Tuesday, May 17, 2022

Getting the cool-down right

Several years ago I wrote about getting the warm-up right, and I still believe that many runners neglect the warm-up to their own detriment. But after you work out, what about the cooldown (or, less commonly these days, a “warm-down”)? How long, how far, and how fast should a cooldown be? Getting to a place where we can answer these questions is going to require getting a framework in place where we understand why you should cool down in the first place. 

Understanding the reasons for doing a cool-down after a workout

Like much of the accumulated lore of running, the common rationales for doing a cool-down at all contain a healthy mix of physiology, bro science, folk wisdom, and true coaching wisdom. Allegedly, the cooldown is supposed to gradually reduce your heart rate, pump “lactic acid” out of your muscles, and condition your legs to running while tired. Failing to cool down is again allegedly supposed to make you feel more sore the next day, and harm your recovery capabilities. Needless to say, each of these rationales has a hefty amount of myth alongside perhaps a kernel of truth. 

Instead of trying to consider and analyze each of these claims in turn, I think it’s better to break the question of “why do a cooldown” into smaller parts: who is doing the cooldown, what workout does it follow, and what training goals are we trying to accomplish, both in this workout and in the longer-term training plan. 

Younger runners should cool down differently than older runners

First off, your level of running experience has a big impact on what kind of cool-down is appropriate. Let’s start at the very young end of the spectrum–I spent a season at Edina as the head middle school cross country coach. That fall, I was working with runners who were young both chronologically and in terms of their training age–very few had ever run before, and certainly not in any kind of structured way. 

For young, inexperienced runners, the primary benefit of a cool-down is structure: it helps reinforce the notion that practice generally, and workout/race days specifically, are comprised of a structure of things (e.g. team meeting, warm-up jog, drills, strides, intervals, cool-down, and a check-in before heading home) that we do every time, no matter what. Building up comfort and familiarity with a structure for training is extremely useful, because later on in your running career this structure can help pilot you through challenging circumstances, like extreme anxiety before a big meet, or disappointment and frustration after a poor performance. 

Another benefit for young runners is that cool-downs provide more aerobic running. When you early on in your running career, more aerobic mileage is pretty much always beneficial, regardless of the form in which it’s delivered. However, younger runners are also generally going to be doing less mileage than older runners, so this pushes in the direction of doing less total cool-down. 

Older runners get less aerobic benefit from a cooldown

Once you are a few years into your running career, you’ve already gained a lot of the low-hanging fruits of improvement from general aerobic running. By “general” here I just mean “non-specific”—jogging around the infield after a track race isn’t really stimulating any specific aspect of your aerobic fitness. This is fine if you’re 16, but if you’re 26 and have been training seriously since high school, your body will not respond much, if at all, to these kinds of general aerobic stimuli. You need specific, targeted aerobic stimuli to improve, which generally means long, fast running–the kind you’d do in the workout proper, as opposed to part of the cooldown afterwards. 

Injury-prone runners should do a shorter cool-down

If you are trying to limit a runner’s mileage for injury-related reasons, the cooldown should be the very first thing you should reduce (or even eliminate). First, as discussed above, it is only providing a general aerobic stimulus, as opposed to a race-specific stimulus, so (even for younger and less-experienced runners) its importance is secondary to that of race-specific training. Second, when you are cooling down, you are doing so on tired and damaged legs. In that sense, you are encountering additional mileage when you are at your most vulnerable. The injury risk associated with cooldowns can be ameliorated to some degree by doing it slowly, but again–this makes it merely a general aerobic stimulus. 

What kind of workout are we cooling down from?

Much of the traditional justification for cooling down has to do with the cool-down’s purported ability to remove metabolic byproducts (“lactic acid” being the often-misused label here) from your muscles and from your bloodstream. I suspect there is some truth to this, so it’s worth considering the metabolic milieu inside your muscles and bloodstream after a workout.

Do longer cooldowns after faster interval workouts

If you’ve just completed a fast interval session (or “HIIT” session as you might hear it called elsewhere), there are two distinctive differences in the physiological state of your body. First, your muscles are bathed in metabolic byproducts like hydrogen ions, lactate, and calcium ions, plus probably some smashed-up bits of muscle that were damaged during the workout (creatine kinase, a biomarker of muscle damage, increases markedly following fast interval workouts). Second, your blood contains many of these same metabolic byproducts, because they are diffusing from your muscles out into your bloodstream. 

