(Photo by Kevin Frayer/Getty Images)
In order to really get a good idea of what happens to your body as it gets out of shape, it helps to have an understanding of how it gets into shape.
Now, for all the different types of fitness we'll look at, change occurs depending on the amount of time from your last workout. Immediately after working out, your body will go into a growth mode. It'll first seek to replace all the energy you used during the exercise. Then it will rebuild the muscles (your heart and blood vessels are muscles too) and adapt them to better service the activity you put them through. Depending on the activity, you may have to wait as many as threedays before working out again or you'll risk structural damage.
After you're completely recovered, that's when things can go bad. First, I'll cover what happens when you get into shape. Then I'll look at what happens when you stop working.
When you exercise, your body adapts in a number of different ways to help cope with the stresses you place on it. When you stop working out, these adaptations are scaled back at different rates depending on the adaptation.
When you do cardiovascular exercise, you work your muscles a little bit. Initially, you'll see some development, but those gains plateau pretty quickly.The main thing that improves is your body's energy systems. The harder you push yourself, the more your anaerobic systems improve, the longer you run, the more you work your aerobic systems .
Aerobic respiration is very efficient in creating energy in terms of energy per fuel, but very slow and dependent on oxygen, which isn't simple to get into your system. Anaerobic energy is used when your cells don't have access to much oxygen, but actually uses more energy than it creates, net. When you're using energy in bursts where the load is greater than 20% of your 1 rep max, blood flow will be temporarily cut off to the muscles preventing them from getting oxygen. This happens during the concentric phase of the movement. It can happen during fast runs, every time your foot lands, during intense exercise, or weightlifting in general.
In terms of aerobic capability, the major thing that your body adapts for is gas exchange with your cells:
- More alveoli in the lungs (more pockets for the oxygen to enter the blood through, and more pockets to transfer out CO2).
- More capillaries in the lungs (bigger pipes for your blood to connect with the alveoli.
- More capillaries in your muscles (more places for your blood to drop off oxygen AND clean out CO2).
- More blood volume.
- More red blood cells (to carry more oxygen and clean out the CO2) -- this is also one way that endurance athletes "blood dope", they add red blood cells to make their body's more oxygen efficient.
- Stronger heart (the heart is able to pump more volume in fewer pumps).
- More efficiency at the point of exchange due to improved gradient between blood and tissues.
- Higher mitochodrial content in muscle fibers necessary for the muscles to process the added energy demands.
Anaerobic energy piggybacks on a lot of those points. It doesn't specifically need gas exchange as much, but it does need to be able to more efficiently turn pyruvate and hydrogen ions into lactate for recycling via the Cori cycle . A note on the hydrogen ions, you know the burning sensation in your muscles when you're working hard, particularly when you're doing endurance resistance training? That's the hydrogen ions building up and interfering with signals from your nervous system telling it to work.
- Anaerobic fitness increases capacity of oxygen and carbon dioxide capabilities, while aerobic fitness improves the efficiency of the exchange.
- Anaerobic fitness increases your ability to get the lactate out and buffer the hydrogen ion allowing you to do more work without the burn.
How fast do you lose it? Why do you lose it?
If you're out of shape to begin with, just getting into shape and you stop, most of these adaptations aren't in place yet, so you'll go back to ground zero relatively quickly. If you are in great shape and you stop suddenly, different things happen.
In terms of fitness, the first thing to go is your cardiovascular maximums and endurance. You'll lose your VO2 max and endurance pretty quickly: minutes off of your 5k within threeweeks.
This is mostly because of your body scaling back the extra red blood cells it created when you got in shape. You don't need them anymore so it won't continue creating them at that rate. It creates millions of them daily, so after a week or so you'll be back to normal, out of shape levels. It can take three to fourweeks for your capillary density to fade. Additionally, your mitochondrial content in your muscle mass can decrease by 50% over the course of a week. 
I had to take a year off for various reasons. I went from being able to do a nineteenminute 5k to not being able to finish a 3k in that time. I'm back to being able to do a twenty-oneminute 5k six months later (my course is much more difficult, so I have that excuse).
