Out of Shape: Why We Lose Our Fitness

Originally posted on LIFEApps

Why do we lose our fitness when we stop exercising?

It’s a deceptively simple question. The explanation is derived from a time where energy, in the form of food, was the most precious resource around. Throughout our evolutionary history, our bodies developed a way to build and maintain fitness when physical exertion was required to hunt food. But on the flip side, our bodies developed a way to shift our metabolic state when food supply was plentiful.

The system may have worked well in the past. But in modern times, the over-availability of energy-dense food and the reduced imperative to exercise has thrown this system into disarray.

Keep reading to learn more about the evolutionary perspective on fitness and why we lose it so damn fast!

The easiest way to maintain your ability to do a pushup? Training pushups. Photo by Annie Spratt on Unsplash.
The easiest way to maintain your ability to do a pushup? Training pushups. Photo by Annie Spratt on Unsplash.

How Trees Exercise

Biosphere 2, a research facility located in Arizona, is a fully enclosed, multi-acre artificial ecosystem built to “explore the complex web of interactions within life systems” [1]. At the start of the first research mission within Biosphere 2 in the 1990s, trees were provided an exceptional growth environment with optimal light, nutrients and rainfall. But the trees kept falling over before they fully matured.

The missing ingredient? Wind; an environmental stressor that triggers the tree to produce a load-bearing type of wood. The lesson here is that trees need to be stressed* with wind to fortify the strength of their trunks as they mature.

The same concept applies to us, in the case of exercise. Like trees that need wind to grow strong and resilient, humans need exercise (a “good” stress or “eustress”) to strengthen our heart, muscles and mind. However, unlike the strength that trees gain once wood is deposited in the trunk, which lasts a lifetime, we lose the benefits that we gain from exercise at an alarming rate.

Two young adult women weight lifting outdoor with a barbell. When is the last time you pumped some iron? Don't be afraid - just start.
When is the last time you pumped some iron? Don’t be afraid – just start.

*Incidentally, other agricultural issues during this mission resulted in a food shortage that left the crew members in another state of stress – caloric restriction – that reduced their body weight, cholesterol levels, blood pressure and insulin levels in a similar fashion as intermittent fasting!

Evolution of Fitness

We’ve all heard or experienced the maxim for fitness that what is gained in months is lost in weeks. How cruel the world can be that our body requires consistent physical stress to maintain health, yet the benefits are reversed faster than they are gained!

Why is this the case? Surely for something so intrinsic to our survival and reproduction over thousands of generations, we would have developed the ability to maintain our fitness over our lifetime – much like the cumulative strength of a tree as it matures. To appreciate why it is difficult for us to maintain fitness, we will first look at what changes occur when we exercise and how these functions diminish back to baseline without regular exercise. Then we can dive into why and how we need to keep up physical activity to maintain our fitness.

So what are some of the physiological changes that occur in our body when we start exercising for an extended period of time?

Your Body on Exercise

Exercise has an illustrious impact on our health and wellbeing. It is even recommended as a complementary therapy to improve cancer treatmentoutcomes. As a general overview, you can expect the following changes to occur as you become more physically fit:

  • Your blood volume increases – you literally have more blood pumping through your heart! You have more red blood cells, meaning a greater ability to carry oxygen around in your body.
  • Your heart muscle mass increases.
  • You develop a higher density of capillaries (blood transport vessels) in your lungs and muscles.
  • You develop a higher number of alveoli (for oxygen transfer into blood) in your lungs.
  • In your muscle cells, the number of mitochondria – the ultimate energy producers of your body – increases.
  • Your metabolic rate increases  – you burn more energy at rest!
  • More efficient control of the buildup of lactate (the compound that makes you sore after a workout) in your muscles.

These improvements manifest themselves in many different measures of fitness, such as a lower resting heart rate, faster heart rate recovery and higher VO2 Max.

Your VO2 Max, also known as maximal oxygen uptake, is the maximum amount of oxygen your body can use during intense exercise.

Tough Gain, Quick Wane

Unfortunately, when the stress of physical activity is lifted, our body adapts in an opposite manner to match the new workload (or lack thereof) that we are putting it under. In as little as five days after our last bout of exercise, there are measurable changes in the functioning of our body, starting with a drop in our cardiovascular performance.

