Heart Rate Training For Trail Running 101

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There is a thin line between love and hate when it comes to running training. You can be out there, frolicking through meadows, thinking you are all about rainbows and unicorns. It might feel easy, your soul filled with joy and your stomach filled with Cinnamon Toast Crunch (a synonym for “joy”). But while your psychology might be blaring the love siren, your physiology might be quietly conducting a hate symphony.

Every life-affirming uphill might be a bit harder effort than you think. That little bit of extra exertion each day adds up over time. The stress hormone cortisol may combine with musculoskeletal breakdown and hormonal deficiencies to slow down the love train in your head. Then, all of a sudden, it comes screeching to a halt.

Because of a little bit too much trail love, your body decides it hates you. Whether it’s injury, overtraining, stagnation, or burnout, the biggest workout becomes just walking up stairs. And a good, loving run? Impossible.

That story plays out over and over for trail runners. Training based on perceived exertion can be effective, but it also can be self-destructive if your brain hasn’t properly calibrated effort. Heart-rate training provides an objective method to quantify exertion and help control the love-hate narrative.

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Two female trail runners check their running data on their smart watches

Heart Rate Training Basics

During sub-maximal running, oxygen consumption increases and blood flow needed for working muscles goes up proportionally. Cardiac output thus increases to respond to the demand for blood in the form of higher stroke volume (amount of blood pumped per beat) and heart rate (beats per minute). This reads like an erotic novel for vampires.

Other variables play a role too. Heat increases blood flowing to the skin for cooling, raising cardiac output. Dehydration reduces blood volume, requiring an increase in heart rate to make up the difference. Someone jumping out of the bushes and yelling “I AM A VAMPIRE AND I LIKE THE ARTICLE YOU WROTE” increases heart rate too.

In other words, heart rate integrates lots of complex variables to tell a story about how hard your body is working. It’s not a perfect metric, though. There is a lot of variance depending on external factors, and a few-beat difference is probable noise, not signal. But generally, understanding how your unique cardiac physiology responds to training can let you calibrate your perceived exertion. That way, when you think a run is a beautiful love story, you’ll know you are right even when you aren’t wearing a monitor.

Calibrating Your Physiology

Not every heart is created equally. Some people are hummingbirds who can hold extra-high heart rates for hours. Others are whales who can sprint their brains out and not have their heart rate rise that much at all. The variance is due to heart size, individual background, and genetics.

Because everyone varies, it’s essential to calibrate your heart rate zones prior to using a heart-rate monitor for training. Some methods use set formulas based on age, but those can be inadequate if you don’t fall smack-dab in the middle of the bell curve. A whale that trains with an equation will overdo it, while the hummingbird will probably have to walk to stay within range.

There are two main methods to calibrate heart rate (plus lots of others that you can Google if you want to go deep into the rabbit hole). First, you can do a max heart rate (MHR) test. There are lots of ways to do it, all of which come with a world of delightful discomfort. I prefer the 3-minute hill test.

After an easy 15-minute warm-up, find a moderate-but-not-steep grade that averages around 8%, and run up once moderately hard (an effort you could sustain for 20 minutes or so). Run down to the base, then do it again, but this time, go hard the last 30 seconds, finishing with your hands on your knees. Finally, do that one more time, but go really hard from the beginning with a final all-out sprint of 30 seconds. The highest recorded number will probably be within a few beats of your MHR.

To take it up a notch, get MHR calculated with a physiologist in a lab, running on a treadmill until you upchuck your Cinnamon Toast Crunch. Using MHR is risky because how it relates to heart rate zones is heavily dependent on fitness level and background.

A second method that is usually preferred is a lactate threshold test. One protocol pioneered by Coach Joe Friel is to do a 30 minute time trial around the hardest effort you can. At 10 minutes, lap out your watch so that you have the average of the last 20 minutes of the tempo. That average approximates your LTHR (lactate threshold heart rate). Many athletes can also use average heart rate from a 10k race. This test is also most accurate in a lab, combining finger pricks to measure blood lactate and respiratory monitoring.

If you have a slide rule and abacus handy, you can also use the Heart Rate Reserve (HRR) method. Each morning for a few days, take your heart rate right after you wake up to find a resting value. Subtract that from your MHR to find your HRR. Next, using the Karvonen formula, you can calculate zones. I was never great at my minuses, so we’ll skip this one (all the calculations are similar to those for MHR, just with different percentages).

how to heart rate training trail running roche
An old male happy sportsperson standing and resting after the run.

