What type of cardio should I be doing? HIIT? Zone 2?
It can be hard to know what to do with all the mixed information that's out there online these days. A lot of people are trying to sell their online programs and spreading misinformation, some saying you should only be doing HIIT, others saying you should only stay in 'zone 2'. When the reality is it's a little more nuanced than that.
Today my goal is to show you the science, as the science isn't open to interpretation, it's objective, so you know I'm not trying to sell you anything.
Now without delving too much into the different energy systems (which could be a whole blog in itself), we will go through the benefits of both aerobic and anaerobic conditioning. Firstly, strength is the ability to generate force, conditioning is the ability to use force efficiently and continuously.
Anaerobic conditioning is typically higher intensity, shorter duration training, often done in intervals (I'm sure you've heard of HIIT - high intensity interval training). Aerobic conditioning is typically longer and slower (you may have heard of LISS - low intensity steady-state).
Let's start with anaerobic style training, or HIIT. Because HIIT is done at a higher intensity, it generally requires greater power and force production. HIIT is considered ≥90% of maximal oxygen uptake or >75% of maximal power.
Because it uses a different energy system, it also builds up metabolites, which is the burning feeling you get from continued exercise (particularly due to hydrogen ions as they have a low pH). With the build up of metabolites, this also sends inhibitory signals to the muscles to stop them from contracting effectively, this is why your force output decreases as you fatigue. Generally speaking, anaerobic exercise is not sustainable for a long period of time roughly around 0-3 minutes (Patel et al, 2017).
Some of the notable physiological benefits (among many others) you will get from anaerobic training are as follows:
Improved lactate threshold - meaning you're able to buffer out the metabolites more efficiently, allowing you to continue for longer.
You will be able to hold higher heart rates for longer, and have lower heart rates at sub-maximal exercise.
Increase in power production.
Increase in VO2 max (maximal oxygen uptake)
Increase in insulin sensitivity (good for keeping blood sugar in check)
(Atakan et al, 2021)
One of the big draw cards of HIIT is that it doesn't require as much volume to get results. It is especially good if you are time-poor, there are studies showing that even 20 minutes per week can give you significant benefits. (Li & Siegrist, 2012)
Keep in mind, we are talking true high intensity here, not high density. For example, not 60 seconds of half-arsed burpees or star jumps. Ideally, it is best done as some form of continuous exercise where you can track your pace (bike/ski/row/running etc).
Now aerobic exercise is exercise that can last a much longer time and is more sustainable. It uses a different energy system that requires oxygen and generally uses more fat as a fuel source (this doesn't mean you automatically burn more fat, you still need a calorie deficit for that).
Some of the notable physiological benefits you will get from aerobic conditioning are as follows:
Increased capillarisation (more efficient oxygen delivery to working muscles)
Increased mitochondrial density
Lower resting heart rate
Improved pulmonary function (eg lung capacity)
(Atakan et al, 2021)
The aforementioned physiological benefits also work to allow you to recover better between sets and even between sessions.
The main drawcard for aerobic training is that it is lower impact (generally speaking), and it allows you to build plenty of volume, provided you have the time, as it will not be as taxing to the whole system.
So now that you know a bit more about the two, which should you be doing?
Both aerobic and anaerobic exercises have unique and positive correlations towards improved cardiovascular health, and 1/3 of adults and 4/5 of adolescents - approximately 1.4 billion people - do not meet public health guidelines for recommended levels of exercise (ACSM, 2023).
So realistically, anything is better than nothing.
Obviously which type you should be doing somewhat depends on your goals.
But the aerobic system underpins everything, it is technically always working, so you need at least to build a base there first if you had to choose. Then maybe you can look to build your anaerobic fitness thereafter.
If doing both, how you split it might be up to your preference, or your schedule, but you could start with 2 longer/slower aerobic and 1 shorter/higher intensity anaerobic stimulus each week to get all of those benefits mentioned above
So, shock horror, to have the best of both worlds and reap the unique benefits from each, our advice is to do both!