Chronic Training Load

A few months ago, “Kevin” posted a concern about his training in the 80/20 discussion forum. He explained that lately his heart rate had seemed rather high relative to his pace, and the VO2max estimates he got from his watch had dipped slightly. A newer runner, Kevin was becoming discouraged and beginning to doubt the 80/20 method’s effectiveness. In my reply, I asked Kevin how he actually felt while running, and whether he would be noticing anything amiss if he ran without his watch.

“Now that you ask,” he answered, “I think I feel fine while running, maybe even good, but when my pace is right and my heart rate seems high, I feel down about it and frustrated during my run and also after.” Just as I had suspected, Kevin’s supposed fitness crisis was in fact an entirely emotional phenomenon triggered by information that had a variety of possible explanations, all of them more plausible than the one he’d assumed in defiance of how he was actually feeling and performing.

The story of Kevin’s illusory fitness crisis is an example of what I call a measurement problem, or a fake problem created by measuring. If you’ve ever had the Check Engine light come on in your vehicle and taken it in for service only to discover that the problem was with the Check Engine light, not the engine itself, you know what a measurement problem is.

The physicists reading this article know all about measurement problems. Indeed, in physics there is something known as the measurement problem, which has to do with how wave functions behave when observed. Simply put, wave functions behave differently when observed than they are believed to behave in the absence of observation. This problem is famously exemplified in the thought experiment known as Schrödinger’s cat. In it, a hypothetical observer opens a box with a cat inside and discovers that the cat is either alive or dead, but until the box is opened the cat is both alive and dead.

This paradox sounds crazy if you’re not familiar with quantum physics, but a good many physicists believe that wave functions really do exist in an indeterminate state until they are forced essentially to “choose” a definite state upon being observed. What is certain is that, as German physicist Werner Heisenberg articulated in his eponymous Uncertainty Principle, you cannot measure any system from the outside, hence to measure something is to change it. An amusing example from the realm of physical exercise is the discovery that male subjects in exercise science studies give lower perceived effort ratings when the researcher collecting the data is female. Ah, men. We’re hopeless!

Most of the measurement problems I see in endurance sports are caused by heart rate monitors—or rather by athletes’ failure to recognize the flaws and limitations of these devices. All too often athletes treat their heart rate monitors like the voice of God, when in fact they’re more like the voice of a bossy backseat driver with macular degeneration, giving you too much information, much of it unreliable. The most common example of a heart rate measurement problem relates to cardiac lag, or the delayed response of the heart muscle to changes in exertion. I’ve lost count of the number of athletes who have told me that, no matter how fast they go, they “can’t get into Zone 5” during a 30-second Zone 5 interval. It’s all I can do sometimes not to ask these folks, “Did it not cross your mind that if you’re running literally as hard as you possibly can, then you’re probably not in Zone 2, or that if your pace is consistent and your heart rate is changing, it’s probably your pace that’s telling you the truth about your intensity, not your heart rate?” But that would be mean.

Heart rate doesn’t have a monopoly on measurement problems in endurance sports, though. I’ve heard a number of athletes express genuine worry that they’re losing fitness, despite feeling fantastic and crushing their workouts, because their Chronic Training Load (CTL) score on TrainingPeaks has declined. CTL is a mathematical construct—an abstract measurement of fitness, not fitness itself—yet many athletes put more trust in it than they do in the evidence of their own senses. To me this makes about as much sense as skipping dinner despite a rumbling stomach because your glucose monitor says you’re not hungry.

In all seriousness, I am equal parts fascinated and horrified to look on helplessly as growing numbers of athletes increasingly accept technological measurements as more real than the physical things they measure. An athlete who is running as fast as he can but believes he’s running at low intensity because his heart rate monitor says so is an athlete who believes his heart rate measurements are more real than his own body. Likewise, an athlete who is crushing workouts and feeling great yet believes she is losing fitness because her CTL says so is an athlete who believes that this technologically mediated variable is more real than her lived experience.

