Billy Sperlich is one of the world’s leading experts in the area of training intensity distribution (TID) in endurance sports. I’ve often cited his research, which he conducts out of the University of Würzburg, in my books, articles, and blog posts. Recently, Sperlich released a series of eleven tweets summarizing the “experiences and takeaways” he’s accumulated in studying TID over the past few years, with links to the studies he’s been involved in. It’s a tidy little resource for endurance athletes and coaches, so I’ve taken the liberty of repackaging it here, with supplemental commentary.

1. “TID between endurance sport and time of season vary considerably”

This tweet includes a link to a comprehensive 2015 review of existing research on training intensity balance in endurance athletes that Sperlich collaborated on with colleague Thomas Stöggl. Although the text of the tweet conveys the impression that the findings are all over the map, honestly, if I knew nothing about endurance training and I read this review I would come away feeling quite confident that I would get good results from a high-volume, mostly low-intensity training approach, regardless of my specific sport or current phase of training.

2. “The TID quantification method substantially influences the proportion of low/medium/high intensity training”

 This tweet includes a link to a new study involving endurance kayakers that Sperlich conducted with three other researchers. It showed that individual athletes’ training intensity balance varied significantly depending on whether it was measured with performance metrics or physiological metrics. My takeaway as a coach is to avoid mixing and matching intensity metrics in measuring TID.

3. “Maybe different TID quantification methods are necessary depending on the time of season”

This tweet links to the same study as the previous one. On the basis of their findings, Sperlich et al. speculated that each of three methods of measuring intensity—velocity, heart rate, and blood lactate—has advantages and disadvantages, and that it may be sensible for coaches to prioritize different ones at different times in a training cycle.

I’ve actually found it useful to prescribe individual workouts with different intensity metrics from day to day, based on each metric’s strengths and weaknesses and the type of workout. For example, I might give a runner a power-based hill repetitions run on Tuesday, a heart-rate based easy run on Wednesday, and a pace-based tempo run on Thursday. This is somewhat different from using different intensity metrics to monitor and regular intensity balance, though.

4. “The seasonal analysis of TID reveals extensive inter-individual variability”

 This tweet includes a link to another study involving elite paddle sports competitors. Sperlich and his collaborators found a high degree of variability in individual athletes’ training intensity balance at different periods of training. But again, all of the athletes spent the bulk of their time at low intensity in all phases.

5. “Published ‘Polarized’ TID observations are not necessarily ‘polarized’”

A polarized TID is one in which little time is spent at moderate intensity. This tweet links to a study coauthored by Sperlich that describes and validates a tool called the polarization index, which quantifies the degree of polarization in a given period of training. This tool is useful in determining how effective a polarized approach to TID is compared to other approaches.

6. “Athletes with pyramidal TID during preparation may (automatically) shift to polarized TID when entering competition period”

The primary alternative to a polarized approach to TID is a pyramidal approach, wherein more time is spent at moderate intensity than at high intensity. This tweet links to a prospective, controlled study, again coauthored by Sperlich, in which elite rowers were separated into two groups, one of which trained with a polarized intensity balance while the other trained with a pyramidal intensity balance for eleven weeks. Neither group improved more than the other, which isn’t shocking because the two programs were the same in most respects, containing equal volume and similar amounts of training at low intensity. At the elite level, both polarized and pyramidal training are dominated by low-intensity training. (There seems to be a theme emerging here . . .)

7. “The analysis of waking hour TID (training & off-training TID) shows a broader more holistic perspective to understand the TID-dose-response”

 This tweet links to an interesting study published last year that explored the influence of non-scheduled activity on training intensity balance, training volume, and performance in elite male rowers. In essence, Sperlich and his collaborators sought to find out what difference it makes, if any, if daily activities outside of formal workouts are measured the same way formal workouts are. What they found was that such activities had a small but statistically significant impact on training volume and TID but no impact on performance.

8. “Not much TID analysis exist in female endurance athletes.”

 This point is underscored by the fact that there is no study linked to from the tweet! Kudos to Sperlich for drawing attention to the problem.

9. “Too much ‘black and white’ in the ‘80:20 TID story’. This specific low:high-intensity TID may work in one sport or for one athlete or during a certain period of the season but is far from the obtained data of the last years and surely no universal best-practice TID.”

