Consider this age-old dilemma: you’re at the bottom of a hill, and you want to get to the top. Should you head straight up the steepest slope or switchback back and forth at a gentler incline? The answer depends on the context. If you’re on a marked trail, for example, you should definitely stick to the prescribed switchbacks. But a more general answer involves digging into the physics.

That’s the goal of , from a research team led by David Looney and Adam Potter of the U.S. Army Research Institute of Environmental Medicine. Previous researchers have found that “steeper is cheaper” for runners, meaning that it takes less energy to ascend directly up steeper slopes. But it wasn’t clear whether the same is true for walkers and backpackers, or whether the answers change depending on how hot or cold it is.

The Best Slope for Trail Runners

For starters, it’s worth looking back at the trail-running data. In 2016, researchers at the University of Colorado decided to the increasingly popular world of . The total elevation gain in these races is set at 1,000 meters, or 3,281 feet, but every course is different. A steep slope will have a shorter course distance but be harder to run up. A gentle slope will be easier to run up but cover a longer total distance. For a given finishing time, what’s the sweet spot?

The Colorado researchers built the world’s steepest treadmill (video ), capable of reaching a slope of 45 degrees—a 100-percent grade, in other words. To put that into perspective, a black diamond ski run is typically about 25 degrees, and gym treadmills rarely go more than 9 degrees. They had to line the treadmill belt with sandpaper for grip, and even then runners couldn’t stay balanced beyond 40 degrees.

They tested runners at a variety of slopes, with the treadmill speed adjusted so that they were always gaining elevation at the same rate, equivalent to a vertical kilometer in a very respectable time of 48 minutes (the world record is just under 30 minutes). Here’s what the results looked like for walking (black circles) and running (white circles), with metabolic rate (basically how quickly they were burning calories) on the vertical axis:

At gentle slopes like 10 degrees, it takes a lot of energy to climb, because the treadmill is moving really fast to gain the required elevation. At steeper slopes, the calorie burn decreases: steeper is indeed cheaper, at least up to a point. Beyond about 30 degrees, calorie burn starts increasing again, presumably because the incline is now so steep that it’s hard climb efficiently. The sweet spot, then, is between 20 and 30 degrees—which, as it turns out, corresponds to the average slopes of the courses where the fast vertical kilometers are held.

(You might also notice that walking burns less energy than running for most of the steeper slopes. That’s a truth that most mountain and trail runners eventually discover for themselves. However, it doesn’t necessarily mean that you should only walk up hills, as I explored in this article on the walk/run dilemma in trail running.)

The Best Slope for Hikers

Climbing a kilometer in 48 minutes is really fast, the aerobic equivalent of running as hard as you can for 10 kilometers, so it’s not clear that the Colorado results have much relevance for backpackers or military personnel. Looney and his colleagues decided to run similar experiments at a range of much slower climbing speeds. The Colorado study had a climbing rate of 21 vertical meters per minute; Looney’s study looks at four different climbing rates of between 1.9 and 7.8 meters per minute, a much more realistic range for hikers.

The overall results are similar to the running results: steeper was once again cheaper. For each climbing rate, choosing a steeper slope corresponded to burning fewer calories. As with the running data, there’s probably a point where getting too steep becomes counterproductive. But the steepest slope in Looney’s study was only about 13 degrees, and in that range steeper was always better.

There was an additional wrinkle in Looney’s protocol: the military is on Arctic operations, so they ran the same protocol at three different temperatures: 32, 50, and 68 degrees Fahrenheit. The two warmer temperatures were basically the same, but the data at 32 degrees was slightly different.

At slower vertical climbing rates, calorie burn rates were higher than normal at 32 degrees, because the subjects were spending extra energy keeping themselves warm by shivering and activating their . At higher vertical climbing rates, calorie burn rates were roughly the same regardless of temperature, presumably because they were working hard enough to stay warm even at 32 degrees. In cold temperatures, in other words, pushing harder can sometimes be more efficient because it saves you the energetic cost of keeping yourself warm. (Conversely, you might imagine that the steepest slopes would cause problems in really hot conditions because you’re more likely to overheat.)

The Takeaway

The most important caveat to keep in mind when interpreting these results is that the comparisons are based on a fixed climbing rate. If you’re at the bottom of a hill and want to get to the top in a given amount of time, then choosing a steeper route will generally save you energy. If you’ve got all the time in the world and don’t care how long it takes you to reach the summit, then you might well choose to take a gentler route that will feel easier as you climb.

Most of us, though, live in a world where time is scarce. Even if we’re not racing vertical kilometers, we’re hoping to make it to the summit and back, or to the next campsite, before dark. In that situation, if you’re choosing between two routes, remember this: If one route is twice as steep as the other, you’d have to walk twice as fast up the gentler route to reach the top in the same time. Counterintuitive though it may sound, that data shows that under most circumstances, twice as steep is easier than twice as fast.

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There’s tons of evidence, from hundreds of studies with hundreds of thousands of participants, showing that exercise is an effective tool to combat depression and other mental health issues like anxiety. These studies find that it’s at least as good as drugs or therapy, and perhaps . It’s now recommended in official guidelines around the world as a or treatment. Still, there’s an important caveat to consider: is all this evidence of a connection between exercise and mental health any good?

That’s the question debated in in Medicine & Science in Sports & Exercise, based on a symposium held at the annual meeting of the American College of Sports Medicine. Four researchers, led by Patrick O’Connor of the University of Georgia, sift and weigh the various lines of evidence. Their conclusion is mixed: yes, there’s a relationship between exercise and mental health, but its real-world applicability isn’t as clear as you might think.

The Observational Evidence on Exercise and Mental Health

O’Connor and his colleagues assess three main types of evidence. The first is observational studies, which measure levels of physical activity and mental health in large groups of people to see if they’re connected, and in some cases follow up over many years to see how those relationships evolve. The headline result here is pretty clear: people who are more physically active are less likely to suffer from depression and anxiety now and in the future.

Observational studies also suggest, albeit more weakly, that there’s a dose-response relationship between exercise and mental health: more is better. is enough to produce an effect, but higher amounts produce a bigger effect. It’s an open question, though, whether doing too much can actually hurt your mental health. Some studies, for example, have found links between overtraining in endurance athletes and symptoms of depression.

The big problem with observational studies is the question of causation. Are active people less likely to become depressed, or is it that people who are depressed are less likely to be active? To answer that, we need a different type of study.

The Evidence from Randomized Trials

The second line of evidence is from randomized control trials, or RCTs: tell one group of people to exercise, tell another group not to, and see if they fare differently. Overall, the evidence from RCTs lines up with the observational evidence: prescribing exercise improves or prevents the occurrence of depression and anxiety.