The following timeline describes our diminishing fitness after sudden inactivity:

Day 5 – An 8% drop in VO2 Max [2], the maximum amount of oxygen our tissues can use during intense exercise (see Measures of Fitness for a description of VO2 Max).

Day 14 – The amount of small blood vessels or capillaries feeding your muscles drops by 6% [3].

Day 21 – Your blood volume drops by 9% and your plasma volume drops by 12% [4].

Day 28 – By this time, you can expect a 14% drop in your original VO2 Max [5]. Studies have also shown that by this time there is a 64% decline in the activity of oxidative enzymes that produce energy in our muscles [6], a 40% decrease in muscular stored sugar (glycogen) stocks [7] and a decrease in insulin sensitivity [8].

Day 40 – Your cardiovascular performance can drop to as low as 20% of your original VO2 Max [9].

What accounts for this drop in cardiovascular fitness? At the muscular level, small blood vessels called capillaries transfer oxygen and nutrients to our muscles and remove metabolic by-products such as lactate. Without exercise stimuli, our body neglects the maintenance of these capillaries that feed and flush out our muscles.

In addition, dwindling muscle glycogen stocks (the starting material for energy production) and muscle proteins (oxidative enzymes that convert glycogen to energy) during inactivity mean that our muscles don’t perform like they used to.

All of these factors – fewer capillaries, sugar storage and energy-producing enzymes – are intimately linked to efficient energy production within our muscles during exercise. A drop in this efficiency limits our ability to maintain intense exercise for a given amount of time. You might have been able to climb a mountain a month ago, but with physical inactivity, you could struggle significantly a second time around.

Your body is great at adapting to the physical stress you put it under - but also great at reverting out of physical fitness when the demands disappear for even a short time. Photo credit: Phil Gates-Idem, LifeOmic. Rockclimbing.
Your body is great at adapting to the physical stress you put it under – but also great at reverting out of physical fitness when the demands disappear for even a short time. Photo credit: Phil Gates-Idem, LifeOmic.

But Why Don’t We Maintain Fitness?

Unfortunately, evolution is not an omnipotent force. Its power is context dependent – it depends on our environment.

If food was in short supply, your body wouldn’t waste its precious resources on physical fitness unless it were the key to unlocking more food. In the case of our ancestors, food was almost always a scarce resource. Our ancestors’ physiological state was limited by this lack of food supply. Increasing their muscle mass, blood volume and metabolic rate (all measures of fitness) would have been energetically costly. Fitness was only useful or adaptive if it allowed our ancestors to unlock a larger hunting area and the ability to secure new, more nutritious food sources.

For our ancestors, this was a complex resource allocation problem. In other words, their bodies found a way to only increase their muscular fitness, for example, for a short while when needed. Much like for the evolution of aging, where energy resources needed to be effectively split between survival and reproduction, our ancestors had to find the optimal balance between maintenance of fitness and energy conservation, while also saving resources for all-important reproduction.

Because calorific energy was limited, our ancestors benefitted from avoiding unnecessary exertion. (Why run around on off days when you could be a couch potato?!) Most of our body’s systems evolved with the requirement for physical stress (in other words, exercise) to stimulate growth and biochemical activity.

Although evolutionary history specially adapted humans to be endurance athletes, we are just as adapted to be physically inactive. – Daniel E. Lieberman, PhD

What does this all mean? Our bodies are adapted to respond positively to physical exertion, but only just enough to satisfy the requirements of the physical activity we undertake. In the absence of exertion, we economically remobilize this energy towards survival and potential reproduction. In times of plentiful food supply (which usually, at least according to our evolutionary history, comes with a reduction in physical activity), our body prepares itself for the possibility of future famine by depositing fat, which is cheap to store and energetically dense, not muscle.

How to Maintain Your Fitness

Now consider the present day. For the majority of us, it is eternally characterized as an environment of plentiful food supply. In this case, the only way to divert your body away from its natural response of fat storage is to engage in consistent physical activity (along with healthy eating).