The cooldown can function as a way to keep your blood circulating, which should help at least some of your body’s recovery systems do their job. Excess lactate, for example, can be metabolized by the liver and the heart, so keeping blood flow going should accelerate this process. Indeed, research bears this out: “active recovery” following high-intensity intervals increases the rate at which lactate levels drop over the 10-15 minutes following a workout [1].

So, there’s a good justification for cooling down after fast interval sessions that generate a lot of metabolic byproducts. However, what about workouts that don’t generate a lot of metabolic byproducts, like Daniels-style cruise interval workouts, marathon pace workouts, and Canova-style long fast runs? In this case, much of the justification for a longer, traditional cooldown falls apart. These sessions are already a big, specific aerobic stimulus to your body, so the added benefit of a bit of slower running is minimal or nil. It’s wiser to do a short jog of just a few minutes at a very easy pace to cap off these kinds of workouts, then call it a day. This will keep your blood flow up for a few more minutes but won’t put any extra stress on your legs, which already sustain a lot of damage on high-volume aerobically-oriented training sessions. 

What are we trying to accomplish in training, both short-term and long-term?

The final thing to consider when choosing a cool-down is what our training goals are, both for today’s workout and over the long term. When it comes to short-term goals, getting some aerobic stimulus from a cooldown is going to be more relevant when the focus of the day’s workout is not on aerobic fitness. This might seem paradoxical, but keep in mind the distinction made earlier between a specific stimulus to aerobic fitness (like a 6mi aerobic threshold run) versus the general aerobic stimulus of a cool-down. After an aerobic threshold run, the marginal aerobic benefit of a cool-down is negligible, but that’s not necessarily the case after a session of 600m repeats at 3k pace, whose focus is not on purely aerobic fitness. 

In terms of long-term training goals, we should also consider things like mileage goals, managing injury risk, and whether we are focusing on building overall fitness through consistent daily training, or are trying to generate a big, specific stimulus to the body, then recover well afterwards. For a beginner or intermediate runner early on in a training cycle, hitting a weekly mileage total is going to be a much bigger priority than a more experienced runner who is six weeks out from a marathon and has just completed a long fast run at 95% of marathon pace, and hence the beginner would do a longer cooldown (if necessary for a mileage goal) than the experienced runner. 

Long cooldowns usually do not make sense

Among runners trying to hit certain mileage targets, you’ll commonly see cooldowns of 5-8 miles after workouts or races. In most cases, I am against this. First, you’re putting quite a lot of volume into your legs when they are at their most vulnerable. Second, workout or race days are trying to accomplish a different thing in training than an easy run, and I think it’s generally a mistake to conflate these. 

In most cases, I’d rather have the athlete in question do a double the same day, or adding more mileage to other days of the week. This creates some separation in the training stimulus between sessions, though there are some cases where a moderate-length cooldown (maybe 4 or 5 miles) makes sense, especially for higher-mileage runners who are doing early-season track races. I illustrate an example of this below. 

Another alternative I like more than long cool-downs are post-race workouts. I would have sworn I wrote a blog post about this, but it looks like I haven’t! I put these in a different category than mere cool-downs, so we’ll leave that topic for another day (and hopefully I will remember to add a link here once I’ve written that post).

Illustrating principles of cooling down with some examples

The conceptual arguments above might seem a little abstract without some concrete examples of runners of various experience levels at different points in their training cycle and in their running career. These are fictionalized, composited, and semi-hypothetical versions of real runners I have worked with in the past, with names and some identifying details changed: 

Athlete: Matt, 16 year old male, 9:50 3200m PR 

Workout: 5x1200m at 96-102% of 5k pace

Cool-down: 10min easy run + 10min easy jog barefoot on grass or turf

This workout is a pretty typical “specific endurance”-type session you might do in April of an outdoor track season ending in June. This athlete is young and talented, and with no history of significant injury, so our eye should be towards setting up a foundation for better performance in the future. That’s the rationale for (1) doing a somewhat longer cooldown, and (2) doing some of it barefoot, as a way to get some ancillary benefits for foot strength. 

Neither of these things are likely to affect this runner’s performance this year, but can be a foundation to build on later. There’s a slight aerobic benefit to keeping the pace honest at least for the non-barefoot part of the cool-down, and since the workout is going to generate a decent amount of lactate, we do want a cool-down of a reasonable duration to start reprocessing that and other metabolic byproducts. 