With muscular strength and fitness, your body will first improve the central nervous system message processing. Your initial strength gains when you start working out? It's all in your head. They're not really associated with any muscular adaptations, more neurological. It can take two to eightweeks to fully get your CNS in gear from working out.
Your body has twotypes of muscle, type I (oxidative, which is used for endurance activities), and type II (glycolytic, which is used for intense activities). Type II have greater mass potential, while type I is improved upon, mostly the same way that cardiovascular training improves your body, through improved pathways to get blood and gas to your muscles.
For your type II muscles, it doesn't appear that your body builds new muscle fibers, it merely makes the muscle fibers you have larger by increasing the size and quantity of myosin and actin filaments, making the myofibrils (the containers for the myosin and actin), more fluid in the muscle cells, and increases in the connective tissue. Type IIb muscles (they have large bursts, but the power doesn't last long) convert to Type IIa muscles under training.
Additionally, your body can increase bone density as a result of resistance training to better support the progressive loads you're putting on it.
It can take years for your musculoskeletal adaptations to fully take place, but for hypertrophy to really begin, it takes about sixteensessions to really see lasting change for an untrained person (the pump you feel after a workout is called transient hypertrophy, it goes away).
How fast does it take for them to go away? Why do they go away?
Your body will stop building them up. You're demonstrating to your body that you don't need those muscles anymore. If you're otherwise eating fine, your body will not consume your muscles, but it won't repair them. Over time your body will revert to a stable state that's adapted to the workload that you're giving it.
Your body will also start shifting more attention to type I fibers away from the high burning Type II muscles. At this point, it really depends on who you are and how well you're trained:
- Some athletes see a loss of about 6% muscle density after threeweeks.
- Some power lifters see losses of as much as 35% after sevenmonths.
- Young women who trained forseven weeks and gained two pounds of muscle mass, lost nearly all of it after detraining for sevenweeks.
The longer you go without training, the more you lose.
Because it's not actively eating away at your muscles, they can last for months to years depending on how strong you were to begin with; the fitter you are the longer they last. When you start lifting again, you'll be able to start from a higher spot from when you started last time. Part of this is because your muscle goes away slowly, the other part is that your nervous system still knows how to lift that much weight, that was half what you were working out when you lifted.
Your body does this because we've evolved to be prepared for famine. Your body strives to keep an optimum amount of high energy parts for the amount of work applied to them. There's biological limits to this, if you try to work too much volume you'll start to cause more damage than they can repair in time.
If you keep eating the same amount of calories as you did when you were working out, most of that will turn to fat. You'll be consuming more than you need. Going along with famine preparedness, fat is cheap to store and extremely useful when food is scarce, so your body will stock pile it if you aren't giving it a reason not to. It's this reason that most people think muscle turns to fat, it doesn't, it's just that when people stop working out, they usually don't compensate for the calorie usage change properly and end up putting on fat.
Finally, if you do not consume enough calories to maintain your metabolism, your body will begin to catabolize (consume for energy) your muscles. When people starve themselves, they'll lose weight fast at first, mostly from water and your body consuming muscle. They may appear to be fat but skinny at the same time because the body will consume muscle until it has the minimum required to function before it goes in full force on the fat.
The good news!
The better shape you were in, the less time it will take to get back into shape. Your muscle memory remains for a long time after your muscles have faded. Your body remembers how it was able to run and lift, you just have to remind it and get those muscles, blood vessels, and lungs back in shape to make it happen again and maybe lose a few pounds in the process.
When you start working out again, your type II muscles remember things much quickly. While they myofibrals may not build up immediately, it's possible to gain much of the lean mass you got back through fluids in the muscle fibers soon after starting training again.
Your endurance will come, but it takes longer.
Of course, it will take more time the longer you go without exercise. For cardio, it takes less time to break down your fitness; as stated above, it could take years to lose all your muscle.
Before my year off, I was lifting I was able to do 90 lb dumbbells (with shaky form and with a spotter) on bench press. When I went back to the gym I was able to do 60 lbs. Now I'm able to do 80 lbs without a spotter a month later.