"Polarized training" is a macro-scale approach to training plan structure that emphasizes either very high or very low intensity training, with little-to-no time spent at moderate intensity. For a great review of the physiological concepts, TrainerRoad has a very thorough article with many linked research articles. The strictly quantitative approach to polarized training is important, and it's a very logical foundation for something like TrainerRoad, which exists (almost) solely in the quantitative space. But I think that it's a limited approach, and I think it misses out on including a major aspect of successful training - what I'll call a "qualitative" approach. Brett Sutton is notorious for a seemingly purely qualitative approach (at least superficially) with his "Medium, Moderate, Mad" classification of "zones"; Sutton's approach is way more complex than that, but as a generalization, he's a great counterpoint in terms of being more qualitatively focused in training, especially when it comes to measuring intensity. For most of my own career, I was very wedded to the quantitative approach, but as I've gotten older and stopped being a professional, I've come to have more of an appreciation for the qualitative, and I actually think that there was a whole lot more quantitative in my periods of success as a professional than I ever really acknowledged.
As a forty-year old Masters (gasp...) cyclist, I find that I can try hard - objectively - once a week. These workouts take place (after a year absence) on the velodrome, or on Zwift, usually in the form of a race. During these workouts, I care about my quantified performance. How fast did I go? What sort of watts did I produce for a given duration? These are not necessarily the most physically taxing workouts, but they are the most mentally challenging. I care a lot about how I perform during these workouts. They are, essentially, pseudo-races (or, in the case of a Zwift race, an actual race). And I think it's incredibly important to have workouts where success and failure is objectively defined. Where simply trying hard is not enough. Because that's what it's like on race day. The clock does not lie. I strongly dislike the concept that failure doesn't exist. That everything is a learning opportunity. That's ridiculous. Sometimes the only lesson is that you weren't good enough. But holding yourself accountable to objective measurement is taxing. And asking this of yourself too often is a recipe for burnout.
Subjective intensity does have its place though. In addition to my once per week objectively measured workout, I have one workout that I measure subjectively. Once a week, I will try hard, but I don't really care what that means in a quantitative way. Sometimes this means doing maximal effort starts or sprints. Other times it's just giving a hard effort - or efforts - on a given climb. In this case, I only care that it felt hard. That I gave a good effort. The numbers might be useful for gauging where I'm at. Sometimes it's a sign that in spite of being tired, I still had good legs. Other times, I just ignore them.
The rest of the time, I simply go according to feel. If I feel good, the watts will be higher. If I don't, they'll be lower. But I don't really care about anything other than being out there, getting away from my desk, about repetition and consistency. I did something, and something is always better than nothing. (N.B. this isn't to discredit the importance of rest; it's simply reflective of the fact that any training provides some stimulus.) I've found that polarization of mental effort is much more relevant to me than polarization of physical effort. Now, to a certain extent, there is almost no difference here. There's no clear distinction between the mental and the physical; they are intrinsically linked. There's a reason why the best efforts are often the ones that feel easiest.
When I worked at Zwift, there was a fair amount of criticism of the workouts and training plans as being designed simply to "look appealing" or being structured simply to keep people engaged as opposed to being grounded in sound physiology. But to me, this always missed the mark. My guiding mantra when I was working with coaches to develop the plans and workouts was, "The best training plan is the one you will actually do." This isn't a new concept of course; I think I first came across this concept in a great assessment of various fad diets. The best diet is the one you'll actually follow. To me, getting - and keeping - people interested wasn't besides the point; it was entirely the point.
I've evolved that thought process into what I think of now as a funneling approach to training. There are three funnels, each of which narrows down the prescription for training. The first is what you actually could do. For most age groupers, this is overwhelmingly a matter of time. If you have an hour to train, it doesn't matter if the "best" workout for you (from the standpoint of pure physiology) is a 2hr "base" ride. That's just not an option. And this is where I think the biggest struggles for most age-groupers trying to adapt high-volume professional programs comes from. You can't just scale down the volume; the training load becomes insufficient. But you also can't just scale up the intensity; it's not sustainable. A 3hr base ride is not necessarily "half" as productive as a 6hr base ride. But likewise a 3hr ride at 75% of FTP is not necessarily as productive as a 6hr ride at 50% FTP, even though the TSS score is the same. TSS does have value when it comes to scaling, but it does start to break down, especially as you get to higher intensities. E.g. a 6hr ride at 50% of FTP and a 2hr ride at 86% FTP as a better example; energy systems are not quite so interchangeable. And this is where polarized training really brings value; it does a great job of capturing why intensities are not purely transitive.