Athlete: Alex, 25 year old female, 1:20 half marathon PR

Workout: 11 sets of (1km at 100-103% HM, 1km at 90% HM)

Cool-down: 8-12min easy jog

This is a half marathon-specific session that you might do six or eight weeks out from your target half. It’s a fairly high volume session already, and though it will generate a bit of lactate especially in the final few fast kilometers, the amount will be pretty small compared to Matt’s workout. The intensity of the cooldown is immaterial—the workout itself is already a big aerobically-dominated stimulus, so this can be as slow of a jog as desired. The duration can be tuned a bit to help hit mileage goals, but on a session like this I definitely would not do more than 15 minutes, even for a very high mileage runner.

Athlete: Gabby, 18 year old female, 4:54 1600m PR

Workout: 800-600-500-300-200-200 at 2mi→800m pace 

Cool-down: 12min easy on stationary bike or 8-10min easy on AlterG

Gabby is a very talented runner with a recent history of stress fracture and some nagging pain in her ankle. This is a session we might do a few weeks out from a major track race. Since this would be at the end of her senior year of high school, the focus here is entirely on avoiding any extra biomechanical stress on top of the workout itself (which is itself designed for maximum benefit / minimum volume*intensity). 

The stationary bike provides the ‘active recovery’ needed to reuptake metabolic byproducts, but without the biomechanical stress of extra mileage. Notably, the stationary bike is included here because this is something Gabby would be familiar with, having used it to cross-train during her injury. You don’t want to have a runner suddenly start using the stationary bike right before a big meet if she hasn’t been using it recently. If available, an AlterG would be a fantastic and more running-specific alternative. 

Athlete: Trevor, 38 year old male, 3:20 marathon PR

Workout: 16 miles at ~90% of current marathon fitness, or ~80% of current 5k fitness 

Cool-down: None

Trevor runs 40-55 miles a week, has a busy schedule, and wants to break 3:00 in the marathon. To reach his goal, he needs to lean on marathon-specific work to a greater degree than his higher-mileage peers. This is a classic “long fast run,” which we might do around 12 weeks out from the marathon. Trevor would already be comfortable doing 19-20 miles at an easy effort, and will have done previous long fast runs at this same speed but 10, 12, and 14 miles in duration, spread out over the past several weeks. After finishing this run, Trevor’s legs have received a pretty good thrashing, and his cardiovascular system just got a huge aerobic stimulus, with little to no lactate generation. Here, the benefits of a cool-down are pretty much nonexistent—so we just don’t do it! 

Athlete: Natalie, 22 year old female, 2:07 800m PR

Workout: 3x450m at 98-102% of 800m pace w/ 6-10min rest

Cool-down: 15-20min very easy jog

This is a classic 800m-specific session that is going to flood the body with lactate. To set ourselves up for recovery, we want to get a good solid cooldown at a very light intensity. There’s little aerobic benefit to be had here (and aerobic fitness is not at all the focus of this session) so the speed does not matter on the cooldown. Fast mid-distance runners like Natalie may nevertheless prefer to cool down at a faster pace, just because the mechanics of running, say, 7:30 mile pace feel more comfortable for her than 8:30 or 9:00 pace.

Athlete: Christopher, 13 year old male, 5:35 1600m PR

Workout: 10x300m at 5k-ish effort, 100m walk or jog recovery

Cool-down: 12min easy run on track with faster teammates

Workouts for middle schoolers can’t usually be structured as well as those for high schoolers and beyond. Beyond “start this workout at 5k effort and try to get faster,” there’s little benefit in trying to prescribe specific paces, since middle schoolers often aren’t able to pace themselves very well and you have a lot more variation in running ability with this age group. It’s better to choose workouts that naturally encourage consistency.  300m reps with a 100m recovery are one of my favorites, since more fit runners can jog the 100m and less fit runners can walk it (if you make them all jog the recovery, slower runners can’t do the 300m much faster than their recovery jog!). The “cool-down” here for Christopher is really part of the workout. By running with faster teammates, probably at a pace faster than he’d go even for his easy runs, he gets a much-needed additional aerobic stimulus that will help him improve over the long-term. 

Christopher also benefits from the structure of doing a cool-down. Even if he just slogs around at a barely-above-walking-pace talking with his friends, he’s still learning that all workout (and race) days have a common structure: a warm-up run, mobility drills and strides, the workout or race itself, and a cool-down jog. When Christopher is a senior in high school and is running at the state cross-country meet, being able to ‘run the script’ is going to be immensely useful for dealing with the stress and anxiety of a big meet.

