Training Impulse

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.

QED.

Unless you fell onto this blog through a trapdoor and you have no clue what you’re doing here, you know that I am a proponent of the 80/20 training method, which entails spending about 80 percent of your training time at low intensity and the rest at moderate and high intensities. This does not mean that I believe every athlete should always do exactly 80 percent of his or her training at low intensity. There are more general, non-quantitative ways of stating my core philosophy of endurance training that do a better job of getting at its essence. For example:

Intensity balance is the single most important variable in endurance training. The single most beneficial thing you can do in your training is to consistently maintain an intensity balance that is heavily weighted toward low intensity yet does not neglect high intensity. The single most common and costly mistake that endurance athletes make in training is to spend too much time at moderate intensity, way too little time at low intensity, and also too little time at high intensity.

These statements are strongly supported by both real-world evidence and scientific research, and the last of them in particular has gotten further scientific support from a cool new study published in The Journal of Strength and Conditioning Research. Conducted by a team of researchers at Belgium’s Ghent University led by Jan Boone, the study involved 11 recreational cyclists training for a mountain-climb event. Over a 12-week period, each subject trained as he or she saw fit while wearing a heart rate monitor to collect data that was then passed on to the researchers. Before and after this 12-week period, all of the subjects underwent testing to assess various aspects of their fitness level.

The main purpose of the study was to test the power of certain ways of measuring training load to predict changes in fitness. Training load is a function of both the volume and the intensity of training. Because there was a great deal of variation in the volume and intensity of the training that the 11 cyclists involved in this study did in preparation for the mountain-climb event, it was expected that there would also be significant inter-individual differences in the amount of fitness they gained. What remained to be seen was how well the four ways of quantifying training load that were being put to the test in the study were able to account for these differences.

I don’t want to get too deep into the mathematics involved. If you’d like to go deeper on your own, open up a web browser and run a Google search on training impulse (TRIMP), of which there are four competing versions. These four methods of calculating TRIMP were the specific tools used by Boone’s team to quantify training load. What’s important to know is that all four of them allow athletes to achieve equal training loads, hence equal levels of predicted fitness, through different combinations of volume and intensity. For example, a cyclist who increases the average intensity but not the volume of his training might end up with the same TRIMP score as a cyclist who does the reverse. The Ghent researchers questioned the validity of this allowance, and the results of their experiment justified their skepticism. While the cyclists did demonstrate improvements in power output at the aerobic and anaerobic threshold and in maximum power, these improvements correlated weakly with changes in TRIMP values.

In addition to tracking TRIMP, Boone’s team calculated the relative amounts of time each athlete spent at low, moderate, and high intensity. Interestingly, this data proved to be a better predictor of fitness gains. In particular, those athletes who spent the least time at moderate intensity exhibited the greatest improvements in power output at the anaerobic threshold. Combining the data on training intensity distribution with the data on training load accounted for almost all of the inter-individual variance in fitness improvement. The authors concluded that the TRIMP formulas should be modified to factor in training intensity distribution.

The lesson for you, as an athlete who cares most about your fitness improvement, is that increasing your training load won’t do you a heck of a lot of good unless you’ve got your intensity balance right. By taking some of the time you’re currently spending at moderate intensity and moving most of it into the low-intensity bucket and the rest into the high-intensity bucket, you will feel and perform better without increasing your training load. And by continuing to apply the 80/20 rule as you add minutes to your weekly training, you will ensure that those minutes aren’t partially wasted.

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