For example, here’s a graph from a 2024 meta-analysis of 218 RCTs with a total of over 14,000 participants, :

Dots that are farther to the left indicate how much a treatment aided depression compared to a control group. Notice that walking or jogging ranks slightly above cognitive behavioral therapy and far above SSRI drugs. That’s an encouraging picture.

The evidence still isn’t bulletproof, though. One problem is that it’s very difficult to avoid placebo effects. Participants who are randomized to exercise know that they’re exercising, and likely also know that it’s supposed to make them feel better. Conversely, those who sign up for an exercise-and-depression study and are assigned to not exercise will expect to get nothing from it. These expectations matter, especially when you’re looking at a difficult-to-measure outcome like mental health.

Another challenge is the timeframe. Exercise studies are time-consuming and expensive to run, so they seldom last more than six months. But a third of major depressive episodes spontaneously resolve within six months with no treatment, which is in part why FDA guidelines suggest that such trials should last two years, to ensure that results are real and durable.

Why Context Matters When Studying Exercise and Mental Health

The third and final body of evidence that O’Connor and his colleagues dig into is the contextual details. Exercise itself seems to matter, they write, but “who we play with, whether we have fun, whether we are cheered or booed, and whether we leave the experience feeling proud and accepted, or shamed and rejected also matters.”

For example, most of the research focuses on “leisure time physical activity,” meaning sports and fitness. But there are other types of physical activity: occupational (at work), transportation (active commuting), and domestic (chores around the house). Is there a difference between lifting weights in the gym and lifting lumber on a construction site? Between a walk in the park and a walk down the aisle of a warehouse?

One view of exercise’s brain benefits is that it’s all about neurotransmitters: getting the heart pumping produces endorphins and oxytocin and various other mood-altering chemicals. If that’s the case, then manual labor should be as powerful as sports, and working out alone in a dark basement should be just as good as meeting friends for a run on a sunny day. Both intuition and research suggest that this isn’t the case.

Instead, some of exercise’s apparent mental-health benefits are clearly contextual. Doing something that creates social connection and provides a feeling of accomplishment is probably helpful even if your heart rate doesn’t budge above its resting level. And conversely, an exercise program that leaves you feeling worse about yourself—think of the cliché of old-school phys ed classes—might not help your mental health regardless of how much it boosts your VO2 max.

This is where the big research gaps are, according to O’Connor and his colleagues. It’s pretty clear at this point that exercise isn’t just correlated with mental health; it can change it. But the best ways to deploy it in the real world remains understudied. For now, the best advice is probably to follow your instincts. Don’t stress about what type of exercise you’re doing, how hard to push, or how long to go. For improving mental health, these variables seem to have surprisingly weak effects. Instead, focus on the big levers: whether you’re enjoying it, and whether you’ll do it again tomorrow.

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Earlier this month, the Journal of Applied Physiology published a paper with the title “Evidence on Sex Differences in Sports Performance.” Seems pretty straightforward, but of course it’s not. The gap between male and female athletes has become a major flashpoint in debates on whether transgender women and athletes with differences of sexual development, like the South African runner Caster Semenya, should be able to compete in women’s sports.

Three scientists—Michael Joyner of the Mayo Clinic, Sandra Hunter of the University of Michigan, and Jonathon Senefeld of the University of Illinois Urbana-Champaign—present a series of seven statements on the topic of sex differences in sport, along with the evidence to support them. Some of them seem obvious, others less so. Whatever your opinion on the debate, I think it’s worth considering each of these statements (which I’ll paraphrase) in turn, in order to understand what the current evidence says and where the gaps are. The full paper, including references, is free to read .

A note on terminology: the article deals with differences in sex rather than gender. Although it’s an oversimplification, I’ll use the terms male and female to refer to people with XY and XX chromosomes, respectively.

1. Males outperform females in events that depend on strength, speed, power, and endurance.

The evidence cited here is primarily performance data from sports like running, jumping, and weightlifting, where outcomes are easily measured. Among elite adults, the male-female gap is typically above 10 percent. The largest gaps are seen in sports that depend on explosive power, like high jump and long jump, where the gap approaches 20 percent. Field sports are harder to measure, but to the extent that they involve running and jumping and lifting, similar conclusions should apply.

Are these gaps biologically determined, or, , the result of social factors like the limited opportunities for women in sport? Elite performance data, on its own, can’t answer this question. But there’s no question that the gap exists, and is nearly universal. There may be some exceptions in activities like , where the determinants of performance are more complex. Overall, though, this statement should be uncontroversial.

2. This male-female gap shows up before puberty.

This seems like a significant claim, because it suggests that males may have a performance advantage that isn’t erased even if a transgender woman has undergone hormone therapy to lower testosterone levels. The evidence, once again, is primarily from performance data. Take a look at this graph of age-group track and field results for boys and girls between 7 and 18 years old:

Between the ages of 7 and 9, boys seem to be ahead, on average, by 4 to 5 percent. The gap narrows between the ages of 10 and 12, presumably as girls start puberty earlier than boys. After the age of 13, male puberty gets going and the gap widens rapidly.

So what gives 8-year-old boys an edge? As Joyner and his colleagues acknowledge, it’s once again hard to distinguish between biological and social factors. There is a possible hormonal explanation. We undergo a “minipuberty” during the first few months of life, with a temporary increase in sex hormones that is associated with a subsequent increase in muscle and decrease in fat accumulation in boys. But it’s also true that boys tend to spend more time running and jumping in unstructured play, and this may reflect gendered social expectations rather than sex differences.

Overall, the small gap in pre-puberty performance doesn’t seem like strong evidence of ineradicable differences between males and females. Instead, it’s the subsequent shape of that curve that, as we’ll see, turns out to be more significant.

3. The gap widens with puberty, along with changes in body structure and function.

In the graph above, male-female differences accelerate dramatically after the age of 13 and continue all the way to adulthood. Now it gets harder to attribute the changes to social factors, because there are a host of other changes that accompany puberty and are associated with sports performance: males see a greater increase in muscle, airway and lung size, heart size, oxygen-carrying capacity of the blood, and so on.

Perhaps the most obvious difference is height: by the age of 20, the average male is taller than 97 percent of women. Differences in lung size or hemoglobin levels are invisible to us; differences in muscle mass could conceivably be because boys are encouraged to work out more. But height? We see it all around us, and accept that it’s driven by biological sex differences.

4. The main driver of the male-female performance gap in adults is the surge in testosterone during male puberty.

Here’s when things get more contested. Where, you might ask, is the randomized controlled trial proving that males who go through puberty without testosterone are worse athletes, or that females who go through puberty with male levels of testosterone are better athletes? Such studies haven’t been done, for obvious practical and ethical reasons.