Athlete: Joe, 28 year old male, 31:40 10k PR

Workout: 20mi at 95% of target marathon pace

Cool-down: 4min very easy jog

This long fast run is one of the final ‘capstone’ workouts you’d do during a block of marathon training, with this run happening 3 or 4 weeks out from the goal race. Joe is a talented athlete who’d be looking to run close to 2:25, and he’s comfortable with 90+ miles a week, but even for him, this is one of the toughest sessions he’ll do during this training cycle. The cooldown here serves no real physical benefit, but a few minutes of easy jogging serves as a good opportunity to psychologically decompress after nearly two hours of focused, high-intensity running. 

Athlete: Camille, 17 year old female, 10:50 3200m PR

Workout: 1600m indoor track race

Cool-down: 30-40min easy to moderate w/ 5x25sec strides in last mile

Camille is a long-distance specialist who has built up to 50-55 miles a week over the winter, and is doing an early-season indoor track race. A warm-up and typical cool-down is only going to total a few miles, and only one mile at a fast pace isn’t much of a stimulus to the body in any direction. This is one case where a long(er) cooldown makes sense. Notably, this would also be a good opportunity for a more formal post-race workout—something like 6x3min at the anaerobic threshold with 50-60sec jog recovery would be a good choice too. 

Recap: A principled approach to cooling down

Are cooldowns going to make or break your training? Probably not. Still, it’s worth having a principled approach to how you structure them, and to understand when and why you do them. John Kellogg once likened the benefits of sprint drills to choosing the right spices for seasoning for a meal. Do you still get most of the nutritional benefits without seasoning? Sure. But with just the right choices, you augment the benefits of the meal in a way that’s both tasty and more nutritious. 

So too with cool-downs: being thoughtful about the cooldown helps you understand what you’re trying to accomplish in training, and the right choices can set you up for long-term success in running. Keep in mind the various roles a cool-down can play: adding structure to a young runner’s training, keeping blood flow high to flush out metabolic byproducts after high-intensity sessions, serving as an additional aerobic stimulus to the body, and putting (sometimes undesirable) additional biomechanical stress into your legs.


  1. Wiewelhove, T., Schneider, C., Schmidt, A., Döweling, A., Meyer, T., Kellmann, M., Pfeiffer, M. and Ferrauti, A., 2018. Active recovery after high-intensity interval-training does not attenuate training adaptation. Frontiers in physiology, p.415.

Friday, May 28, 2021

Cadence lock: Why GPS watches have a hard time measuring heart rate during running

 Does your GPS watch report bizarre, dramatic spikes in heart rate in the middle of your run? I received a text message from a friend recently with this screenshot of his heart rate and elevation data from an easy run:

This was from a typical run on pretty mild to moderate hills. At first glance, you might think that running uphill triggered an abrupt increase in heart rate, but unless this coincided with an abrupt surge in speed, such a big spike in heart rate seems very strange.

Looking at this plot, it seems like one of two things is happening: either (a) my friend’s watch is seriously mistaken about his heart rate, or (b) my friend should see a cardiologist.  

Even though companies like Garmin or Apple or Fitbit keep their data processing algorithms pretty close to the chest,  my PhD research involves a lot of work with wearable technology, so I have a pretty solid understanding of what’s going on under the hood when a GPS watch is estimating things like running speed or heart rate.

I immediately noticed that the supposed “heart rate” being reported by the GPS watch in the screenshot above looked suspiciously similar to the range of values you’d see for cadence—around 165 to 185 steps per minute. So, I asked him to send me an overlay of his heart rate and his cadence (which is also measured by the watch). An, lo and behold—at halfway through the run, a near-perfect match! 

This phenomenon is something that’s been dubbed “cadence lock” in the running scene—sometimes, your watch seems to “lock onto” your cadence, confusing it with your heart rate. Why does this happen, and how can you fix it? To answer these questions, we’ll need a bit of background on wearable tech.

How do GPS watches measure your heart rate when you run?

Heart rate sensors on GPS watches are a relatively new innovation, but they rely on techniques that date to the mid-20th century. If you have one of these watches, you know they have a bright green LED on the wrist-facing side, like this: 

The watch above has three green LEDs, and one photosensor (the black square in the middle). This approach is called photoplethysmography, or PPG, and is the same technology used in pulse oximetry (which is used in hospital “finger clips” to track blood oxygenation.

