Endurance is super important for martial artists for 2 sets of reasons: 1) not getting tired while sparring or performing your kata makes your sparring or kata better; and 2) not getting tired while training to improve your martial arts skills makes your training more effective. Practicing skills while you're fatigued is, simply, not very effective (remember, it's not 'practice makes perfect,' it's 'perfect practice makes perfect.' Practicing sloppy technique just makes you very good at doing sloppy technique.)
Endurance is also super important for life. I've explained before - if daily tasks cause your heart rate to go really high and leave you gasping for your air, not only is that unseemly, but it means your body is going into a high stress response to handle an everyday thing. That's really, really bad for your health, since you're pushed into a sympathetic state very frequently.
I used to think I had a strong handle on the kind of training that pretty much anybody should do to improve their endurance. You can scan old posts on this blog to see examples of that. Sadly, I had an overly simplistic view. The real story is more complicated, and training for endurance is harder (and easier, as I'll explain later) than I thought.
[Note: this post is background information, meant to be a reference. It isn't really about training, it's to set up later posts on training. Also, the information here is not particularly controversial, and has been pulled from many, many sources - this is the consensus right now, and as such I'm presenting it with a very high degree of confidence. Later on I'll get into more theoretical conclusions where my confidence is not as high.]
What is energy?
Energy in this context means biological energy, the energy used by all your bodily systems to do pretty much anything. And almost all of your body's chemical reactions, when they need it, get energy by splitting molecules of ATP into ADP and a phosphate (ATP is adenosine triphosphate, meaning it has 3 phosphates - when one gets knocked off it turns into ADP, adenosine diphosphate, an adenosine with 2 phosphates stuck to it, and a loose phosphate, and a bunch of energy.) ATP turning into ADP + P is kind of like burning wood in the sense that it releases energy but it is UNLIKE burning wood because the ADP and P can get stuck back together, giving you another ATP, ready to use, as long as you can get the energy for THAT from somewhere else. So your ATP is more like a rechargeable battery - it can discharge, giving you energy, and then recharge, as long as you have a power source to recharge it with.
From a martial arts or movement perspective, you can think of it like this: To execute any movement, your nerves carry a signal to the muscles, which contract, which exerts a force on the body. The action of the nerves (carrying the signal) and the muscles (to contract, then relax) depends on ATP - ATP is 'used,' fueling the action, and you end up with ADP + P.
Also worth noting: your cells use energy all the time, not just when exercising. They need a constant stream of energy (that's why you burn calories even at rest). All that happens when you exercise is that the amount of energy you need goes up, or you could say the rate at which you use energy goes up.
What are energy systems in general?
If you took high school biology you have a general sense of how this works from a big picture perspective - your body takes in energy from food, 'burns' it with the aid of oxygen, and uses that energy to turn that ADP and P back into ATP, where it can be used again (and again and again) to fuel chemical reactions, like muscle contractions.
More specifically, what we call energy systems are the chemical processes that directly regenerate ATP.
Or, you could say that the energy systems are the systems that provide ATP for use by your cells (and by provide, we can mean store a bunch of it, or restore the ATP by combining ADP and P back into usable ATP).
What are the 3 energy systems?
1) Alactic anaerobic system (also called PCr, AtP_CP, ATP_PCr, Creatine Phosphate Energy System, Oxidative Independent Energy System, Short Term Energy System) is the FIRST place your cell 'looks' for ATP. You are ALWAYS using the alactic anaerobic system - ALWAYS. It consists of a bunch of ATP, just sitting around waiting to be used, and some Creatine Phosphate (you can get ATP back from ADP by breaking down Creatine Phosphate super fast, so these two are sort of counted together). It's super fast, but super small - you only store somewhere between 4 and 10 seconds worth of energy in the alactic anaerobic system. And it doesn't depend on anything else - the alactic anaerobic system is just atp and creatine phosphate, to 'work' (i.e. provide energy) it doesn't need oxygen or any other substrates.
So think of your alactic anaerobic energy system as a pool of energy that is always, immediately available, in full. It's the fastest energy system of all, and once it's run down close to empty nothing can replace it as fast as you can use it - that's why if you look at Olympic sprinters, they are ALL slowing down by the time they get past 70 or 80 meters.
If all you had was alactic anaerobic, you wouldn't need to eat or breathe, and you could exert maximal power for maybe 10 seconds, but then you would DIE. That would be bad. So:
The next two energy systems are there ONLY to replenish the Alactic Anaerobic System. They don't really make sense on their own.
2) Aerobic energy system (also called oxidative dependent) is the PRIMARY way your body replenishes that ATP as it gets used. The aerobic energy system requires oxygen (that's literally what 'aerobic' means), so how fast it can pump produce energy depends on how fast your body can get oxygen to your cells, as well as a few other factors.
The aerobic system is SLOW. It can only replace a little ATP every second. When you exert yourself, you can pretty easily use up ATP faster than the aerobic system can replenish it (though your aerobic system can get better, and faster, with training).
The aerobic system can go for a LONG time, even at its maximum output. It burns fat and/or glucose, and requires oxygen.
The aerobic system is working ALL the time. Just like you are ALWAYS using ATP from the pool in the alactic anaerobic system, your aerobic system is ALWAYS working to refill that pool. When people say that their workout is not aerobic, or that it is only anaerobic, they are not really correct. The aerobic system is always involved (it may not be by itself, is all).