Am I wrong to feel personally targeted by this one? Arguably, I’ve done more than anyone to promote the “80:20 TID story”. Yet my own thinking about the 80/20 intensity balance is far from black-and-white, and I’ve taken pains to express this nuance. Exhibit A is the following excerpt from my 2014 book 80/20 Running:

There’s no reason to tie yourself in knots trying to aim for perfectly round numbers. What is important is that you avoid ratios that are way off the mark, such as 100/0, 30/70, and the 50/50 ratio that is the norm for recreational runners.

In short, the ideal balance of training intensities is a narrow range rather than a precise ratio. But that range may be slightly different for individual runners. The 80/20 Rule is what Seiler has referred to as a population optimum. This means that a training intensity distribution that is very close to 80/20 is best for most, but not all, runners. A few runners respond better if they do a little less or a little more of their training at low intensity. There is no evidence, however, of extreme “outliers” who respond poorly to 80/20 training and much better to either a heavily speed-based program or to an always-slow regimen that lacks any work at higher intensities. So you can’t go wrong by following the 80/20 Rule. It’s certainly the place to start. As you gain experience, though, you may find that you respond better to a 70/30 ratio or a 90/10 ratio, in which case you’ll want to make that your personal rule. But it’s more likely that you will find your sweet spot closer to 80/20.

The sweet spot shifts, however, as the training process moves along…

Exhibit B (and I could easily adduce exhibits C through Z if I weren’t fearful of boring you) is a blog post I wrote last year, titled “How to Practice 80/20 Training Without Really Trying,” where I lament, “I do see a fair number of athletes overthinking the whole 80/20 thing, and it concerns me,” before going on to advocate a nonliteral interpretation of 80/20 defined by two rules: 1) Be sure you’re actually at low intensity when you intend to be, and 2) Devote roughly one out of every three training sessions you do to moderate or high intensity.

To the extent that I am guilty of overselling 80/20, I’m not sorry. I’m a coach, not a scientist, and like any good coach I love truth, but I love results even more. As the legendary basketball coach Billy Donovan said, “Believe it your system, and then sell it to your players.” Buy-in is critical to athletic success. A good system that an athlete believes in will always yield stronger results than a better system an athlete hasn’t fully bought into. Scientists are understandably uncomfortable with this reality, but it is the reality. “You lied!” says the scientist to the coach. “I exaggerated,” answers the coach, “and I won because of it.”

10. “Single case TID observation of elite athletes are interesting for the examined athlete but not a blueprint for all other athletes => My experience: The day-by-day decision-making forms a TID signature which depends on several internal and external factors.”

I agree. But who is actually suggesting that single-case observational studies of training intensity balance in elite athletes are a blueprint for all athletes?

11. “Seeking for a best practice (a priori) universal TID pattern most likely will not assist personalized training prescription… (and probably does not exist…)”

This statement seems to suggest that coaches need to start completely from scratch with each new athlete, behaving as if nothing that any athlete has ever done before has any relevance whatsoever to the next athlete to come along. While I certainly agree that the optimal training recipe for each athlete is unique in the fine details, the last time I checked, all endurance athletes were human, and certain things are generally true for all humans, and for Pete’s sake, a coach has to start somewhere with a new athlete! And yes, I do believe that elite best practices are the best starting point.

What’s missing from Sperlich’s tweets is any acknowledgement of just how poorly the vast majority of endurance athletes manage their training intensity balance. The typical recreational endurance athlete is so far away from optimal TID it’s not even funny. Also missing from the thread is any kind of nod to the reality that the vast majority of recreational endurance athletes do not have one-on-one coaches.

To be more specific, the typical recreational endurance athlete is caught in the moderate-intensity, doing far too little training at low intensity. When such an athlete goes from muddling along on their own to following one of the readymade 80/20 plans available in my books or on this website, they almost invariably experience breakthroughs in fitness and performance. Most of them also feel better in training and enjoy the process more, and many report a reduction in injury frequency.

Does this mean that athletes can do no better than follow a readymade 80/20 plan? Of course not. There is another level, and it is the personalized approach Sperlich advocates. I love to see athletes take this last step in the process of true training optimization, but it’s not a realistic step for many.