Joyner and his colleagues argue that we can instead piece together the evidence from studies showing links between testosterone levels and increased physical performance during puberty; the various studies in humans and animals showing testosterone’s effects on muscle, bone, and blood parameters; doping studies where volunteers took testosterone; and strong circumstantial hints like the graph above showing the widening performance gap during puberty. The evidence here isn’t perfect, but as a whole it’s convincing.

5. Body changes during female puberty can have negative effects on sports performance.

This is an angle I hadn’t thought much about. The discussion usually focuses on the advantages conferred on males by testosterone, but there are a distinct set of changes that females experience during puberty. For example, they accumulate more body fat; their growth plates fuse so they stop growing taller; they develop breasts, which can alter balance and movement patterns; their hips widen, which may increase injury risk; they experience hormonal fluctuations associated with the menstrual cycle that may (or may not!) affect performance; they may eventually miss training time during pregnancy and face increased injury risk when returning to training after childbirth.

There’s no doubt that all these changes occur, and that they have the potential to influence performance. Whether they collectively make a significant contribution to the gap between male and female athletic performance is less clear. It’s worth considering, but I’d classify it as an open question for now.

6. Suppressing male testosterone levels after puberty only partly eliminates the male-female performance gap.

There’s a smattering of case studies and comparison studies to support this statement. A 2023 U.S. Air Force in Military Medicine, for example, tracked fitness test scores for nearly 400 transgender servicemembers for up to four years after they began hormone therapy. For transgender women, performance on some tests, like the 1.5-mile run, ended up corresponding to average female times by the fourth year of hormone therapy. But for other tests like push-ups, there were still differences.

Here’s how push-up scores evolved in transgender women over the course of four years of hormone therapy. The red band shows the range of male scores within one standard deviation of average; the blue band shows the corresponding women’s range. Scores are still higher than average even after four years.

One reason for the retained advantage is that some of the changes that occur during puberty are irreversible. Those who go through male puberty will, on average, be taller and have bigger lungs. They’ll lose muscle mass during hormone therapy, but still retain more than the female average. There’s also evidence for “muscle memory,” a phenomenon that makes it easier to build muscle if you’ve previously had it.

It’s worth noting that the significance of retained advantages will vary from sport to sport. Greater height and muscle mass matter a lot in sports like basketball and rugby; they may matter less in, say, marathon swimming.

7. Adding testosterone improves female performance, but doesn’t eliminate the male-female gap.

This claim is the mirror image of the previous one: transgender men improve various facets of sports performance after beginning hormone therapy, but they don’t gain the full ten percent. This supports the idea that testosterone matters for performance, but that timing also matters: it plays its most significant role during puberty.

These are the seven claims that Joyner and his co-authors make. Some are stronger than others. But even if you take them all at face value, they don’t tell you what the rules for transgender or intersex athletes should be. That involves a difficult balance between fairness and inclusion. Maybe the male-female differences discussed here are the most important consideration; maybe they’re outweighed by other factors. I don’t think there are any easy answers here, but any compromises we reach need to acknowledge that these differences exist and are persistent.

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Researchers in Germany recently published one of those studies that, now and then, make me question my core beliefs. I’m a supplement skeptic, but I try not to let that identity prevent me from assimilating new data. And if there’s one supplement whose possible benefits I’ve been on the fence about in recent years, it’s vitamin D.

The new study, , is part of a major initiative to improve the performance of German elite athletes. A research team led by Sebastian Hacker of Justus Liebig University in Giessen studied 474 athletes on German national teams in a range of sports including hockey, table tennis, and three-on-three basketball. They tested vitamin D levels and measured (among other outcomes) handgrip strength.

Here’s the money shot:

This graph shows handgrip strength as a function of 25(OH)D levels, which is how vitamin D status is assessed in the blood.  The two dashed lines indicate the thresholds between vitamin D deficiency (below 20 ng/mL), insufficiency (between 20 and 30 ng/mL), and sufficiency (above 30 ng/mL). There have been long debates on where these thresholds should be set, but that’s the current thinking. Note that you’ll sometimes see 25(OH)D levels expressed in nmol/L; to get to those units, multiply the values above by 2.5.

The key point: there’s a clear slope to the line. Higher levels of vitamin D are associated with stronger grip strength, which in turn has been associated with health, longevity, and (less clearly) athletic performance. For every 1 ng/mL increase in 25(OH)D, handgrip strength increases by 0.01 N/kg, which means that going from 20 to 30 ng/mL should boost your strength by about three percent.

The Case for Vitamin D Supplements as a Performance Aid

Vitamin D plays roles in a whole bunch of body systems, including bone health, immune function, and—perhaps most notably for athletes—muscle performance. If you’re truly deficient in vitamin D, there’s no doubt you should get your levels up. But the evidence in the “merely insufficient” range is less clear, even in this data. If you took all the values below 20 mg/mL out of the analysis, would there still be a relationship between vitamin and handgrip strength? It’s not clear.

This isn’t the first time researchers have shown a relationship between vitamin D and strength. In fact, pooled data from 28 studies with 5,700 participants and concluded that there’s a positive relationship between vitamin D levels and quadriceps strength. At least, that’s the headline result—but when you look closer, it’s less convincing. The positive relationship was for quad strength when contracting the muscle at a specific speed of 180 degrees per second. But there was no relationship at a slower speed of 60 degrees per second. Worse still, there was a negative correlation for maximal contractions against an immoveable force: higher vitamin D levels were associated with smaller max force.

In other words, we shouldn’t be too quick to assume the new German data is definitive. Instead, it’s another data point in an ongoing debate. Another review, , finds “mixed results” in studies on the relationship between vitamin D levels and muscle mass and strength.

Causation or Correlation?

Even if we eventually conclude that there is a positive relationship between vitamin D levels and strength, it doesn’t necessarily follow that we should all start popping vitamin D pills. First of all, there’s the possibility of reverse causation. People who are strong and healthy may choose to spend more time exercising outdoors, which in turn may produce higher vitamin D levels. That’s actually one of the strengths of the new German study: since all the subjects were elite athletes, we can assume that they have similar levels of general fitness and physical activity.

There may also be confounding factors. Back in 2019, şÚÁĎłÔąĎÍř contributing editor Rowan Jacobsen wrote a surprising article in which he argued that the benefits of sunlight extend beyond merely raising vitamin D levels, most notably in triggering the release of nitric oxide from your skin into your bloodstream. If that’s the case, then taking vitamin D supplements won’t necessarily fix whatever problems are associated with lack of sunshine.

