What is it?
Cardiorespiratory fitness is the ability of the lungs and heart to supply oxygen to the skeletal muscle for energy production and to the brain and organs for life. Often aerobic capacity is included within the definition of cardiorespiratory fitness, but I believe it is so foundational (and subject to different training modalities) that it should be trained on its own.
If you have not read the section on aerobic capacity, I suggest you do that first. The aerobic and anaerobic systems are two different energy systems of the human body that need to be trained differently. While there are plenty of overlaps that doesn’t mean that just engaging in elevated heart rate efforts a few times a week is going to train both sufficiently.
Cardiorespiratory fitness impacts aerobic capacity, because it involves the systems that deliver oxygen to the mitochondria for energy production. However, it also goes beyond aerobic capacity, because it involves adaptations that allow the body to function when aerobic capacity is exceeded. So when we talk about training cardiorespiratory fitness, we mean training both the system that delivers oxygen to the mitochondria and training the system that allows us to produce energy without sufficient oxygen. And while some of this benefits our aerobic capacity to be sure, some of it is wholly different.
Why do I want it?
First off, training for cardiorespiratory adaptations will improve your oxygen delivery system, which in turn will help aerobic capacity. It won’t strictly cause adaptations in your mitochondria, but it will cause adaptations in your cardiopulmonary system that in part are necessary to raise your aerobic capacity/threshold.
This is also the system we use for medium to high threshold efforts. And that means different things for different people, because we all have different thresholds. For some it might be a fast run, but for others even swimming can be so demanding as to qualify as medium to high threshold. For some people the energy to walk up a hill may indeed be partly or even fully funded by our anaerobic system. If we are not adequately paying attention to our aerobic capacity and if we are getting older and that system is declining, then we are coming to rely on the anaerobic system more and more. So having that system trained will be helpful in many efforts and in later life.
In addition, when we are using our anaerobic system our bodies are producing something called lactate as well as hydrogen ions. We used to believe that lactate was some sort of garbage product that built up when we were doing very high intensity work, because it causes our muscles to burn. It turns out that lactate is actually incredibly useful. The muscles can recycle lactate and reprocess it to produce more energy (again using the anaerobic energy system). There are also plenty of studies suggesting that it improves cognitive function and acts as a neuro-protective agent. And lactate is part of a process called sarcoplasmic hypertrophy which may be a separate path to building muscle (or at the very least a supportive function to the more classical myofibrillar hypertrophy). It turns out that it is the hydrogen that burns our muscles by lowering the ph and making the environment more acidic, but that burning sensation is a signal to you that the lactate is building up and doing great things. So don’t just feel the burn in your high intensity efforts, love the burn, because it’s benefitting your health and longevity.
Finally, there are a host of chemical benefits from elevated heart rate efforts, including the release of beneficial neurotransmitters (endorphins, dopamine, serotonin), the lowering of the risk of certain diseases of advancing age, the increasing of metabolism and the increasing of anabolic hormones (which may trigger higher muscle growth potential).
How do I train it?
To train the anaerobic system we need to operate in the anaerobic system. There are various modalities to do this, but in general we are targeting higher heart rate activities. Again if you like to run, we run. If you like to swim, we swim. If you like to lift, we lift hard with little rest. We can do long and hard efforts or short bursts of intense efforts. There are a lot of ways to skin the proverbial cat, but unfortunately the intensity does need to be higher than when we are training aerobic capacity.
So in the end we just need to find something you like. But make no mistake, high threshold training is tough, and because it is tough it is one of the fastest ways to build a strong health identity.