constantly are building our muscles but also breaking them down. Sounds confusing? Well… what happens is that we constantly have to make new (functional) proteins in our muscles because some proteins may have been damaged and need replacement. We call this process Muscle protein synthesis. The process of breaking down (dysfunctional or damaged) proteins is what we call Muscle protein breakdown. Whether you like it or not this is happening the entire time (and trust me, it’s a good thing!).

Now, there are 2 main common anabolic strategies that influence these processes. Those are (obviously) resistance exercise and protein (or amino acid) intake. But what specific effects do they then exactly have?

Exercise influences the processes of muscle protein synthesis and breakdown, by increasing both of them after exercise. So, you are increasing muscle protein synthesis to build new (functional) proteins, but also breaking down damaged proteins that your muscles want to get rid of. Interestingly, the effect of exercise on muscle protein synthesis is greater than the effect on breakdown. So, in other words, you become a bit more ‘anabolic’. If we happen to be fasted during exercise, we would still lose tiny bits of muscle as in total your breakdown would still override your synthesis (See Figure 1 to make this easier to understand). So, what we now need is….

Protein… yes, because if we would then also eat something which contains proteins we can further stimulate muscle protein synthesis and also in fact reduce muscle protein breakdown. So together they (exercise and protein) form an essential duo to be able to gain muscle mass in the long-term (as you make more proteins than you break down in your muscles). Probably makes sense to all of you. 

Fair to say here that performing exercise also makes your muscles more sensitive to the “anabolic” properties of (protein-derived) amino acids (Note: all proteins are made up of amino acids, same as your house is made up of building bricks). So, performing exercise will allow our muscles to make much better use of the proteins that you ingest!

Figure 1. Conceptual idea of muscle protein synthesis, breakdown and net balance. If you are in rest and fasted (for example after waking up in the morning), your muscle protein breakdown will be higher than muscle protein synthesis. So, in that situation you will lose some muscle (negative net balance) See top figure. If you are then performing exercise, you will increase both muscle protein synthesis but also breakdown. But because you will increase muscle protein synthesis more than breakdown, you become a bit more “anabolic”. So net balance is still negative, but less negative than in rest See middle figure. If you would then eat protein or amino acids, you can further stimulate muscle protein synthesis, lower muscle protein breakdown and thereby having a positive net balance. So, then you are effectively gaining muscle See lower figure.

Now we have this bit of background (and know that exercise is good for you, duuhh), let’s get back to the main point of this whole blog post… what happens when we are inactive?

Recap: we are mostly interested now in step reduction and bed rest as this is what currently many people, maybe you as well (I hope not!), are facing…

Step reduction

In science we often use step reduction models where we ask participants to lower their daily activity to about ~1000-1500 steps. This may seem quite drastic, but is exactly what can happen when someone is not active at all. Think about it, you wake up and first decide to walk to the fridge to get some milk and the rest of your breakfast, then you sit down on the couch or on a chair to either chill or start working… in the mean time you get up a couple of times to go to the toilet, often to walk to the kitchen (out of complete boredom) and start snacking something you feel bad about quite soon after… and then sit down again etc… you get my point... you are very inactive.

This inactivity has several negative health outcomes. So, what happens is that quite soon after you’re inactive, your muscle protein synthesis will be lowered substantially. So, that anabolic process that drives muscle growth is reduced. This is caused both by the fact that your muscle protein synthesis is lowered in the fasting state, but your muscles now also appear to respond less to the anabolic effects of protein (or amino acid) ingestion. Within a week of inactivity you can already see an overall decline in this muscle protein synthesis of 27% (1)! Mainly via this mechanism you will then also lose substantial amounts of muscle mass and the longer you’re inactive the more (absolute) you will obviously lose. Of course, you will also lose muscle strength rapidly as you are simply inactive.

Another important aspect to consider is that with inactivity you are becoming more insulin resistant. This means that your body has to produce more insulin to get the glucose from your blood stream into the muscles. Apparently, this can already occur within 1 day of simply being inactive (2)! So, the longer you are inactive the higher the risk of developing a metabolic disease such as Type II diabetes (of course it will take longer than 1 day to really be at that stage, but it shows you the importance).

So, I think this is already enough information why you should stay active (apart from other reasons that we won’t discuss now, such as a reduction in your physical condition etc.). Maybe a final note here, these effects are occurring in both young and older people, but where young people seem to eventually return to normal levels (but it still can take long to get back to your old level!), old people may not fully recover from these periods of inactivity, bringing them at increased risks for developing metabolic diseases, falls, fractures, and even mortality. So older people should most definitely also pay attention here!

Bed rest

For those people that are bed ridden it may even be worse. In that case you may not move your limbs and body at all. And indeed, after only a few days of bed rest you can already see severe (negative) changes. To bring this in context, after 1 week of bed rest you can already lose 1.4 kg of lean body mass (3), which equals about 7 beef steaks. So, in other words, in 1 week of bed rest you have stripped 7 pieces of beef away from your body. That’s pretty serious. To make it even worse, it will take you much longer to (re)gain that amount of muscle, probably around 12 weeks of intense resistance exercise (4, 5).