What we really want are intervention studies, where we give extra vitamin D to people and see if they get stronger. And we don’t want subjects who already have sufficient levels of vitamin D, because they stand to benefit less; instead we want people with insufficient levels. That’s what , this one from Estonia, did.

The Estonian researchers took 28 volunteers with “insufficient” 25(OH)D levels in the low 20s mg/mL. Half of them got a placebo, and the other half took 8,000 IU per day of vitamin D, which eventually got their 25(OH)D levels up to a healthy 57 ng/mL. Both groups did 12 weeks of resistance training, but there were no discernible differences in their results, which were published in the journal Nutrients. Here are the gains in one-rep maximum for various exercises for the two groups:

In fact, the further you dig into the literature, the less convincing the data looks for vitamin D as an athletic supplement. For example, there was that found no significant benefit of vitamin D supplementation on muscle strength but a trend in the right direction. But even that weak finding was tainted by “key errors in the analytical approach,” according to : the true effect is close to zero.

Of course, vitamin D’s merits as an athletic supplement are distinct from its potential for more general health purposes. Might it be that taking vitamin D supplements helps prevent cancer, heart disease, or type 2 diabetes; increases bone density; or reduces your risk of falls? No, no, no, no, and no, according to . More than 60 Mendelian randomization studies, which use genetic data to divide people into pseudo-randomized groups with high or low vitamin D levels, have generally found no difference in health outcomes.

Put it all together and the overall case for taking vitamin D supplements doesn’t look very compelling to me—assuming, that is, that you don’t have a genuine deficiency. Defining that threshold is the tricky part. Is it below 20 ng/mL, which health authorities consider deficient? Is it below 30 ng/mL which they label insufficient? Is it somewhere higher or lower or in between? I’m not sure, so for now I’ll hedge my bets: despite all my skepticism, I’m going to arrange to get my levels tested at my next doctor’s appointment.

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One of the funny little details in Eliud Kipchoge’s attempts to run a sub-two-hour-marathon was the electric pace cars. In Nike’s Breaking2 race in 2017, they used a Tesla. In the INEOS 1:59 Challenge, where Kipchoge actually broke the barrier, it was an Audi e-tron equipped with a specially built cruise control that was accurate to within less than a meter over the entire marathon distance. “There will be no emissions out of the back to upset any of the runners,” one of the INEOS engineers .

Was this just window-dressing, like the strips of aerodynamic tape that the Breaking2 runners pasted to their calves? Or does a bit of exhaust in the air actually slow you down? over the years have attempted to answer this question, but the results have been unclear, in part because it’s difficult to get accurate readings of air quality on the racecourse itself. But a new study from a research team at Brown University, led by Elvira Fleury and Joseph Braun, offers a more definitive answer: it matters.

The Problem with Particulate Matter

Fleury and Braun used a “spatiotemporal machine learning model” to produce detailed hyperlocal estimates of exactly how much fine particulate matter was in the air at each mile marker along the courses of nine major marathons in the U.S. for each year between 2003 and 2019. The model integrates readings from nearby air sensors with satellite data, weather, topography, and other inputs.

Fine particulate matter—also known as PM2.5 or, more familiarly, soot—refers to particles that are less than 2.5 microns in diameter, and is produced by internal combustion engines, forest fires, and other sources. It’s easy to inhale, and can cross from your lungs into your bloodstream, triggering inflammation and oxidative damage that raises the risk of heart disease, diabetes, and other conditions. When you’re running, you breathe more air than usual, and suck it in through your mouth, which bypasses the nasal filtration (i.e. hairs) that would otherwise catch some of the particles. This triggers a variety of problems, including constricting the blood vessels that supply your muscles with oxygen—bad news for a marathoner.

The machine learning model showed that PM2.5 levels varied widely from place to place and year to year. Levels in Boston and Chicago were as high as 20 micrograms per cubic meter in some years, and as low as 2 or 3 micrograms per cubic meter in others. Other courses like New York, Houston, and Los Angeles were in a similar range. The study, , combined this pollution data with 2.5 million finishing times, adjusting for other factors like heat and humidity.

How Does Air Pollution Affect Marathoners?

Before digging into the results, it’s worth pausing to consider what we’d expect to see. On a superficial level, there are two big trends to consider. The obvious one is that slower runners are out there longer, so we’d expect the total amount of time lost to increase with finishing time. The other one is that faster runners tend to breathe more heavily, so they suck in more particles per breath and lodge them more deeply in their respiratory systems—so we might, conversely, expect the effects to decrease with finishing time.

The most important question, though, is whether there are any effects at all. Overall, male marathoners at a given percentile finishing position were 32 seconds slower for each increase of 1 microgram per cubic meter in PM2.5 levels; female marathoners were 25 seconds slower. That may sound like a modest effect, but it seemingly suggests that average Chicago Marathon times in a low-pollution year like 2019 (~3 micrograms per cubic meter), might be around eight minutes faster than in a high-pollution year like 2011 (~20 micrograms per cubic meter). Even if that turns out to be an overestimate—I’ve taken the most extreme comparison I could find—it suggests that we’re talking about minutes rather than milliseconds.

Here’s how the effect varied depending on finishing position. The graphs below show finishing percentile on the horizontal axis, with first place on the left and last place on the right. The change in finishing time per microgram per cubic meter of PM2.5 is on the vertical axis. Graph A shows male finishers, graph B shows female finishers.

In both cases, the pattern is roughly the same. The fastest finishers have a relatively small effect; the median (which for most of the races tends to be between 4:00 and 5:00) and slightly-faster-than-median finishers have the biggest effect; and the slowest finishers have a smaller effect.

What explains this curve? It’s hard to know. It could be competition between the two factors I mentioned above: shorter exposure time protects the faster finishers, less heavy breathing protects the slower finishers, but runners in the middle get hammered. There are also lots of other possibilities. Maybe more well-trained runners are less affected by breathing discomfort. Maybe the anti-inflammatory effects of high aerobic fitness confer some protection from the negative effects of pollution. Maybe you actually adapt to polluted air if you train in it enough. There are glimmers of evidence for all these effects, but they remain speculative.

What These Air Pollution Findings Mean in Practice

The idea that air pollution hurts athletic performance certainly isn’t new—recall when U.S. athletes wore breathing masks to protect their lungs with they arrived in Beijing for the 2008 Olympics. What’s different here is that the effects are showing up even at very modest levels of air pollution. The for 24-hour exposure to PM2.5 is 35 micrograms per cubic meter, well above the levels seen in any of the races. The full-year standard was lowered last year from 12 to 9 micrograms per cubic meter. Of all the race-years analyzed, 61 percent of them were below this more rigid 9 micrograms per cubic meter standard—and yet these pollution levels still impacted race times.

