Not long ago, I read an article from Outside Magazine which mentioned an elite cyclist who eschews heart rate monitoring during training in favor of power meters because “power is objective.” While I like power meters and I think they are an important tool in our (presumably much larger) athletic toolbox, I take issue with this view.

While a power meter tells us exactly how much power the body is putting out, it doesn’t tell us a lot about how the body is arranging for that power to be produced. That’s a problem.

The body has 2 main energy systems, both of which are used in varying amounts during a bout of activity: a capability to create energy by breaking down glucose, or sugar (call this “LO gear”—producing the low-end torque needed for power and acceleration) and a capability to create energy from lipids, or fat (call this “HI gear”—producing the high-end torque necessary for endurance). LO and HI gear can be subdivided into more energy systems—ATP-PC, anaerobic glycolysis, glucose oxidation, fat oxidation, and ketosis—but I believe that the beginner (particularly the beginner distance runner) should first master the distinction between fat and sugar usage, and how to apply it concretely to training and racing.

As I’ve discussed before, our heart rate is hardwired to our stress levels (a.k.a. the intensity at which our body is operating or expects to operate), and therefore to whether we are breaking down sugar or breaking down fats—in other words, which gear we are using at any given time.

The higher our heart rate, the more we are using the LO gear necessary to produce lots of power. The lower our heart rate, the more we are utilizing the HI gear necessary to sustain activity for protracted periods of time.

This doesn’t mean that we are burning the most fats when our heart rate is lowest—it just means that the greatest percentage of our energy comes from fats. By increasing our heart rate from its lowest point, we increase the amount of fats burned until the requirements of the task (as reflected by the heart rate) are high enough that a threshold is crossed—and the body is forced to switch to sugar in order to produce the necessary power.

(My favorite way of estimating at which point your body is most likely to switch from HI gear to LO gear is Phil Maffetone’s 180-Formula.)

Consequently, the problem with the power meter is that it doesn’t tell you whether the body is getting this energy from LO or from HI. The issue is simple—and it’s the very same one you would have if you’ve ever tried to go on a long roadtrip with a car that doesn’t shift up above second gear: you’re going to run out of fuel, blow the engine, or both.

During a marathon, just like during a road trip, your success depends on how well you’ve developed HI gear. Nobody argues that power (from LO gear) is incredibly important in a marathon—I often quote Owen Anderson, author of Running Science, who (in my opinion) famously said that “the marathon is a power race.” But the ability for you to get to the finish line (which is a precursor to racing to the finish line) is predicated on how well you developed HI gear.

“Hitting the wall” is a ubiquitous experience in the running community. I myself have hit the wall a dozen times. It’s almost a rite of passage—the badge of a “true” endurance athlete. It also means that muscle glycogen (a.k.a sugar) was depleted too soon: the runner was utilizing LO gear too much, and ate through all its fuel.

(HI gear—“fat-burning,” loosely speaking—draws from a massive fuel source. A 150 lb marathoner with 12% body fat has some 45,000 calories in the tank. LO gear has perhaps 2,000.)

Supposing that 60% of entrants at any given marathon are hitting the wall—although it wouldn’t surprise me if the real percentage was far higher—there is an epidemic of runners who despite their best intentions and best efforts, either (1) have not developed HI gear well enough, or (2) do not understand how to pace themselves in order to use just enough LO gear to go fast but not enough that they bonk at the halfway mark.

How do we factor this into training?

Let’s use the most classic bit of marathon training as an example: the long run. Since the marathon is a HI gear, fat-burning race, then we have to make sure that our long run is being fueled primarily by HI gear.

Suppose that some runner X has enough glycogen stores to fuel LO gear for 14 miles. If she’s been using LO gear to fuel the majority of her efforts up to the 14 mile mark, then she hasn’t really used HI gear to run even 1 mile.

This marathoner doesn’t really have a robust, well-developed HI gear to switch to. For her, a 14 mile run and a 16 mile run are extremely different experiences. The 14 mile run can be performed well with a powerful LO gear, but as soon as she bumps the distance up to 16 miles or more, her speed will drop dramatically—particularly towards the later miles.

(Marathon pace for the elite runner is only a few seconds per mile slower than half-marathon pace. In contrast, marathon pace for the recreational runner may be a third slower than half-marathon pace).

The problem isn’t that she hasn’t trained the mileage itself, but rather that the energy system that is supposed to handle high volumes of mileage was never really developed—so when she bumps up to a mileage that requires that fat-burning energy system, she grossly underperforms relative to her expectations.

Considering how many marathoners hit the wall, I believe that most of us don’t train HI gear on most of our long runs. This doesn’t mean we shouldn’t train LO gear or run long and fast in preparation for a marathon. It means that we need a reason for doing so—and we need to know when we’re crossing the threshold. Power meters aren’t enough. We need heart rate monitors: we need a window into what’s happening inside our body in real time.