Why Heart Rate And Pace Fail On Trails

Heart rate or pace?

For years, runners have debated which is better, but both measure different parts of the same effort.

In this episode, CTS Director of Coaching Cliff Pittman explains why RPE is the missing piece, how perceived effort integrates cardiovascular strain, metabolic demand, and mechanical loading, and why learning to trust it can improve both training and race-day pacing.

HOST

Cliff Pittman is the Coaching Development Director at CTS, leading the Ultrarunning and Cycling Coaching staff with a specialty in guiding athletes from first-time ultrarunners to elite competitors at races like Western States 100, Leadville 100, and the Triple Crown of 200s. A competitive trail and ultra athlete himself, Cliff brings firsthand experience and a rare ability to turn complex training science into simple, actionable coaching.

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Find more free resources here: https://trainright.com/blog/

Resources:

• https://drive.google.com/file/d/10eFC_tdi8c_V6_S-m28YLCfp_qmWzZ3r/view
• https://borgperception.se/wp-content/uploads/2019/10/The-Borg-CR-Scales-Folder.pdf

Transcript

Heart Rate Vs Pace Is A Trap

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Runners argue about whether to prescribe training and measure effort off heart rate or off pace. And the argument is pointless because they measure two different things. Heart rate measures cardiovascular strain, pace measures output. So neither one can tell you everything about your effort on its own. You need both together. And you need something that can combine them in real time. And that's RPE. And it's why RPE should drive your effort instead of either one alone. I'm Cliff Pittman, Pro Ultrarunning Coach, and Director of Coaching at CTS. I coach athletes from first-time ultrarunners to professional and world-class athletes. And by the end of this video, you will know exactly how heart rate, pace, and RPE fit together instead of competing with each other and how to actually use that on race day. Now, let's start with what heart rate actually measures because that's where this whole problem begins. Heart rate measures cardiovascular strain. And that's the only thing it measures. It doesn't tell you anything about metabolic cost or mechanical loading. And cycling is actually where this limitation became obvious. Once power meters showed up, coaches could finally hold power output constant and watch what heart rate did over the course of a ride. What they found was that a rider could hold the exact same power output for an hour and heart rate would still climb, drifting from zone two into zone three, while the actual output never changed at all. So same metabolic cost, but rising cardiovascular strain. Now that gap is the reason cycling moved off a heart rate and onto power as its primary training metric, because it proved heart rate couldn't be trusted to represent output on its own and shouldn't be the target for exercise intensity prescription. Now it's still a valid measure of cardiovascular stress, but not metabolic or mechanical. Cycling was able to solve part of that problem with better hardware. A power meter measures mechanical output directly and precisely since watts or watts regardless of wind, grade, or fatigue state. It isn't measuring metabolic cost itself the way that ventilation or gas exchange would in a lab. Because pedaling efficiency stays remarkably stable for a given rider, power output becomes an extremely reliable proxy for metabolic costs, precise enough that a coach can prescribe intensity with something close to surgical accuracy. Running, unfortunately, doesn't have that same option. Running power estimates aren't a true mechanical reading at all. They're modeled approximations built from accelerometer data and assumptions about running economy. And those assumptions break down exactly where our sport gets interesting. Technical terrain, steep grades, and fatigue-altered mechanics. Pace, even great digested pace, is also not reliable on trail because technicality changes what a given pace can actually cost you from one mile to the next on the same run. Even within its own domain, heart rate has problems worth knowing about. Core temperature, caffeine, nervousness before a big effort, hydration status, elevation, and accumulated fatigue can all shift your heart rate independent of how hard you're actually working, which means the same effort can produce different numbers depending on conditions that have nothing to do with the actual training intensity. And the sensor itself matters. Most risk-based monitors use a technology called something that I won't even try to pronounce, but it estimates heart rate from changes in blood volume detected through light absorption rather than directly counting electrical heartbeats the way a chest strap does.