Also, with bed rest you see dramatic changes in anabolic resistance (remember: a reduced increase in muscle protein synthesis after protein intake), insulin resistance etc. So clearly inactivity has an enormous impact on our bodies. But now the main question, do we simply have to accept this? Or can we do something about it?


So, when we think about strategies to counteract muscle loss we are of course thinking of anabolic stimuli that we can apply. So as already discussed, protein (or amino acids) as well as exercise (in any way possible) come to mind. Also, we may consider certain exercise mimetics, so strategies that can cause involuntary muscle contractions. And finally, we can of course also wonder ourselves whether anabolic steroids would work. So what works? And what doesn’t…


Can we do something with our nutrition or supplementation to counteract muscle loss during inactivity?

I can be pretty short here, not much unfortunately. We know that protein, amino acids, creatine, fish oil etc. all may have some benefits around training periods to stimulate muscle protein synthesis or providing some extra energy to make some additional reps possible. However, the science behind supplementation during inactivity is less promising. In short, it seems that if you ingest ~1.2 g/kg/d of protein there is not much benefit of further increasing this with protein (6) or leucine (which is the most anabolic amino acid) (7) during inactivity. Also, despite a few studies showing some benefits of leucine or its metabolite HMB supplementation during bed rest (8, 9), the evidence is not compelling enough in my opinion to advice those supplements in case you ingest already sufficient amounts of protein (³1.2 g/kg/d). Also, creatine has not been shown very effective during limb immobilization (10), but can be effective during rehabilitation following immobilization (11). There are currently promising suggestions that fish oil (also known as omega-3) may offer some benefits (12), but this needs confirmation in further studies.

So, what is the best nutrition advice for now?

Eat sufficient protein (aim for minimally 1.2 g/kg/d), which I honestly think most (if not all) of you already do at the moment, and keep in energy balance. Don’t overeat (which happens easily in these times; as it makes you fat which can have a negative influence on your muscles), but also don’t under eat as both will be detrimental for your health and may both in fact lead to increased muscle loss (13, 14).

So now we have this advice, what else can work?

Anabolic steroids

Anabolic steroids are of course well known to stimulate muscle growth, especially when combined with exercise. It is, for example, known that when combined with strength training, 600 mg/week of testosterone enanthate can increase lean body mass by 6 kg in just 10 weeks (15). So yes, clearly anabolic steroids are effective when being active. But what if we are inactive? Can we then apply anabolic steroids to counteract the losses in muscle mass? Well, unfortunately this does not seem to be the case (if we are completely inactive). At least not with injecting nandrolone decanoate (200 mg) during 1 week of immobilization (16). Please keep in mind here that my goal is not to try and advise the use of anabolic steroids in general here, I just want to show you what we know about it from a scientific (and pure informational) point of view.

So, given that both nutrition and anabolic steroids don’t seem to be very effective during inactivity (but are when we are active!), it seems that those muscle contractions are having a massive effect on muscle health. So, what can we do with exercise or exercise mimetics (to also stimulate those muscle contractions)?

Exercise (mimetics)

There is good evidence now that for those people that really cannot do any kind of exercise during inactivity (think about those that are very ill and are required to rest in bed), that an exercise mimetic known as neuromuscular electrical stimulation (NMES) works (6). This is a small device that can provide some electrical impulses to your muscles (via patches placed on the skin on top of the muscles), via those electrical impulses it can cause your muscles to contract without you even “knowing” it. So, it is completely involuntary. Again, this would be an effective strategy to reduce some loss in muscle that would otherwise occur. However, keep in mind that this is not something you should do yourself at home (but it can be applied with an appropriate protocol by trained people, such as nurses, in the hospital for example).

And for those that are able to do some (home-based) exercises?

Keep active! Make sure to get your daily steps (³8000 steps per day, so preferably a bit more) and do some home exercises such as push-ups, body squats, and, if possible, go out to a park and find a bar to do some pull ups and other exercises. If you make sure to go to fatigue with those exercises then you are likely to maintain your muscle mass (yes not joking here!), you may still lose some strength, but you can definitely avoid losing a lot, so when back at the gym you already start from a better position than if you did not perform any exercises at all. So, keep moving and keep an exercise routine! And remember always: Use it or lose it!

Practical advice (for those that are not hospitalized):

-       Stay as active as possible (at least 8000 steps per day and preferably a bit more).

-       Keep doing home-based exercises (push-ups, body weight squats, lift a bag of water bottles etc.) and try to go to fatigue with the exercises you do (note: this is for people that are experienced lifters, if you are unexperienced try first to incorporate some exercise routine before going to the max and fatigue).