Here’s the basic idea: the way the hemoglobin proteins in red blood cells absorb light is affected by whether or not they are currently carrying oxygen. Further, because blood vessels contract and expand along with the beating of your heart, the way certain wavelengths of light (such as green light!) are absorbed, or not, varies predictably as your heart beats.

Because each beat of your heart sends a wave of freshly oxygenated red blood cells rushing through your lungs and into your blood vessels, there is an ebb and flow to the amount of light absorbed by the tissue close to the surface of your wrist.

So, in theory, it should be possible to look at the rise and fall of the light absorption at the wrist to detect the heart rate. And indeed, most of the time this works just fine! The only problem is, in running, things get a bit messy.

Why is wrist-based heart rate so inaccurate during running?

Running causes problems for wrist-based optical heart rate monitors because it creates strong acceleration signals that interfere with the optical data coming into the watch. There’s a nice open-source dataset at PhysioNet that beautifully illustrates the problem. Check out the following plot: 

We’ve got three sources of data: a chest electrocardiogram (also known as an ECG) measuring the true contraction of the heart via electrical activity, an optical sensor at the wrist measuring the optical signal on the back of the wrist, and a wrist-worn accelerometer, which measures the motion of the wrist. The person in this plot is running at a pretty easy pace on a treadmill. A few things to notice:

  1. On the chest ECG (top panel), heart rate jumps out plain as day. This runner’s heart beats nine times in five seconds, meaning his or her heart rate is about 108 beats per minute during this five-second window.

  2. In the acceleration data at the wrist (middle panel), cadence jumps out pretty clearly as well. It’s a bit harder to see if (unlike me) you don’t look at this stuff all day long, but based on the repeating pattern of acceleration (which comes from the forward-backward swinging of the wrist and the up-down bouncing of the body), it’s pretty clear this runner takes about 11 steps in ten seconds, meaning his or her cadence is about 132 steps per minute.

  3. The optical data from the wrist (bottom panel) is pretty messy! 

You can probably start seeing where we’re going here. The problem that a GPS watch faces is trying to extract the heart rate signal from all the noise generated from your cadence. One way to get a handle on the difficulty of this problem is to take a look at the different frequencies present in each signal. That’s what the plot below is doing: 

For the engineers out there, this comes from a Fourier transform; if this means nothing to you, don’t worry about it. Notice the problem? The heart rate signal at the wrist is tiny compared to the noise from the motion of your wrist! Thus, it’s easy to see where “cadence lock” comes from: the watch locks onto the noise generated by the motion of your wrist, instead of the true signal generated by your heart.

As you might imagine, the problem of tracking your heart rate amid all this noise gets even nastier in two situations:

  1. When the wrist acceleration signal is very strong—which is what happens when you are running fast

  2. When your cadence gets close to your heart rate—which also happens when you are running fast

This, of course, is quite annoying for people who do heart rate-based training, since you’d normally be especially concerned with your heart rate during your workouts.

We might imagine some ways to attack this problem: use multiple optical sensors, try to somehow subtract the acceleration signal out of the optical signal, and so on. You can bet this is what companies like Apple, Garmin, and Fitbit try to do. But, given what we’ve seen above, it’s likely that even the best tricks won’t always work.

How to improve the accuracy of wrist heart rate measurements during running

First off, if knowing your heart rate is absolutely critical to your training, use a chest strap instead. That’s the best way to guarantee accurate readings. That being said, there are a few strategies you might use to improve the quality of wrist-based heart rate measurements.

For starters, strap your watch on tightly and make sure the sensor is clean. When the optical sensor is pressed against your skin, it will move around less when you run, and have a better chance at reading your heart rate accurately. 

Another trick you can try is holding your arm out in front of you for several seconds before checking your heart rate. The idea here is to reduce the acceleration on the wrist from the swinging motion of your arms. I suspect this will work less well than you’d think, because even when you don’t swing your arms when you run, they are still bouncing up and down quite a lot with respect to the ground (because your arm is attached to your torso).

An optical heart rate monitor that experiences less acceleration—for example, the kind that goes on your forearm or upper arm—will likely have an easier time detecting your true heart rate and not falling prey to cadence lock.

Still, as I mentioned above, even the upper arm experiences quite strong accelerations during fast running, so even an arm-based optical heart rate sensor is not likely to be a perfect solution for everyone.

Lastly, if your watch supports it, change the settings on your watch face to display your cadence on the same screen as your heart rate. If you check your heart rate and see it closely tracking your cadence, you’ll know that you’re experiencing cadence lock, and you should disregard what your watch says. 