Why Heart Rate Drifts And Misleads

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Now that's a reasonable estimate most of the time, but it's still an estimate layered on top of a metric that only covers one domain to begin with. So in ultrarunning, we don't get to solve the heart rate problem the way that cycling did. And what we have instead is RPE. Now I want to make the case for why that's not a constellation prize. But before we get there, we need to define our terms clearly because the rest of this video depends on understanding how these three things differ. Metabolic cost is the energy your body is burning to produce the work. It's the fuel side of effort and how hard your muscles are working to generate force and what substrate, carbs or fat, is being used to do it. Cardiovascular strain is the load on your heart and circulatory system, since your heart's job in this context is to pump oxygen-rich blood to working muscle and clear out the byproducts of that work. Heart rate is a direct measurement of this one system and nothing else. Mechanical loading is the physical wear and tear on your muscles, tendons, and connective tissue from impact and repeated force production. And every foot strike, every downhill stride, every mile on technical train adds to it. This is durability territory, and it happens to be the one domain none of our watches or metrics measure directly. So we're talking about three genuinely different things: fuel burned, strain on the heart, and physical wear on the tissue doing the work. A singular number can't capture all three at once, which is exactly why heart rate, built to reflect only one of them, runs into trouble the moment you ask it to do more. Here's a scenario that shows why this matters practically. Now imagine you're on a climb in a cold rainstorm at altitude six or seven hours into a hundred mile race. What should your target heart rate be in that exact moment? Now there's no formula that answers that question because the conditions are stacking cold, altitude, and accumulated fatigue on top of whatever your legs and your lungs are already actually doing. But you already know what a six out of ten feels like in any weather, at any altitude, at any point in a race. That's the entire advantage RPE has over every other metric. It doesn't need the conditions to cooperate in order to still be accurate. The same problem shows up in a more common scenario too. So let's go back to the heat example from the top of this video now that we've defined our terms. When it's hot, your cardiovascular system is doing two jobs simultaneously. It's delivering oxygen to working muscles and it's shunting blood to the skin to help dump heat. And both of those demands show up at the same singular number on your wrist. Heart rate climbs, and it climbs for a legitimate

Three Strains One Effort Signal

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reason. Because your cardiovascular strain genuinely is higher at that pace than it would be on a cool morning. But your threshold session was never targeting cardiovascular strain in the first place. It's targeting a specific metabolic stimulus, the one that drives the mitochondrial and lactate clearance adaptations you're actually trying to build. And that stimulus is dictated by output, meaning pace, on a controlled surface, not by heart rate. If you slow down to bring heart rate back into a certain range, you end up reducing the actual training stimulus below target in order to manage a number that rose for a reason that has nothing to do with the adaptation you're chasing. You'd finish the session with a comfortable heart rate and an undertrained metabolic system, which is the exact trap. Heat makes the wrong metric look like it's telling you to back off. And athletes who don't understand why end up undertraining their threshold work all summer long. This is exactly why RPE earns its place. Because you can be in genuine heat, cardiovascularly strained, watching a heart rate number that looks wrong, and still know through RPE that the pace and the effort are correct. RPE lets you separate from my heart is working harder because of the heat to my threshold effort has changed. A distinction heart rate simply isn't built to make. Late in an ultra, your legs can lose the ability to produce the force they were producing early in the race, independent of anything happening in your cardiovascular system. That's a mechanical failure, and heart rate has almost nothing useful to say about it, because you can still be moving at a heart rate and effort level that looks aerobically manageable on paper while your legs are the actual limiter. So heart rate ends up diverging from the real target in two different directions across the sport. In the heat, it reads high while the actual training stimulus hasn't changed. Late in an ultra, it can read low and controlled while the legs are failing underneath. Both cases come from the same root problem. Heart rate is telling you about one system, and we've been asking it to tell us about all three. This is the problem RPE was actually built to solve and is worth understanding where it came from. Gunnar Borg introduced the original 6 to 20 scale in the 1960s, deliberately anchoring it to heart rate so that multiplying the number by 10 would land close to expected beats per minute for most people during steady state exercise. It was built specifically as a bridge to cardiovascular strain. In 1982, Borg published a follow-up in Medicine and Science and Sports and Exercise, introducing the category ratio scale, the 1 to 10 version, most of us use today, and this version was designed to capture respiratory and metabolic strain more directly rather than heart rate alone. Now that shift matters because the modern RPE scale was never built to stand in for one system, it was built to reflect the body's combined internal state, pulling signal from multiple systems at once. Sports scientist Samuel Marcora's research on effort and exercise tolerance along with the broader psychobiological model built on his work supports this directly,