-       Eat sufficient amounts of protein ³1.2 g/kg/d, which can be more of course if you are still training a lot (guidelines go for an average of 1.6 g/kg/d, with an upper limit of approximately 2.2 g/kg/d for serious lifters).

 -       Try to stay in energy balance, don’t over eat but also don’t under eat.

-       Stay safe and avoid close contact, so you keep healthy and, given the restraints, can still live an active and healthy lifestyle.

Some provided references:

1.               Shad BJ, Thompson JL, Holwerda AM, Stocks B, Elhassan YS, Philp A, et al. One Week of Step Reduction Lowers Myofibrillar Protein Synthesis Rates in Young Men. Med Sci Sports Exerc. 2019;51(10):2125-34.

2.               Stephens BR, Granados K, Zderic TW, Hamilton MT, Braun B. Effects of 1 day of inactivity on insulin action in healthy men and women: interaction with energy intake. Metabolism. 2011;60(7):941-9.

3.               Dirks ML, Wall BT, van de Valk B, Holloway TM, Holloway GP, Chabowski A, et al. One Week of Bed Rest Leads to Substantial Muscle Atrophy and Induces Whole-Body Insulin Resistance in the Absence of Skeletal Muscle Lipid Accumulation. Diabetes. 2016;65(10):2862-75.

4.               Holwerda AM, Overkamp M, Paulussen KJM, Smeets JSJ, van Kranenburg J, Backx EMP, et al. Protein Supplementation after Exercise and before Sleep Does Not Further Augment Muscle Mass and Strength Gains during Resistance Exercise Training in Active Older Men. J Nutr. 2018;148(11):1723-32.

5.               Snijders T, Res PT, Smeets JS, van Vliet S, van Kranenburg J, Maase K, et al. Protein Ingestion before Sleep Increases Muscle Mass and Strength Gains during Prolonged Resistance-Type Exercise Training in Healthy Young Men. J Nutr. 2015;145(6):1178-84.

6.               Dirks ML, Wall BT, van Loon LJC. Interventional strategies to combat muscle disuse atrophy in humans: focus on neuromuscular electrical stimulation and dietary protein. J Appl Physiol (1985). 2018;125(3):850-61.

7.               Backx EMP, Horstman AMH, Marzuca-Nassr GN, van Kranenburg J, Smeets JS, Fuchs CJ, et al. Leucine Supplementation Does Not Attenuate Skeletal Muscle Loss during Leg Immobilization in Healthy, Young Men. Nutrients. 2018;10(5).

8.               Deutz NE, Pereira SL, Hays NP, Oliver JS, Edens NK, Evans CM, et al. Effect of beta-hydroxy-beta-methylbutyrate (HMB) on lean body mass during 10 days of bed rest in older adults. Clin Nutr. 2013;32(5):704-12.

9.               English KL, Mettler JA, Ellison JB, Mamerow MM, Arentson-Lantz E, Pattarini JM, et al. Leucine partially protects muscle mass and function during bed rest in middle-aged adults. Am J Clin Nutr. 2016;103(2):465-73.

10.             Backx EMP, Hangelbroek R, Snijders T, Verscheijden ML, Verdijk LB, de Groot L, et al. Creatine Loading Does Not Preserve Muscle Mass or Strength During Leg Immobilization in Healthy, Young Males: A Randomized Controlled Trial. Sports Med. 2017;47(8):1661-71.

11.             Hespel P, Op't Eijnde B, Van Leemputte M, Urso B, Greenhaff PL, Labarque V, et al. Oral creatine supplementation facilitates the rehabilitation of disuse atrophy and alters the expression of muscle myogenic factors in humans. J Physiol. 2001;536(Pt 2):625-33.

12.             McGlory C, Gorissen SHM, Kamal M, Bahniwal R, Hector AJ, Baker SK, et al. Omega-3 fatty acid supplementation attenuates skeletal muscle disuse atrophy during two weeks of unilateral leg immobilization in healthy young women. FASEB J. 2019;33(3):4586-97.

13.             Biolo G, Agostini F, Simunic B, Sturma M, Torelli L, Preiser JC, et al. Positive energy balance is associated with accelerated muscle atrophy and increased erythrocyte glutathione turnover during 5 wk of bed rest. Am J Clin Nutr. 2008;88(4):950-8.

14.             Biolo G, Ciocchi B, Stulle M, Bosutti A, Barazzoni R, Zanetti M, et al. Calorie restriction accelerates the catabolism of lean body mass during 2 wk of bed rest. Am J Clin Nutr. 2007;86(2):366-72.

15.             Bhasin S, Storer TW, Berman N, Callegari C, Clevenger B, Phillips J, et al. The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men. N Engl J Med. 1996;335(1):1-7.

16.             Horstman AMH, Backx EMP, Smeets JSJ, Marzuca-Nassr GN, van Kranenburg J, de Boer D, et al. Nandrolone decanoate administration does not attenuate muscle atrophy during a short period of disuse. PLoS One. 2019;14(1):e0210823.
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