There’s a lot more to say about whether heart rate training for runners is even a good idea in the first place (I for one am skeptical), and there’s also more to say about a range of other factors that can affect the accuracy of a wrist-worn heart rate monitor.

The bottom line is that you should keep your sensor clean, make sure your watch isn’t loose, and compare your recorded heart rate against your cadence. If they’re suddenly changing in lock-step, you know something is off, and you shouldn’t trust your wrist-based heart rate data.

Saturday, January 23, 2021

Blog update and podcast/media roundup

Well, it’s been a while! As you may have noticed, Running Writings hasn’t seen an update in quite a while. Perhaps for good reason—I’ve been working on my PhD in biomechanics at Indiana University's School of Public Health, which, as I’ve discovered, leaves very little time to spare.

Every time I think up an idea for a blog post (and I have had dozens!) I realize there’s one more dataset to analyze, one more paper to revise, one more grant to apply for, one more skill to learn. So, though RW hasn’t seen any updates, I haven’t dropped off the face of the earth, and I’ve been learning an incredible amount of new things about the science of running performance and running injuries. But all of that work keeps me very busy, and unfortunately I haven’t had much of a chance to share what I’ve been up to with the people who follow Running Writings. 

Plans for Running Writings

Rest assured, I still have big plans for bringing the science of running out into the real world, and Running Writings will continue to be a big part of that. One of the first steps forward is going to be overhauling the website itself—RW runs on Google Blogger, which is ancient and neglected. You may have noticed that I’ve had comments disabled for several months, as the site was being overrun by spam. Moving to a new hosting platform will help immensely on that front, and will also modernize the look and feel of the site. Look for a big overhaul sometime...“soon”? Early spring perhaps? There is some chance RW will be down temporarily, but I don't intend on deleting or moving any old content.

Once that’s done, I’m hoping to share some of the more useful tidbits of science that I’ve learned over the course of the last 3+ years as a PhD student studying this stuff every single day. They won’t necessarily be the long-format blog posts that I’ve written previously, but hopefully you’ll still find them useful.

Beyond these immediate updates, Modern Training and Physiology is somehow still selling at least one copy nearly every single day, which still blows me away. Somewhere collecting digital dust, I do have outlines for more long-format writing (and even more books) in the future. I’ll be revisiting those as I get closer to finishing my PhD. 

What am I researching these days anyways?

When I was applying to graduate school, I remember putting a clever line in my personal statement about “realizing I didn’t just want to write about running research - I wanted to become a running injury researcher myself.”  Well, I did it! I have a few research papers out, and am working on several more. Seeing something that you originally wrote in a coffee shop using Google Docs show up in a nicely formatted PDF in a real scientific journal is really something else. 

After a lot of voracious reading and some scatterbrained ideas presented at a few conferences, my research interests have settled on four broad topics: running injuries (big surprise there!), wearable technology, physical activity epidemiology, and advanced statistical methods for studying those first three topics. 

This fall, I passed my qualifying exam, which means I am All But Dissertation. So, just as soon as I can cook up a really brilliant PhD thesis, I’ll have my degree! No small matter, of course—especially considering the pandemic (any bets on how long until you can safely sit in a poorly-ventilated room five feet from someone running on a treadmill?). But, I do have some research ideas I’m pretty excited about; yes they involve several of the topics above, and no, unfortunately I can’t share any details yet! 

Podcasts and media appearances

One thing I can share is that the research I’ve done so far has led to some pretty cool opportunities already. In April, my advisor (Dr. Allison Gruber) and I appeared on the Mountain Land Running Podcast to talk about using wearable technology to monitor training loads

Pretty soon after, one of my studies got mentioned in a Runner’s World article about cross-training after injuries (original paper is here—in it, we used Fitbits to study what happens to a runner’s overall physical activity level when he or she gets hurt). Then, this winter, I got to chat about elite marathon training and the science of running injuries with Joe Sell on the Marathon Running Podcast. 

Joe’s podcast was especially fun, since we got to chat both about science and about elite training (especially Renato Canova’s marathon training methods)—since my research is so focused on injuries, I don’t get to do much with elite training these days. The previous episodes of the podcast, especially episode 6 with Nate Jenkins, are awesome, so it was incredibly fun to go on this podcast. 

Brought to you by readers like you

In an odd way, none of these things would have been possible without the readers of this blog. So, allow me to say “thank you!” to everyone who’s been reading RW over the last ten years! I wouldn’t have gotten my start in running injury research, and all of the great things that have come out of it, if this website hadn’t gotten any traction. Here’s to more running, more writing, and more Running Writings! 