How RPE Works And How To Train It

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showing that perceived effort isn't just a passive readout of physiological strain. It functions as a primary regulator of pacing behavior, integrating input for muscle afferents, cardiovascular strain, and central drive simultaneously in real time. What you end up with is one number generated continuously by a system built to synthesize inputs no external sensor combines on its own. Now let's make this practical because this is the part that actually changes how you train and race. Here's the CTS model. We use a 1 to 10 scale mapped to five training zones. And this isn't a subjective add-on sitting next to your data. It's the tool that runs the session when pace and heart rate can't be trusted. And between heat, terrain, and duration, that's most of the time in this sport. Alright, zone one is RPE4, which is a conversational effort you could hold for hours without strain, and this is where recovery runs and true easy aerobic work live. Zone two is RPE five to six, comfortably hard, but sustainable, where you could still hold a conversation in short sentences. This is where the bulk of your training volume, which should sit, since it's your aerobic durability zone. Zone three is RPE 7, which is moderately hard, with breathing noticeably deeper and full sentences becoming difficult. And zone four is RPE 8 to 9, hard and threshold adjacent, sustainable only for a defined and limited duration. This is where repeats and threshold specific sessions live, including that hot day threshold run from the top of this video. And talking beyond a word or two is essentially not possible here at this effort. Zone 5 is RPE 10, maximal to near maximal effort, used for short intervals, hill repeats, and is unsustainable beyond a few minutes. This is where race day pacing becomes the real application of everything we've covered. In a road marathon, you can pace largely off pace and heart rate because the domains stay aligned for the duration of the effort. But in an ultra, that alignment breaks down somewhere in the race. And it breaks down sooner if it's hot, sooner if the train is really technical, and sooner if durability happens to be your limiter to begin with. The athletes who pace ultras well have trained their internal calibration to point where RPE checks are automatic and accurate, so they're not waiting for their watch to tell them that they've gone out too hard or too easy. They feel it in real time across all three domains at once, and they adjust before the data would have caught it. That calibration is a trainable skill, built the same way any skill is built through repeated, deliberate practice, checking perceived effort against your data and training until the two are reliably

Race Day Pacing And Next Steps

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telling you the same story. That's the part of why we structure long runs and training races the way that we do, since it isn't just physiological adaptation, it's teaching an athlete to trust and accurately read their own internal signal, so that on race day, when heart rate and pace both stop being reliable, they still have a tool that works. Here's where this lands. Heart rate measures cardiovascular strain and it doesn't measure metabolic cost or mechanical loading. In the heat, that gap shows up as a number that looks alarming while the actual training target hasn't changed. Late in an ultra, it shows up as a number that looks fine while the legs are failing underneath. And both cases come from the same blind spot pointed in opposite directions. RPE is the tool built specifically to integrate metabolic, cardiovascular, and mechanical strain into one real-time signal, which is exactly why or exactly what a sport with this many variables actually requires. Use your data, test your thresholds, and track your green adjusted pace, but train your RPE like you train your fitness. Because on race day and on that 95-degree threshold run, it's the one tool still telling you the truth after everything else has started to lie to you. If you want to understand what that mechanical piece becomes the deciding factor late in a race, I've got a full breakdown of the durability as a performance variable. Link is on the screen. And if you want coaching that puts this into practice for your specific event, visit trainrite.com to connect with a CTS coach and subscribe for more coach driven ultra running education.