One takeaway, then, is that if you’re going for a big marathon PR and you have a private pace car guiding you, it might be worth going electric. More generally, add air quality to the long list of factors to consider in choosing a race or evaluating a performance after the fact. If you set your PR at Boston in 2004, or Chicago in 2011, or Philadelphia pretty much any year before 2015, your coulda-shoulda-woulda time just got a few minutes faster.

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One of the big debates in endurance sports these days is about “training intensity distribution,” which is a fancy term for how much of your training time you spend going easy, medium, or hard. The dominant paradigm is the polarized distribution, which calls for a lot of easy running, a little bit of hard running, and not much in the middle. But there are various other viewpoints, including the currently fashionable Norwegian training, which puts a heavy emphasis on medium efforts.

One way of exploring which training distribution is best is to look at the training diaries of the best endurance athletes in the world. That’s how the concept of polarized training was born, and it’s why Norwegian training is rising in popularity. Of course, this isn’t as reliable as a randomized trial. Maybe most elite athletes train in a certain way because it’s popular, not because it’s objectively better than the alternatives. And even if we figure out the best way for elites to train, it’s not clear that those insights will apply to the rest of us.

Another option to assess training intensity is to look at how the unwashed masses train: to sift through reams of data looking for the patterns and variables that predict the best race performances. That’s the approach taken in , from a group of researchers led by Daniel Muniz-Pumares of the University of Hertfordshire and Barry Smyth of University College Dublin. They analyzed 16 weeks of training data leading up to a marathon for 120,000 runners who recorded their training in Strava.

To Run Faster, Run More

Before delving into the nitty-gritty of training intensity distributions, we should start with the elephant in the training room. By far the best predictor of marathon time was how many miles a runner racked up. The researchers divided their sample into half-hour finishing groups: the fastest group was the sub-2:30 marathoners, the slowest group was those between 6:00 and 6:30.

On average, the runners accumulated 28 miles per week over the 16 weeks prior to their goal race. But there were big differences. Sub-2:30 runners ran 67 miles per week, about three times as much as those running slower than 4:30 and 60 percent more than even the sub-3:00 runners. Here’s the weekly mileage (in kilometers, on the vertical axis) as a function of marathon finishing time (in minutes, on the horizontal axis):

This is the men’s data; the women’s data show essentially the same pattern. The four different lines show the average mileage during four different four-week blocks before the race. There are some slight differences—mileage is highest five to eight weeks before the race, for example—but the overall pattern is the same throughout: faster runners run more.

What the Training Intensity Distribution Reveals

You could be forgiven for thinking that this is painfully obvious. But what’s interesting is how the faster runners ran more. They didn’t just scale up their training proportionally compared to the slower runners. Instead, the difference was almost exclusively in how much easy running they did.

You can divide the accumulated training into three zones loosely corresponding to easy, threshold, and interval or race pace. (I won’t belabor the details of how they crunched the training data or defined the zone boundaries, but it’s based on calculating each runner’s critical speed using the approach I described in this article.)

When you break out the different training zones, you find that runners of all levels, from sub-2:30 all the way through 6:30 marathoners, did virtually identical amounts of hard zone 3 training. They also did very similar amounts of zone 2 threshold training. There’s a slight trend toward the faster runners doing a bit more, but it’s barely noticeable. All the variation—remember, there’s a threefold difference in total training volume—is packed into easy zone 1 running.

The graph below is a little busy (it once again breaks out the results into four-week blocks, even though the trends in each block are similar). The key point is that the red lines (zone 3) are flat, meaning that all the different pace groups accumulated similar amounts of hard running time. The orange lines (zone 2) are nearly flat. But the green lines curve sharply upward on the left side of the graph, showing that the faster runners do more easy running.

So It’s Polarized Training for the Win?

That depends on what you mean by “polarized.” There’s a fairly convoluted debate (which I summed up here) on the meaning of the term, but there are two key elements. One is the idea that most of your running should be easy. That’s often summed up (as in the title of ) as 80-20 running: around 80 percent of your running should be easy, with the other 20 percent medium or hard. Muniz-Pumares’s new results support this view.

The second element is the idea that you should avoid medium intensities, since they’re too slow to give you the benefits of interval training but too hard to recover from if you’re trying to run big miles. That is where the name “polarized” originally comes from, since most of your training is supposed to cluster at the extremes of easy or hard. But the new data doesn’t back this claim up: very few of the runners, whether fast or slow, were doing truly polarized training.

What the runners were doing instead is called pyramidal training. Classic polarized training might involve an 80:5:15 breakdown of easy, medium, and hard. Pyramidal training, instead, might be 80:15:5. Instead of avoiding the middle zone, you do a moderate amount. In practice, though, the distinction between polarized and pyramidal is hazier than it seems. Previous research has found that the exact same training plan might look either polarized or pyramidal depending on whether you calculate the intensity distribution using running speed, heart rate, or even the intended effort.

The bottom line, from my perspective, is that it’s not worth getting too wound up about the specific nomenclature. This data supports the idea of doing lots of easy running and modest amounts of medium or hard running. It doesn’t support the idea of avoiding the medium zone. Whether you call that polarized or pyramidal is up to you.

What’s Lost in Translation

As I noted at the top, this isn’t a randomized trial. We know that faster runners did more easy running than slower runners. We don’t know if doing more easy running would have turned the slower runners into faster runners. But even if it did, that assumes that the slower runners have the time or desire to run more—and that’s by no means a safe bet.

The fundamental assumption for elites is that their training is primarily limited by what their bodies can handle. Polarized (or pyramidal) training is supposed to be effective because it’s an optimal way of racking up the greatest possible combination of training volume and intensity. To max out what your body can handle in a given week, aim for that 80-20 split.

Meanwhile, out in the real world, the key question isn’t how much my body can handle. It’s how much training I can squeeze in before work or between picking up the kids and making dinner or whatever. The 3:30 marathoners are putting in about four hours of training per week. It’s not hard to believe that adding an extra hour or two of easy running on top of what they’re already doing would make them faster.

The trickier—but also more relevant—question is how to make them faster on four hours of training per week. Switching to an 80-20 split would actually mean doing less total mileage, because they would be replacing a big chunk of their medium or hard running with easy running. Sure, they would recover more quickly from each training session. But would they really end up going faster?

This is an open question, and I don’t think there’s any firm answer at this point. But my takeaway from all this is that we should think carefully about what constraints we’re imposing or accepting on our training. If time is really the issue, then spending more of that precious time running hard might make sense for you. But if “I don’t have time” is just another way of saying “I don’t want to,” or if you’ve been held back by the fatigue and injuries that often accompany hard training, then it’s worth considering doing more easy running. It’s the easiest and least risky type of training—and in this analysis, at least, it’s the one weird trick that distinguishes faster marathoners from slower ones.