And that brings us to the second funnel. Which is what you actually should do. In this case, I'm talking about the purely quantitative. The physiological. What workout will give you the appropriate stimulus to optimize performance. Should you do low intensity? High-intensity? What are the specifics of the workout? And, of course, sometimes, the answer is rest/recover. What you should do is where TrainerRoad really excels. They have great plans that are well designed, and grounded in sound physiology. For a lot of my career, I thought this was the real magic in coaching. In knowing what workouts to do. In what specific things would extract the best performance. A lot of this came from my background in rowing - in particular, American collegiate rowing, which is obsessively preoccupied with ergometer scores. The best coach was the coach who knew the right workouts. Who knew - physiologically and quantitatively - what was required to be successful. Since 2005, I've done essentially every bike ride I've ever gone on with a power meter. And this obsession with quantification was a big part of why I was as successful as I was during my career. But it also is what undermined me a lot during my lost years of 2013 - 2015 and 2016. I cared only about what I should have done, from an objective standpoint, and not at all about the subjective.
With enough time away from the sport - and, critically, a necessary focus by having a job that is not professional sport on what I actually can do because I have to work, I've come to have a deep appreciation for the third funnel, which is what you actually would do. This is the funnel that I think Zwift does the best and what makes it so unique. If you should do a hard workout at threshold, but you have no interest in following a structured block of intervals, then go do a race. If you should do an easy ride, but can't be bothered to just sit idle around, go do a social group ride. Zwift is great because it gives you almost endless convenient options that you are actually willing to do. Of course, you don't need Zwift to do this; Zwift just makes it easy to match up what you will do with what you can and should do. But really, the critical piece here is recognizing and acknowledging that what you are willing to do matters. That what you actually want to do is important.
There was a recent thread in the forum about long-course standout Sam Long and his prospects in Kona. As a bike-runner who also struggled to break through in the swim, my name came up. And I said that, in hindsight, rather than trying to become a much better swimmer, and - ultimately - rather than trying to succeed in Kona, I would have just opted out. I wouldn't have stopped trying to become a better swimmer, but I wouldn't have focused on it. I wouldn't have necessarily stopped going to Kona, but I wouldn't have focused on it. I would have focused more on what I would do rather than what I should do. My best performances in Kona were the years when it was an afterthought. My worst years were when it was a focus, in large part because - at least for me - it also required a fair amount of focus just to get to Kona (interestingly, it was a lot easier to get there when I didn't actually try to do so...). And I think I didn't appreciate the importance of polarization of focus, of emotional investment.
I was never going to win Kona. And for most of my career, I was okay with that. And I had success in Kona when it was just another race. I had a few races that I cared about a lot. And a bunch that I didn't. And that was good. The business aspect of racing really muddied things for me here, because there were races that I only cared about for financial reasons, which presented an interesting conundrum, and one that I managed poorly. Thankfully, that's no longer relevant for me, and it shouldn't be relevant for most age-groupers. And if I can offer some advice to pros (aspiring or current), don't let this weigh you down; the degree to which you care about a race need not be commensurate with financial considerations for your being there; being a professional means a lot more than how hard you race on the course, both for better and for worse. Social media has only intensified this; that shift had just started during my professional career, but it's fully the case now, especially after a year without racing.
Caring about a race is pretty easy. Even though you might care about a given race less, if you're a competitive animal, you're going to care enough. And for many amateurs, myself included, races are a more rare thing. The need to have anything other than "A" races is largely the province of pros, though virtual races and other competitive-events-that-aren't-races can help to balance this out. If polarization of racing is a luxury, polarization of training is a necessity. And here again, I'm talking about polarization of investment. I don't agree with the concept of "ego depletion." The reason for polarization of "investment" is because subjective measures like this are much more all encompassing than something that's purely physiological. Subjective self-assessment is incredibly powerful, much more so than pseudo-quantitative measures like HRV. It's good to have objective measures of output - speed, power, time. But remember those are outputs. Objective measures of input are much more challenging to get right. The reason to think about polarization in subjective terms is that it's a way of actually applying the physiological concepts in a convenient way. Understanding how to apply polarized training purely in physiological terms misses the boat, especially for triathletes who are juggling multiple sports. Polarized training definitely works because of physiological phenomena. But I'd also submit that it works because breaking your efforts into really-hard some of the time and easy most of the time is also more sustainable, mentally, than an approach that's just the same. Polarized training is mentally rewarding, and that's important too.
Ultimately, when you think about applying a polarized approach to your own training, coming at it from the standpoint of effort - of subjective and qualitative measures - offers a much simpler and practical approach than obsessing over time in strictly defined numeric zones. But quantitative measures still have their place. It's how I know that in spite of getting older, this simple approach to training has allowed me to keep getting faster.