Sunday, November 4, 2018

Can improving your 5k time increase your lifespan? A look at extreme aerobic fitness and longevity

During my PhD studies, I try to keep up with the broader scientific literature on the health effects of vigorous exercise (given that I do study running, after all). Just a few days ago, a fascinating new study caught my eye. It was published in JAMA Network Open by a team of researchers at the Cleveland Clinic in Ohio. The study explored the connection between aerobic fitness and longevity. In other words, do aerobically fit people live longer compared to out-of-shape people?

Background: Aerobic fitness and long-term health

For the general population, the answer to this question is a definitive yes, based on previous research. The real innovation of this study was that it specifically examined people with extremely high aerobic fitness. People in the top few percentiles of population-level aerobic fitness don’t get there by genetics alone. As any distance runner is well aware, becoming very fit requires a lot of intense and high-volume training. Some cardiologists have hypothesized that this kind of intense training is unhealthy. They point to research showing that biomarkers of heart damage increase after running a marathon, and other work showing a potential “U-shaped curve” for physical activity levels and cardiovascular disease risk.

The study details

The actual participants in this study were 122,000 men and women who underwent a standardized treadmill test of aerobic fitness. The treadmill test progressed as most do, starting at an easy walk and gradually increasing both the speed and the incline until the participant could not continue any longer. This final stage of the treadmill test was used to determine the person’s “peak METs,” or peak metabolic equivalent energy output.

The researchers tracked each study participant in the Social Security Death Index, which (for fraud prevention purposes) is a registry of all deaths of Americans who have social security numbers. By monitoring which patients showed up in the death index, and when, the researchers could determine who did and did not die during the course of the study.

The findings

As with previous research, the authors of this study found that people with better aerobic fitness, as measured by peak METs, were less likely to die, even after controlling for factors like age, sex, body mass, history of disease, smoking, and other potential confounding variables.

Most interestingly, the researchers found that there was no upper limit to the benefits of physical fitness. The healthiest group of people—in other words, those least likely to die from any cause—were those that the authors classified as having “extreme cardiorespiratory fitness.”

In the context of this study, the authors defined this as scoring in the top 2.3% of all performers for their age and sex. These extremely fit individuals were less likely to die compared to those who scored in the 75th-97.6th percentiles for aerobic fitness, to the tune of a 23% lower risk of death. This pales in comparison to the difference between the most and least fit people, though: Compared to the top 2.3%, those in the lowest 25% of aerobic fitness had five times the risk of death!

Can your 5k time predict your lifespan?

One thing I love doing is trying to translate research findings into something that’s more tangible and practical for people who aren’t clinical researchers. What does it really mean to have a “peak MET two standard deviations above the age and sex mean?

You may have seen the term “MET” before, at the gym on exercise equipment. It’s a standardized unit of energy expenditure, where 1.0 METs is the energy expenditure of sitting still in a chair.  METs are closely related to another unit of energy expenditure that you might be more familiar with, which is VO2.  One MET is equivalent to 3.5 ml of oxygen per kg of body mass. What this means is that if you know someone’s peak metabolic equivalent, you can easily figure out their VO2 max—all you have to do is multiply by 3.5.  So, it’s pretty easy to turn the cut-points for low, average, high, and elite aerobic fitness in this paper from MET thresholds into VO2 max thresholds.

Once we have VO2max, we’re in more familiar territory for runners. If you’ve read my book, Modern Training and Physiology, you know that your VO2 max is a very important predictor for your race performances. Now, coaches like me usually rail against VO2 max as the end-all-be-all of running performance, because VO2 max does not differentiate very well between someone whose 5k PR is 16:00 and someone whose 5k PR is 15:30. That being said, in this case, VO2 max is a pretty useful predictor of running performance, if we are talking about magnitudes like a 30:00 versus a 20:00 5k (or a 30:00 versus a DNF).

You might see where this is going. What I want to do is convert these fairly arcane public health measurements into something that’s understandable for the everyday person. What better way than a 5k time? If we convert the “elite” cutoffs in METs into VO2 max cutoffs, it’s easy to run these through a race time predictor and come up with a goal 5k time for “optimal longevity” (with some very serious caveats!). Let’s take a look at what those elite aerobic fitness cutoffs look like before we talk about the caveats.