The next time one of the fitness devices or apps you use signals a potential problem, do me a favor. Ask yourself the same question I asked Kevin: If you weren’t using that particular technology and were judging your training the way all athletes did before this tsunami of gadgetry came along, would you still see a problem? If not, then there is no problem.


Last year I was contacted by a very interesting person, we’ll call him Brad, who became a professional skateboarder in his teens, then transitioned to professional snowboarding, and then made a go of qualifying for the PGA Tour (making is as far as the Nationwide Tour), and subsequently started getting into triathlon. Now in his 50s, Brad told me he aspired to reach the elite level of Ironman racing despite his age and despite a total lack of endurance training experience. He further explained that he had no interest in short-term competitive goals except inasmuch as they might helped him get to the elite level.

After taking all of this in (and you must admit it was quite a lot to take in), I told Brad that my advice for him was to proceed as if his actual motivations were flipped on their head, which is to say, as if all he cared about was training for and completing races and wasn’t at all concerned about where it all led. Before I explain why I said this, let me tell you about another triathlete, “Mark,” to whom I recently gave similar advice.

Mark is a 40-something triathlete who is chasing the goal of qualifying for the Ironman World Championship and has been held back by a comparatively weak run leg. When I started coaching Mark this summer, we decided to address his Achilles heel by completing a run focus phase culminating in an attempt to break three hours in a solo marathon time trial. A few of weeks ago, after a mildly disappointing marathon-pace training run, Mark realized he wasn’t on track to clock 2:59 on the scheduled time-trial date and asked me what I thought about delaying the attempt several weeks to give him more time to get fitter. I told him I thought this was a bad idea and urged him to stay the course, arguing that doing so would better serve the greater goal of becoming a better runner.

Fitness Building and Athletic Development

Now to explain. Both Brad and Mark were struggling to conceptualize the difference between fitness building and athletic development. We all know what building fitness is—it’s a process by which progressive training is used to stimulate physiological adaptations that increase an athlete’s performance capacity. This process is distinct from the process of becoming a better athlete, which is what athletic development is all about. It goes without saying that you can’t become a better athlete without getting fitter, but the two phenomena operate on different timescales. An athlete cannot build fitness uninterruptedly for more than 24 weeks, give or take. Athletic development, by contrast, can continue for years and indeed requires years to complete.

It’s not a perfect analogy, but the relationship between building fitness and athletic development is similar to the relationship between recovery and building fitness. The physiological adaptations that serve to increase fitness are largely extensions of acute post-exercise recovery processes. Similarly, athletic development is, to a large degree, an extension of fitness-building processes. But here’s the key: You can’t just keep getting fitter and fitter by imposing ever greater recovery needs through larger and larger training stresses. The body needs a reset every now again, during which period some hard-earned fitness is voluntarily given away so the body can achieve a deeper level of recovery than it can during times when fitness gains are actively pursued.

Imagine a runner who completes a well-designed, progressive, 14-week half-marathon training program, races a half marathon, and then takes it easy for two weeks before repeating the same 14-week half-marathon program and racing a second half marathon. I can all but guarantee this runner will perform better in the second half marathon than in the first. Why? Because although the training is the same, the runner is different. By virtue of having gained a lot of fitness in the first training cycle, and having given up only some of it in the following rest period in exchange for deep recovery, the runner will start the second training cycle at a higher performance level than they did the first and will therefore complete it at a higher level than they were at when they completed the first.

Exercise scientists typically measure fitness through inputs. Commonly used measures of endurance fitness such as Training Impulse (TRIMP) and Chronic Training Load (CTL) are calculated as rolling averages of recent training volume and intensity. By such measures, therefore, a runner who completes the same 14-week training plan twice will attain the same level of fitness at the end of each. Yet we know the hypothetical runner in the example I gave above will be a better runner at the end of the second cycle, and that, in a nutshell, is the difference between building fitness and athletic development.