What It Means

None of those numbers mean much on their own. Some world-class athletes have relatively low max heart rates and LTHRs relative to their competitors. It’s not the number that counts, it’s what you do with it.

On top of that, how you measure heart rate matters. Wrist-based optical heart rate is not reliable for many athletes (as outlined in this 2017 Research Letter in the JAMA Cardiology journal), so don’t rely on that number or you may think you are in A-fib or a coma and need a hospital trip. A chest strap is ideal, with the caveat that minor variations day-to-day are likely unimportant. A 155 heart rate on Tuesday and a 158 heart rate on Wednesday might not be indicative of a meaningful difference. Always remember that it’s still an art, not a science.

How to Use It

Knowing that granular differences in heart rate are unlikely to be statistically significant, the best way to use your heart rate is as a general guideline to understand your exertion, rather than as a hard-and-fast rule for pacing. There are two main thresholds to understand to optimize your heart rate training.

First, aerobic threshold (AeT) is the exertion range when your body switches from burning primarily fat to primarily glycogen, increasing respiratory rate and raising musculo-skeletal and neuromuscular fatigue with it. AeT is not a magic number, but in general, easy running should usually be below it (though not always), and recovery running well below it, since going too hard may decrease aerobic adaptations and raise injury risk. Since the bulk of running progression comes from aerobic development, calibrating your “easy” is indispensable.

AeT will usually be around 75-80% of MHR, with variance based on background. So if a runner has a max of 180 beat per minute, AeT is probably around the range of 135-144. That runner probably doesn’t want to go too far over 140-145 on easy days.

For LTHR, it will usually be 85 to 88%, with individual variance. So if that same runner has an LTHR of 165, it’s a range of 140-145.

You can go above that number a bit, or any amount below it. In addition, advanced runners will likely want to subtract a few more beats since it could have them going too fast. The main key is using a calibration of AeT as a systems-check for your perception of effort. If you’re way over the suggested number on your easy runs, your theme song might need to be the Backstreet Boys’ “Quit Playing Games (With My Heart).”

The next thing you need to know is lactate threshold (LT), the tipping point when your body produces more lactate than it can use and waste products accumulate without being cleared. It’s a great metric for calibrating workouts, since most quality, shorter workouts should be above LTHR. You generally want to avoid spending too much time in the “grey-area” well below LTHR but well above AeT where you’re going moderate, but not hard enough to spur adaptations.

LT will usually be around 91-94% of max heart rate, though there are different numbers online so the best way to know for sure is just to do a field test, as outlined above. This could really change with fitness level, so if you’re using MHR, you probably don’t want to use this number for anything beyond a general understanding of effort.

If you used LTHR, you already know your LT. Woohoo! This is like getting a snow day on the morning of an exam.

Heart Rate Training Implications

There are nearly infinite ways to use heart rate training. You could use it in the bedroom if you really wanted to. But I prefer to keep it simple, just focusing on aerobic development and calibration of easy days.

Around 80% or more of your running should be easy. Make sure you are following that universal guideline and not going too far above AeT (especially on uphills or in groups) with occasional heart-rate monitor spot-checks. However, even on easy days, you can exceed AeT on short strides where you develop your speed endurance.

For workouts and races, run by feel for the most part. A machine telling you what to do is less useful when there are error bars built into the measurements. You wouldn’t want to use the speedometer as the sole method to tell you how fast to drive if all you knew is that it was within 5 miles per hour of your actual speed. Heart rate is best used as a tool to calibrate effort, rather than to calculate exact pace.

Occasionally, bring your heart rate monitor along if you want to make sure you are going hard enough on short intervals above LT, and not too hard on tempos where you shouldn’t exceed LT (and to recalculate LT after a training block). Unlike cycling, where athletes are defined in races by whatever power they can put into the machine, running has many more variables and relying on HR (or any other metric) could sell you short.

Most importantly, remember that everyone is different. If the heart-rate monitor stresses you out, burn it in a ceremonial pyre. If the data makes your training feel more purposeful, you can hoard it in a big old pile on your computer.

Just hold onto the biggest rule of all–you are not a set of numbers, you are a unique person, and never stray too far from the feelings that define your humanity.

David Roche partners with runners of all abilities through his coaching service, Some Work, All Play

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