But high volume training will always be the best way to improve endurance in the long run
Over the last several years, high intensity training has become the bee’s knees in the fitness world for its ability to burn calories, improve cardiovascular health, and produce many a hard-bod. What is high intensity training? While there are specific definitions you can look up, we’ll consider it any exercise that you can do continuously for at least ten seconds – like a single sprint, but not longer than two minutes – like a grueling bout of burpees.
High intensity training (sometimes referred to as high intensity interval training, HIIT, or “hee-eet” as I call it) is popular because it gives you more bang for your exercise buck. In less time, it’s possible to get similar adaptations to much longer, lower intensity endurance workouts.
How can that be – did we find a biological loophole in the paradigm of human performance? No. There are real physiological adaptations that occur with high intensity training that are similar to (but not the same as) low-to-moderate intensity endurance training. When you do high intensity training, your heart rate increases, you breathe harder, and your muscles need to make energy to move, just like during lower intensity exercise.
In order to perform high intensity exercise like sprinting, our bodies need to work extra hard in a short amount of time to produce the energy required to move. And while that energy comes from more immediate sources that we have stored in our muscles, our hearts are still pumping like crazy to deliver oxygen and nutrients. Even during recovery, your heart rate stays elevated and your stress hormones and your nervous system are still firing away.
This is where the line between “aerobic” and “anaerobic” exercise gets blurry. In reality, there is no such thing as “anaerobic” exercise: we never stop consuming oxygen. And the stress that high intensity exercise places on your cardiovascular system is a huge reason you can improve your endurance: your heart can use the fitness it got from high intensity exercise to help you during lower intensity exercise.
The other place where high intensity adaptations can improve endurance capacity is in your muscles. In scrambling to restore all that energy that you just used for your high intensity effort, your muscle cells are still working as hard as they would during lower intensity effort, despite the fact that you have stopped moving. They need oxygen to do that work, and they need to make the cellular machinery (mitochondria) to continue to make energy. In other words, recovery from high intensity efforts is like a low intensity workout in and of itself.
This is why any type of training that increases your body’s demands for energy, that increases your heart rate and your oxygen consumption, will improve your cardiorespiratory endurance.
However, if you want to continue to improve your endurance for a long time, don’t go trading in your running shoes for a crossfit membership (figuratively speaking of course; most gyms prefer dollars over used running shoes). While high intensity training will certainly improve your endurance in a shorter amount of time, it requires more recovery, and the ceiling for adaptations is lower.
High intensity exercise is really stressful on your body. Not only on your joints and muscles, but your cardiovascular and hormonal systems as well. Since your body is still working hard after strenuous exercise, even when you’re not moving, it takes that much longer to return to a true resting state. Like, a few days in this case. And remember, your body doesn’t develop those long-term training adaptations until it’s resting.
Another important consideration for high intensity training is that while you will get fit, you cannot improve your fitness to the extent that higher volume endurance training can. This is mainly due to the difference in stimuli on your heart. High intensity exercise makes your heart stronger, but endurance exercise makes your heart longer. The sustained volume-load that endurance training places on your heart allows it to pump more blood for a longer period of time. This is why top endurance athletes like marathoners log tons of miles, and they are the best in their late-twenties and thirties. It takes time for your heart to reach peak capacity.
So, if you want to improve your fitness in a short amount of time, and maybe even have some fun doing it, high intensity exercise is a great choice. But make sure you take plenty of time between workouts to recover, and eventually mix in some longer endurance activities to maintain those adaptations.