Anything done predominantly aerobically is low intensity steady state (LISS). Higher intensity activity exceeds the capacity of the cardiovascular system and the mitochondria in the cells to provide oxygen, so anaerobic glycolysis has to occur: Fuel in the form of muscle glycogen has to be broken without oxygen, a more inefficient process that produces byproducts such as lactate. Moderate intensity steady state (MISS) is continuous activity at a high enough rate that there is significant accumulation of blood lactate. High intensity interval training (HIIT) is intermittent exercise at an anaerobic intensity, with lower intensity recovery in between.
Various authors are coming around to the view that MISS is kind of a no-man’s land that is not an ideal training zone. Spending a lot of time doing MISS is pretty common, though, among serious endurance athletes, and too much of it can have health drawbacks like a risk of developing Afib. Many examples of this were discussed in The Haywire Heart, by Christopher Case, Dr. John Mandrola, and Lennard Zinn (Chris and Lennard themselves developed Afib). Some in the community of endurance athletes have been in a bit of denial about this issue, so this is an important book. I can attest Afib is no fun as someone who has experienced a haywire heart (fortunately temporarily in my case),
I also personally find that too much MISS training can feel good while doing it, but can actually leave me kind of wound up so I don’t sleep as well at night. This is possibly residual cortisol (a stress hormone) produced in excess by too much volume of fairly intense activity. In contrast, relatively short sessions of HIIT leave me feeling great then relaxed later, while longer sessions of LISS leave me feeling mellow, and both lead to better sleep that night.
Spending most of your activity at low intensity, and throwing in some high intensity, but avoiding the no-man’s land of MISS, is a good approach for good health and fitness (as well as athletic performance). I first learned of this on Clarence Bass’s excellent health and fitness website. As Clarence put it “I walk and I sprint. I don’t do anything in between”. This approach has kept Clarence at an elite level of fitness and health for decades, including now at age 80. He does brisk walking in the hills near Albuquerque, not strolls. For high intensity he does intervals on machines like rowers and ski-ergs as well as strength training. He believes in only doing high intensity work less often, like once or at most twice a week, allowing plenty of recovery.
This approach, that uses mostly low intensity work and some HIIT, but minimizes MISS, is also called “polarized training”. This term was introduced by exercise physiologist Dr. Stephen Seiler , who is from Texas but does his work in Norway, a great place to study elite endurance athletes. Polarized training is based on studying the actual training elite endurance athletes do (including runners, rowers, and cross-country skiers), but the effectiveness has also been proven in intervention studies .
I think the take away for us non-elites is to go at an easy pace often (during your activities of daily living or scheduled exercise sessions), go hard occasionally (like a couple of times a week, so you have time to recover in between), and don’t spend too much time in the no-man’s land of MISS in the middle. You can make a big deal out of determining what various training target paces are, using heart rate monitors, or power meters for cycling (they are now starting to be available for running too). But it’s easy to just use your breath. At an easy pace, or LISS, breathing should be comfortable and you should pass the talk test. For high intensity you go hard, and should feel out of breath after each interval, and glad for the recovery period in between.
Legendary coach Bill Bowerman had two other good tips for his runners, both of which are helpful for any activity for the rest of us :
- Easy day/hard day. Always follow a day with a hard workout with an easy day, to allow the body to recover.
- After a hard workout, you should feel “exhilarated not exhausted”. A short and sweet interval session is exhilarating for me. If I work out too hard for too long, though, I can feel exhausted.
There are a lot of books out there that claim that HIIT training is superior to LISS, because it gives so much bang for the time buck. This is a valid point, especially if you don’t have a LISS activity you particularly enjoy. You can be perfectly healthy getting your low intensity activity from your activities of daily living, and get in great shape quickly just by throwing in some HIIT, maybe twice a week depending on how quickly you recover.
Some authors go so far as to claim LISS is actually harmful, which I don’t agree with. It’s fine if you like it and don’t do too much of it, which could lead to overtraining. I can average more than an hour a day of it without problems. I think the problem is more when people do too much LISS aerobics at the expense of not enough (or any) resistance training and HIIT. The top priority is getting those two done, with LISS optional if you enjoy it.
As a final note on pacing, I recommend going by time, not distance, especially during your longer workouts. I first discovered this when I was biking in Boulder. I despised the wind, and if you live near the front range of the Rockies there is a lot of opportunity to despise it. Then I realized it was because I was always biking a set distance, and if I ended up going against the wind, it was frustrating because it slowed me down. So I switched to going by time. Problem solved. On a windier day I might end up not going as far but because I had gotten my time in I knew it was still a good workout. Andy Burfoot gives reasons for going by time for runners in Run Forever, but his reasoning applies to any endurance activity.
I should point out that principles like easy day/hard day are rules of thumb, not principles cast is stone. Some coaches advocate block training which can have a few hard days in a row followed by a few easy days. And some training in the MISS zone, often called tempo training, works well for many people. Everyone is different but the overriding idea is avoiding overtraining or burnout. Easy day/hard day and minimizing MISS have bothed worked well for me.
For those of us that enjoy exercise (whether cardio, strength, or both), two main training errors are unnecessarily doing too much volume of training, and too much intensity. It’s a “more is better” mistake. More can be better, up to a point, but after that you can be breaking your body down too much without giving it enough recovery. Following easy day/hard day is a good start, as is not doing HIIT on too many days. Too high a training volume is another way to overdo. Too much volume or intensity (or both) can lead to overtraining: “Overtraining occurs when a person exceeds their body’s ability to recover from strenuous exercise”. It is the recovery that is key. Deana Kastor said that her renowned coach, Joe Vigil, had a saying “there’s no such thing as overtraining, only under-recovery” .
There are various signs of overtraining (or under-recovery), including your performance getting worse, increased resting heart rate in the morning, persistent muscle soreness, susceptibility to illness, and grumpiness. It’s time to dial the training back when any of this happens. This is more of an issue for pretty serious athletes, but us amateurs can flirt with it if we get overenthused.
- Seiler KS, Kjerland G, “Quantifying training intensity distribution in elite endurance athletes: is there evidence for an “optimal” distribution?”, Scand J Med Sci Sports., 2006.
- Neal C, et al, “Six weeks of a polarized training-intensity distribution leads to greater physiological and performance adaptations than a threshold model in trained cyclists”, J Appl Physiol, 2013.
- Moore, K, Bowerman and the Men of Oregon: The Story of Oregon’s Legendary Coach and Nike’s Cofounder, Rodale Books, 2007.
- Kastor, D, Let Your Mind Run, Crown Archetype, 2018.