It’s also why even the athlete who only cares out long-term development should focus on short-term fitness just as much as the athlete who can’t wait for the next racing opportunity. And it’s why our hypothetical runner is better off completing two separate training cycles separated by a rest period over the next 30 weeks than trying to develop at a sustainable rate over that same period, and why I told Brad to train for and compete in two to three Ironmans a year even though he had no chance of achieving the sort of results he dreamed of in the first several, and why I told Mark to finish what he’d started with his run focus phase and move on.


Injuries are the bane of the runner’s life. More than any other impediment, they thwart the efforts of runners to build fitness and achieve competitive goals. For this reason, injury risk management is a critical component of the training process. If there is a way to reduce injury risk, you want to know about and, if possible, implement it.

A new study in the International Journal of Sports Medicine sheds new light on how manipulating your training workload over time can help you avoid injury. A team of Dutch and German researchers enlisted 23 recreational runners to keep detailed training diaries for two years. An analysis of the data collected revealed that increases in the acute:chronic workload ratio predicted injury, where acute training load (ATL) was calculated as the average of running duration multiplied by intensity over a period of one week and chronic training load (CTL) was calculated as the average of running duration multiplied by intensity over a period of four weeks. What this study found, essentially, was that when a runner’s acute training load exceeded their chronic training load by 10 percent or more, the likelihood of an injury occurring within the next two to three weeks spiked.

The phrase “keep the ball rolling” is a summation of a training philosophy shared by a lot of today’s top running coaches that relates to the study I just described. I reflects the belief that the training process should aim toward slow, steady progress and avoid sudden leaps. Of course, a runner must first get the ball rolling in order to keep it rolling, and there’s inherent risk in this critical phase. But once you’re past it, the goal is to reduce the risk associated with workload increases as close to zero as possible without allowing progress to stall out altogether. This approach works best if you generally keep your chronic training load close to the highest sustainable level, which is to say the highest level you could keep up more or less indefinitely without burning out.

This study helped me better understand something I’ve noticed about my own running, which is that I don’t get injured as much as I used to. I’ve come to think this is largely because I keep the ball rolling. In the past, I kept repeating a cycle of getting injured, taking time off and losing fitness, getting healthy again and resuming training, going after big race goals, and getting injured again. I seldom took foolish risks in ramping up my training, but I reckon my ATL was more than 10 percent greater than my CTL more often than I realized. In any case, over time I learned what my body could and couldn’t handle, what it likes and doesn’t like, and today my personal training formula consists almost exclusively of what my body likes and can handle.

Keeping the ball rolling, for me, entails doing 14 hours per week of training in 12-12 sessions as a baseline. I repeat this routine week after week, with the majority of sessions (a lot of one-hour easy runs, uphill treadmills walks, indoor and outdoor bike rides, and elliptical rides; 30 minutes of strength training every third day) never changing. What do change are the key workouts: the higher-intensity runs and long runs. These become gradually more challenging and more race-specific as I get closer to my next targeted “peak.” The training load does increase, but very gradually, which keeps me healthier than I used to be and is okay from a fitness perspective because it’s pretty high even at baseline (except when I get COVID-19 and am out for an entire month).

Although I rely mainly on experience and tacit knowledge to keep the ball rolling in my training, there are some rigorous, quantitative online tools that runners of all experience levels can use to manage their injury risk by properly managing their training load. One is TrainingPeaks’s performance management chart, which tracks acute training load (“fatigue”) and chronic training load (“fitness”) continuously as you upload your training data. Another, which we’ve told you about in previous newsletters, is PWR Lab, an app that app analyzes smartwatch data to monitor injury risk and help runners make smart training decisions to stay healthy. PWR Lab is offering a coupon code that members of the 80/20 Endurance community.

Every runner should have a collection of mantras to use as appropriate in both training and racing. Add “Keep the ball rolling” to your collection.

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