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Thanks to a wonky hamstring and some bad oysters, I’m currently coming back from a training hiatus of nearly a month. It’s an unfamiliar and somewhat unsettling feeling. So I have a lot of sympathy for Romuald Lepers, a French physiologist and dedicated triathlete who, in the name of science, agreed to take 12 weeks completely off training in 2022.

With the help of his colleagues, Lepers underwent a whole bunch of tests immediately after competing in the Swimrun World Championships in Sweden that fall. After 12 weeks of sloth, he repeated the tests. Then he resumed training and, 12 weeks later, repeated them a third time. The data is presented in a pair of papers in and , offering a detailed look at exactly what happens when you lose then regain fitness, and raising a surprising and tantalizing possibility: maybe a long training break is actually good for you.

Two Theories of Aging

runs a lab at the University of Bourgogne in Dijon, and is one of the world experts on masters athletes, which typically refers to athletes over the age of 40. He’s also an accomplished triathlete. In his younger days, he placed in the top 150 at the Ironman World Championship in Kona. At the time of the study, he was 53, training 10 to 12 hours per week, and still consistently placing near the front of his age category in Ironman 70.3 races.

Over the course of more than three decades, he trained very consistently, never missing more than two weeks of training at a time. So he decided to run an experiment on himself to fill a gap in the literature. There have been various “detraining” studies over the years that measure the loss of fitness when you stop training. A classic 1984 paper, for example, saw a 16 percent reduction in VO2 max after 12 weeks; and this analysis estimated how much a training break of a week or more will affect your marathon time. But there’s very little data on older athletes.

There’s an important debate about fitness loss in older people. We know that it happens, and we know roughly how quickly it happens—on average, at least. Starting in your 30s, you’ll typically lose 0.5 to 1 percent of your VO2 max every year; you’ll also lose muscle mass at a similar rate. One view is that this happens gradually and inexorably. The other view, sometimes referred to as “,” is that we decline at a much slower rate, but every once in a while we have mini-catastrophes—a prolonged bout of flu, a broken hip, a period of intense work or family stress during which we abandon all exercise habits—that lead to a sharp drop. Even if we resume normal training after one of these blips, we never quite make it back to our previous level, so these interruptions contribute disproportionately to our advancing decrepitude.

When you’re 20, you can take 12 weeks off and then, with a bunch of hard work, get right back to where you started. Lepers’s case study offers a test of whether the same is true in your 50s, or whether the body’s adaptive potential is so blunted that some of the losses become permanent.

What Happened After a 12-Week Training Break

The Frontiers in Physiology paper focuses on Leper’s changing fitness. Most notably, his VO2 max, as measured in a treadmill running test, dropped by 10.9 percent. In a similar test on an exercise bike, it dropped by 9.1 percent. That’s a big drop, equivalent to about 15 years of normal aging, but it’s on the low end compared to previous detraining studies. The penalty for time off in your 50s doesn’t seem to be any worse than in your 20s.

When he started training again, there was a surprise. After 12 weeks, his VO2 max didn’t just recover; it was better than when he started. In the running test, it was 4 percent higher than baseline; in the cycling test, it was 6 percent higher. There’s some inherent variability in VO2 max testing (and in all physiological testing, for that matter), but the fact that the same pattern showed up in the running and cycling tests suggests that the effect is real.

For a guy who’s been training and competing at a high level almost continuously for three decades, that’s an unexpected result. There are a couple of possible explanations. One is that his body composition changed. VO2 max is expressed relative to your body weight, so losing weight can create the illusion that you got fitter without changing your oxygen-processing abilities.

That’s not what happened here, though. His body did change: he initially gained 5.5 pounds of fat and lost 4.6 pounds of muscle. Then, after retraining, he lost 9.0 pounds of fat and regained 2.4 pounds of muscle. That meant his body fat went from 10.1 percent to 13.3 percent to 8.4 percent, with a net loss of 5.7 pounds by the end of the experiment. This explains some of the change in VO2 max, but not all of it: his overall oxygen-processing capacity still improved, independent of his weight.

The other possibility is that something changed within his muscles to make them more responsive to training. The second paper, in JCSM Communications, explores this possibility. Lepers underwent muscle biopsies at each stage of the experiment to measure the metabolic properties of his muscles. Detraining ramped up fast-twitch muscle activity and ramped down markers of mitochondrial function and aerobic capacity. Retraining mostly reversed the changes, and in some cases resulted in better-than-baseline muscle properties.

The details of what’s going on inside the muscles are fairly complex, and Lepers cautions that we shouldn’t read too much into a single case report. (That goes for all the findings; we have no idea if he’s just a freak.) But it’s interesting that the two sets of results seem to line up: the microscopic properties of his muscle and macroscopic fitness measures like VO2 max both declined with detraining then bounced back to be better than before with retraining.

So Should We All Take a 12-Week Training Break?

When I asked Lepers this question, he raised a couple of interesting points. One is that the psychology of taking such a long break went better than expected. He kept busy with work, ate normally, and didn’t stress because he knew that retraining would be a fun challenge. Crucially, he knew that once the 12 weeks were up he’d be able to start training again. That’s very different from, say, missing three months with a lingering injury where you’re never quite sure if it’s going to go away.

He also noted that many of his masters friends used to take long end-of-season breaks of a month or more when they were younger. But the older they get, the shorter and less frequent their breaks have become, presumably because they’re afraid that whatever fitness they lose they’ll never get back. If there’s one big headline finding from Leper’s self-experiment, it’s that this isn’t true, or at least wasn’t for him. That should help other masters athletes be a little less paranoid about the dangers of an occasional training break.

We can’t really claim, on the basis of a single case report, that taking 12 weeks off will enable you to break through and reach new levels of fitness. But even if you don’t return better than before, the idea that you can get back to your previous level is very reassuring. Over the years, I’ve found that I enjoy being fit, but what I really love is getting fit: the sensation of steady progress when week after week your times are dropping and your workouts are getting better. That feeling is increasingly hard to come by when you’ve been training for a long time. The best part of training breaks, from my perspective, isn’t vegging out on the sofa during the break; it’s getting back on the horse.

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For the past decade or so, sports scientists have been obsessed with the benefits of heat training. The extra stress of heat triggers various adaptations that help you handle hot conditions, like more sweating. Some of these adaptations, like increased blood volume, may even give you a boost when competing in cooler conditions. As a result, many top athletes now incorporate elaborate heat protocols into their training.