After looking at these times, we can see the disconnect between what’s “elite” at the population level and what’s “elite” for a runner. A 21-year-old male who runs 18:25 for the 5k is certainly in good shape, but he’s still nearly four minutes shy of what he’d need to run to run at Division 3 Nationals in track—much less D2 or D1.  

In the context of health and longevity, this is a good thing: even “extreme cardiorespiratory fitness” is well within the reach of many (though certainly not all) people in the general population. One thing I should reiterate is that the longevity benefits of being in the “elite” fitness group are significant even after adjusting for things like smoking, body weight, and other things you might think could account for differences in fitness levels.

If these times seem out of reach for you, don't fret—the difference in survival between people of "high" and "elite" fitness was statistically significant, but very small compared to the differences between people who with "high" or "elite" fitness compared to those who were not fit. 

As the figure above shows, the biggest differences in longevity are clearly seen when comparing those whose fitness is poor to those who are at least above average, or better.

Not so fast: The caveats

Now, for some caveats. First, my VO2 max to 5k time conversion is a tiny bit hand-wavy. Once you get to high levels of running fitness, most of your improvements don’t come from better VO2 max; they come from improvements in running economy. That being said, I’m fairly confident that my converted times are in the right ballpark.

The more important caveats have to do with the nature of the causal relationship—or lack thereof—between high aerobic fitness and a long and healthy life. This study measured the fitness levels of healthy people, then followed them to see who died. Being extremely fit appeared to be a good predictor of avoiding death in the future. The key question is what biological mechanism is responsible for this association?

The easiest answer would be doing aerobic exercise. After all, there is a strong case to be made that very few, if any, 29-year-old women are going to be in sub-20 minute 5k shape without doing a good amount of aerobic exercise. But there are plenty of alternative mechanisms that could contribute too. What about genetic variation? Some people are born with a lot of what we call “talent,” the state of being highly aerobically fit even when they do not exercise. These same genetic traits which make someone a talented runner may also be responsible for different biological mechanisms that lead to a longer life: more flexible arteries, resistance to metabolic disease, etc.  Strong aerobic fitness might merely be an indicator of these traits, not the actual cause.

Now, the strongest circumstantial evidence still (in my opinion) supports the causal link between intense, high-volume training and a longer lifespan. At low, moderate, and moderately high doses, aerobic exercise is strongly protective against death. Moreover, alternative explanations, like the hypothetical genetic mechanisms I just laid out, would have to account for the fact that it is extremely rare, on the population level, for someone to reach these levels of “extreme cardiorespiratory fitness” with doing a solid amount of training.How many 45-year-old men are such fine genetic specimens that they can run a sub-20:00 5k fresh off the couch, compared to the number of 45-year-old men in sub-20 5k shape who train hard on a regular basis? 

Unanswered questions about fitness and lifespan

When we talk about the potential health effects of training too hard, we aren’t usually talking about a 40-year-old male who runs 19:25 in the 5k. Usually we are talking about ultramarathoners, sub-16 5k runners, Boston Marathon qualifiers, and other people who have a tendency to hammer out hundred-mile weeks and long multi-hour running sessions. If anyone is at risk of health problems from excessive exercise, it would be these extreme outliers. Because of the nature of distributions, you can be sure there are were a lot more 40-year-old men in 19:20 shape in this study than 40-year-old men in 16:20 shape, for example.

While these findings are promising, and suggest that more really is better when it comes to fitness and exercise, there’s still a need for research that focuses on those in the most extreme groups when it comes to exercise volume and intensity.  Fortunately, there is a longitudinal study happening right now on ultramarathoners that’s being run by Stanford University and UC Davis. It might take ten or 20 years before we get solid results from that study, though.

Finally, the question of causality still remains: If you take someone who does not have high aerobic fitness, and train them so they become aerobically fit, will they live longer? This question can only be definitively answered in a randomized controlled trial. Given the difficulty of following people for 20 or 30 years to observe mortality, testing this question directly might be impractical. Instead, it might be possible to correlate changes in aerobic fitness with changes in biological markers that we know are related to longevity—for example, if we hypothesize that lower arterial stiffness is one reason why extremely fit people live longer than those who are not fit, we could conduct a two or five-year clinical trial to see if an intense, high-volume aerobic training program would reduce arterial stiffness, compared to a lower-volume, lower-intensity aerobic training program.

As for me? I’m putting my chips on “better fitness = longer lifespan.” I’m turning 30 this year, so maybe I’ll hit the roads on my birthday, just to make sure I can still crank out a 19:05 5k.