Give your body a reason to believe that it needs to become stronger and it will do just that!

The key to developing a stronger body (and mind) is consistency. Now that we know that our fitness starts diminishing within just five days of inactivity, we can use this knowledge to ensure that we exercise at least once within this timeframe. If we go any longer without activity, we will be struggling against the inertia of inactivity both in terms of our body and mind!

It certainly can be difficult to maintain consistent exercise, especially among a busy work, family and social life schedule. What can you do to keep yourself consistently active?

1. Make it a routine. Whether it’s waking up earlier Monday and Wednesday mornings for a swimming session, or running Thursday and Sunday evenings, set an exercise routine in stone and commit to it. It becomes much easier to follow through when your workout is something you regularly do on that day (try putting it on your e-mail or phone calendar!) as opposed to a choice you have to make each time.

2. Track your progress over time. Improvement is a potent motivator for fitness! Seeing results, whether it’s in the form of weight loss, strength or endurance, can really help you find the energy to work out when you really don’t feel like it (which is something that we all experience). Try tracking your progress in your favorite fitness app such as LifeOmic’s LIFE Extend app!

3. Make exercise social. Exercise can actually be a fun bonding experience if it’s done with friends. Seemingly insurmountable goals become a challenge to overcome together, and motivation and enthusiasm can be more even infectious than influenza. It’s also easier to stay committed to a schedule when you don’t want to let someone else down besides yourself.

So now, armed with the knowledge of the effect that exercise has on your body, the evolutionary mechanisms behind why our fitness diminishes so fast, and some tips on how to maintain a consistent exercise routine, go be like a tree and grow strong! For the rest of your LIFE.



  1. Allen JP. Me and the Biospheres: A Memoir by the Inventor of Biosphere 2. Synergetic Press; 2009.
  2. Olivier N, Rogez J, Berthoin S, Weissland T. Effet du déconditionnement suite à une chirurgie du genou sur l’aptitude aérobie. Sci Sport. 2005;20(5–6):308–10.
  3. Houston ME, Bentzen H, Larsen H. Interrelationships between skeletal muscle adaptations and performance as studied by detraining and retraining. Acta Physiol Scand [Internet]. 1979 Feb 1;105(2):163–70. Available from: https://doi.org/10.1111/j.1748-1716.1979.tb06328.x
  4. Coyle EF, Hemmert MK, Coggan AR. Effects of detraining on cardiovascular responses to exercise: role of blood volume. J Appl Physiol [Internet]. 1986 Jan 1;60(1):95–9. Available from: https://doi.org/10.1152/jappl.1986.60.1.95
  5. Ghosh AK, Paliwal R, Sam MJ, Ahuja A. Effect of 4 weeks detraining on aerobic & anaerobic capacity of basketball players & their restoration. Indian J Med Res [Internet]. 1987 Oct;86:522—527. Available from: http://europepmc.org/abstract/MED/3443487
  6. Chi MM, Hintz CS, Coyle EF, Martin 3rd WH, Ivy JL, Nemeth PM, et al. Effects of detraining on enzymes of energy metabolism in individual human muscle fibers. Am J Physiol Physiol. 1983;244(3):C276–87.
  7. Costill DL. Metabolic characteristics of skeletal muscle during detraining from competitive swimmers. Med Sci Sport Exerc. 1985;339–43.Mujika I, Padilla S. Detraining: Loss of Training-Induced Physiological and Performance Adaptations. Part I. Sport Med [Internet]. 2000;30(2):79–87. Available from: https://doi.org/10.2165/00007256-200030020-00002
  8. Mujika I, Padilla S. Detraining: Loss of Training-Induced Physiological and Performance Adaptations. Part I. Sport Med [Internet]. 2000;30(2):79–87. Available from: https://doi.org/10.2165/00007256-200030020-00002
  9. Martin WH, Coyle EF, Bloomfield SA, Ehsani AA. Effects of physical deconditioning after Intense endurance training on left ventricular dimensions and stroke volume. J Am Coll Cardiol [Internet]. 1986 May 1;7(5):982 LP – 989. Available from: http://www.onlinejacc.org/content/7/5/982.abstract

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