What if the opposite is also true? At in Montreal last month, a physiologist named Dominique Gagnon presented new data suggesting that cold training might offer some unique metabolic benefits that translate into enhanced health and endurance performance. It’s just a hypothesis at this point, based on a decade’s worth of incremental research. But as we head into the darkest, coldest months of the year, it’s kind of nice to think that our winter training might pack an extra punch.

Gagnon is a Canadian who recently moved from Laurentian University, in northern Ontario, to Finland’s University of Jyväskylä, three hours north of Helsinki. He knows cold, in other words. At the annual Canadian Society for Exercise Physiology conference, he presented comparing the training effects of working out in either warm (77 degrees Fahrenheit) or cool (32 degrees) conditions. The goal was to figure out whether training in the cold would boost levels, which is one of the key adaptations that underlies aerobic fitness.

What’s So Great About Cold?

Gagnon’s research on exercise in the cold goes back over a decade. Back in 2013, for example, he published showing that cold-weather exercise relies on a different fuel mix than warmer conditions, burning more fat and less carbohydrate. He suspects that this is because when you’re exercising in comfortable temperatures, there’s actually some local overheating in the muscles themselves.

Human metabolism is only about 25 percent efficient—comparable to the internal combustion engine in your car—so three-quarters of the energy in your food is released as heat in the muscles. That means that the temperature inside your muscles can be high even when the rest of you is cool. The advantage of exercising in the cold, then, is that it prevents your muscle cells from overheating and enables them to keep burning more fat for aerobic energy, which relies on the mitochondria in your muscles. In the long run, that should boost mitochondria levels and train your body to become more efficient aerobically.

There are various other hints supporting this view. Researchers at the University of Nebraska at Omaha, for example, that exercise in the cold produced a bigger spike in the cellular signals that tell the body to produce more mitochondria, though the difference wasn’t statistically significant. And have shown that they get a bigger fitness boost from exercise when the air is mildly cold.

The New Findings on Cold Training

In Gagnon’s new study, 34 volunteers trained three times a week for seven weeks, doing interval workouts on an exercise bike. Before and after the training period, they had muscle biopsies, which involve removing a small chunk of muscle from the leg, in order to analyse how much mitochondria was present. Sure enough, the group that trained in 32-degree air had a significantly greater increase in several different markers of mitochondrial content. Gagnon is still analyzing the VO2 max data, but initial signs are that those training in the cold were more likely to see a significant increase.

Those are encouraging findings. But even if the results (which have not yet been peer-reviewed) hold up, the next big question is whether this approach is practical. How cold do you have to be? Gagnon’s subjects performed their cold training in the equivalent of shorts and a T-shirt, which is less than I would typically wear at that temperature, but not totally unreasonable. Would the effects be nullified if you wore a long-sleeve shirt and tights? Gagnon’s not sure yet—but he emphasized that the goal isn’t to be cold, with measurably lower muscle and body temperature. Instead, it’s to avoid letting your muscles get too hot.

At this point, it’s worth flashing back to some findings I wrote about earlier this year. Stephen Cheung and his colleagues at Brock University in Canada showed that getting superficially cold, with no drop in core temperature, reduced time to exhaustion in a cycling test by about 30 percent. That involved sitting in a 32-degree room with a light breeze for half an hour before the subjects even started cycling. Staying in the room for longer, so that their core temperature actually dropped by a degree, reduced endurance by another 30 to 40 percent. This is not what Gagnon is aiming for.

Instead, the goal of cold training seems to be to let yourself get just cool enough that your muscles don’t overheat. Where that threshold is remains to be determined, and the results will need to be replicated before anyone takes them seriously. Gagnon is in discussions with the Finnish military, which has lots of personnel engaging in physical activity in perennially cold conditions, about further studies. Maybe it will turn out to be the next big thing in endurance training. Or maybe not. To be totally honest, I normally wouldn’t write about such preliminary results—but the idea that it might be true will help get me through some cold training runs this winter.

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If you could patent and sell the idea of holding your breath before exercise to boost performance, it would be a bestseller. Not because it works, necessarily—the jury is still out on that. But because the logic is so good, the physiology is so fascinating, and the technique is so simple.

Instead, without a commercial imperative behind it, the idea has been floating around for years with no clear answers about whether it really works or not. Now , from a team led by Yiannis Christoulas of Aristotle University of Thessaloniki in Greece, offers the most encouraging sign yet that breath-holding might function as a legal form of do-it-yourself blood doping to temporarily enhance your endurance.

The idea is based on the mammalian diving reflex, which is the suite of physiological responses that automatically takes over when you dunk your head underwater. In a whole bunch of different ways, your body switches into oxygen-conserving mode to make sure you don’t run out while you’re submerged. For example, your heart rate slows down, and blood-flow to your extremities ramps down.

How Divers Get a Boost in Red Blood Cells

Most relevant here is that your spleen stores an extra reserve of oxygen-carrying red blood cells. When you dive, your spleen contracts, squeezing these extra red blood cells out into general circulation. This is the bit that is “similar to blood doping interventions,” Christoulas and his colleagues explain: instead of giving yourself an IV with fresh red blood cells, you get them from your spleen. Previous experiments suggest that spleen contraction could boost hemoglobin levels and VO2 max by as much as 5 percent.

Nobody is suggesting that you should go freediving a few minutes before your next marathon. But you can elicit some aspects of the diving response simply by holding your breath; and you can ramp up the response by doing it with your face submerged in cold water. This is the idea that prompted by researchers in France suggesting apnea—that is, breath-holding—as “a new training method in sport.”

Since then, there have been several attempts to harness the benefits of breath-holding for athletic gain, with different activities (swimming, cycling) and protocols (single breath-holds, repeated breath-holds, various recovery durations). Earlier this year, Jan Bourgois and his colleagues at Ghent University in Belgium an overview of these attempts in Experimental Physiology. The overall picture is that the physiology is real, but the practical effects seem to be too small to measure. Hard exercise makes your spleen contract anyway, so it may be that pre-contracting it with breath-holds doesn’t offer any additional benefit.

Dunking Your Face in Water May Be the Key to Breath Hold Success

Christoulas’s new study takes a different view, which is that previous studies haven’t gotten the protocol quite right. In particular, the failed studies have asked athletes to hold their breath, but haven’t dunked their faces in water, so they didn’t fully contract their spleens. The new study had 17 volunteers complete an incremental cycling test to exhaustion, lasting roughly ten minutes, with and without a series of five maximal breath-holds with face submerged in water at 50 degrees Fahrenheit. They took two minutes recovery between each breath-hold and then started the cycling test two minutes after the final hold.