The aerobic system is relatively 'clean burning.' It can work without involving a lot of stress hormones or negative effects on your body. This is sort of obvious - since it's working all the time, every second of your life, your body can't interpret aerobic activity as a threat or an emergency.
Your aerobic system isn't always fully 'on.' It's working a little bit all the time, but it takes time to 'ramp up' when you put more demands on it. That's one (in my opinion, a big) reason why warming up is important - your aerobic system needs time to get into gear so it can replenish your ATP faster. If you start a workout by immediately going into high intensity work, you'll greatly outstrip your aerobic system's ability to replenish your energy.
With training your aerobic system can become able to produce energy faster, but there are limits.
When you're just sitting around or going for a walk or doing something leisurely, the ATP you burn is replaced as fast as you burn it by your aerobic system (ADP is converted back into ATP as fast as you are turning ATP into ADP). BUT once you start using energy at a faster rate, and your aerobic system can't keep up (because at that moment the aerobic system can't replenish ATP as fast as you're using it), then...
3) Lactic Acid system (also called anaerobic lactic, Lactate Energy System, Lactic, anaerobic energy system, non-oxidative glycolytic energy system, glycolitic energy system) is normally more or less dormant, but it kicks in when your alactic anaerobic system gets depleted (runs low) and your aerobic system can't keep up with the demand. This typically happens either because it's very early in your workout, and the aerobic system hasn't ramped up yet, or because your workout is hard enough, depleting energy fast enough, that the aerobic system can't keep up.
The lactic acid system is FAST. It can produce energy much faster than the aerobic system, though not as fast as the alactic anaerobic system. It burns glycogen (glucose stored in the muscles). It produces lactic acid, which will build up in your system and make you feel like crap (it's the hydrogen ions, not the lactate, that causes problems when you produce lactic acid). It doesn't require oxygen. It doesn't last very long, but it lasts MUCH LONGER than the alactic anaerobic system.
The lactic acid system is sort of an emergency system, linked to your sympathetic nervous system. When you need it a lot - when you work in such a way that you get a big accumulation of lactic acid - your body thinks "there is something happening, we need more energy than we can comfortably provide, even if only for a little while." In other words, when it is used, there's a lot of associated stress on the body. You can expect elevated cortisol, activation of the sympathetic nervous system, all the other things that happen with fight or flight.
Here's the big thing - the lactic acid system produces lactate. So as it works, lactate is made, and starts to build up in your body. The lactic acid system itself can't get rid of that lactate - only the aerobic system can do that, and it can't do it nearly as fast as the lactic system can produce it. And the more lactate builds up, the worse the lactic system gets at producing energy (it gets inhibited), and the more tired you feel. In other words, the lactic system is self limiting.
What's the big picture with how these interact?
You're always sort of sipping energy from the pool of ATP and creatine phosphate in your alactic anaerobic system. If you start to sip faster (say, by exercising), the pool shrinks, your lactic acid system starts working a little bit, and your aerobic system starts working harder to replenish the pool. If your aerobic system can't keep up, and you take energy out faster than it can be replaced, the lactic acid system works harder, and lactate starts building up in your bloodstream. At that point your aerobic system is desperately working to keep your alactic anaerobic system topped off AND to reset the effects of the lactic acid system.
The harder you work the lactic acid system, without taking the time to completely recover (meaning take some time where you're using up energy slower than the aerobic system can replace it), the worse you're going to feel and the more your performance will suffer (your body starts to say, "whoa, you're using energy way faster than I can replace it, I'd better make those muscles slow down"). It's the lactic system that really causes fatigue.
Is this meaningful for your whole body or also for individual muscles?
Imagine a workout where you're only working one small muscle (or a couple of small muscles). Something like seated cable curls - your biceps and maybe forearms are working very hard, but the rest of you isn't.
In that sort of case the muscle fibers in your biceps are going to use ATP at a high rate, but your body as a whole isn't. The muscle cells in your biceps will start pulling in oxygen so they can replenish that ATP, but your heart and lungs won't register this demand, because your biceps are pretty small, and even if they're pulling oxygen out of your blood as fast as they can, it won't impact your blood oxygen levels nearly as much as if, say, you were sprinting, and two thirds of the muscles in your body were pulling out oxygen.
So in that case you can have a workout that heavily involves the lactic system in one part of your body (the biceps) but NOT in your body overall.
How does training help?
With training, your aerobic and lactic acid systems can become much, much better. You can develop a much higher tolerance for lactate, get much better at quickly removing lactate, and greatly increase how much energy you can get every minute from your aerobic system. Your alactic anaerobic system can be increased a little bit but probably not much.
As to what kinds of training improve which system, that gets more complicated.
I'm going to address training the energy systems at some point in the future, it's too much for this post. But here's the important principle:
Developing the aerobic system competes with developing the lactic acid system, and vice versa.
In other words, if a workout really develops the lactic acid system, it does NOT develop the aerobic system and may impair it a bit. And workouts that develop the aerobic system do NOT develop the lactic acid system.
What's the take home message for martial arts training?
Ideally, you would have a maximally developed lactic system AND aerobic system. But that's not really possible - training for one is different from training the other. So you have to find the right balance. And the types of training that are good for the aerobic system are not the same as the kinds that are good for the lactic system.
The good news is that, if you are kind of stuck in your training, or if you feel that you're working really hard but not making much progress, understanding these systems might help you break through and make progress again.
I will talk more about training the different systems in future posts.
If you want a more in depth understanding of these issues, read Joel Jamieson's work - here or in his book (now out of print, unfortunately).