The subjects were recreational athletes with no training in freediving or breath-holding. Their average breath-hold time was 71 seconds—though it’s interesting to note that the duration of each successive hold got longer. The first hold averaged just over 40 seconds; the second one was over 60 seconds; the last couple averaged close to 80 seconds. Here are the average breath-hold times (BHT), plus and minus standard deviations, for the five holds:

This progression is partly a result of the spleen’s extra red blood cells in action. Sure enough, blood tests showed that hemoglobin and red blood cell count were both up by 4 percent by the end of the last breath-hold.

The key performance result was that the subjects lasted, on average, 0.75 percent longer in the cycling test after the breath-holds, which was a small but statistically significant difference. It also took longer before they hit the second ventilatory threshold, which is the point when your breathing gets really labored.

It’s More than Just Red Blood Cells Increasing Subjects’ Endurance

It’s worth noting that there are several other mechanisms that might play a role in addition to spleen contraction. Breath-holding raises levels of carbon dioxide in the blood, which in turn (through a mechanism called ) makes it easier for your muscles to unload oxygen from circulating red blood cells. This boosts your aerobic metabolism, and helps explain why the blood tests also showed that resting lactate levels dropped by 15 percent after the breath-holds. The full physiological picture gets quite complicated, but the bottom line is that the subjects in the new study had better—slightly ˛ú±đłŮłŮ±đ°ů—e˛Ô»ĺłÜ°ů˛ą˛Ôł¦±đ.

What does this mean in practice? Personally, I can’t imagine completing a set of five maximal breath-holds two minutes before a race. But some researchers have suggested that a single hold should be enough to get most of the benefits. If you look back at that graph of breath-hold times, it does appear that the biggest change occurs after the first bout, and there are fewer changes after the second one. Maybe two breath-holds a few minutes before competition is feasible.

The other big question is whether a good, hard warm-up accomplishes the same thing. In the new study, all subjects did a ten-minute warm-up that included jogging and “dynamic whole-body stretches.” But it’s possible that a longer and harder warm-up might trigger spleen contraction on its own. These are questions that future studies will have to answer—and I hope they do, one way or the other, because it’s refreshing to consider a weird and wonderful source of potential “marginal gains” that, for a change, is free for everyone.

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To predict your longevity, you have two main options. You can rely on the routine tests and measurements your doctor likes to order for you, such as blood pressure, cholesterol levels, weight, and so on. Or you can go down a biohacking rabbit hole the way tech millionaire turned did to live longer. Johnson’s obsessive self-measurement protocol involves tracking more than a hundred biomarkers, ranging from the telomere length in blood cells to the speed of his urine stream (which, at 25 milliliters per second, he reports, is in the 90th percentile of 40-year-olds).

Or perhaps there is a simpler option. The goal of self-measurement is to scrutinize which factors truly predict longevity, so that you can try to change them before it’s too late. A new study from biostatisticians at the University of Colorado, Johns Hopkins University, and several other institutions crunched data from the long-running National Health and Nutrition Examination Survey (NHANES), comparing the predictive power of 15 potential longevity markers. The winner—a better predictor than having diabetes or heart disease, receiving a cancer diagnosis, or even how old you are—was the amount of physical activity you perform in a typical day, as measured by a wrist tracker. Forget pee speed. The message to remember is: move or die.

How to Live Longer

It’s hardly revolutionary to suggest that exercise is good for you, of course. But the fact that people continue to latch on to ever more esoteric minutiae suggests that we continue to undersell its benefits. That might be a data problem, at least in part. It’s famously hard to quantify how much you move in a given day, and early epidemiological studies tended to rely on surveys in which people were asked to estimate how much they exercised. Later studies used cumbersome hip-mounted accelerometers that were seldom worn around the clock. The , published in Medicine and Science in Sports and Exercise, draws on NHANES data from subjects recruited between 2011 and 2014, the first wave of the study to employ convenient wrist-worn accelerometers that stay on all day and night.

Sure enough, it turns out that better data yields better predictions. The study zeroed in on 3,600 subjects between the ages of 50 and 80, and tracked them to see who died in the years following their baseline measurements. In addition to physical activity, the subjects were assessed for 14 of the best-known traditional risk factors for mortality: basic demographic information (age, gender, body mass index, race or ethnicity, educational level), lifestyle habits (alcohol consumption, smoking), preexisting medical conditions (diabetes, heart disease, congestive heart failure, stroke, cancer, mobility problems), and self-reported overall health. The best predictors for how to live longer? Physical activity, followed by age, mobility problems, self-assessed health, diabetes, and smoking. Take a moment to let that sink in: how much and how vigorously you move are more important than how old you are as a predictor of the years you’ve got left.

These results don’t arrive out of nowhere. Back in 2016, the American Heart Association issued a scientific statement calling for cardiorespiratory fitness, which is what VO2 max tests measure, to be considered a vital sign that doctors assess during routine checkups. The accumulated evidence, according to the AHA, indicates that low VO2 max is a potentially stronger predictor of mortality than usual suspects like smoking, cholesterol, and high blood pressure. But there’s a key difference between the two data points: VO2 max is about 50 percent determined by your genes, whereas how much you move is more or less up to you.

Fitness Trackers Are Key to New Longevity Findings

All this suggests that the hype about wearable fitness trackers over the past decade or so might be justified. Wrist-worn accelerometers like Apple Watches, Fitbits, and Whoop bands, according to the new data, are tracking the single most powerful predictor of your future health. There’s a caveat, though, according to Erjia Cui, a University of Minnesota biostatistics professor and the joint lead author of the study. Consumer wearables generally spit out some sort of proprietary activity score instead of providing raw data, so it isn’t clear whether those activity scores have the same predictive value as Cui’s analysis. Still, the results suggest that tracking your total movement throughout the day, rather than just formal workouts, might be a powerful health check.

The inevitable question, then, is how much movement, and of what type, we need in order to live longer. What’s the target we should be aiming for? Cui and his colleagues track the raw acceleration data in increments of a hundredth of a second, which doesn’t translate very well to the screen of your smartwatch. The challenge remains about how to translate that flood of data into simple advice regarding how many minutes of daily exercise you need, how hard that exercise needs to be, and how much you should move around when not exercising.

To be honest, though, I’m not sure the quest to determine an exact formula for how much we should move is all that different from the belief that measuring your urine speed will give you actionable insights about your rate of aging. Metrics do matter, and keeping tabs on biomarkers backed by actual science, like blood pressure, makes sense. But it’s worth remembering that the measurement is not the object; the map is not the road. What’s exciting about Cui’s data is how it reshuffles our priorities, shifting the focus from all the little things our wearable tech now tracks to the one big thing that really works—and which is also a worthwhile goal for its own sake. Want to live longer? Open the door, step outside, and get moving.

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