Though many runners treat treadmills as a necessary evil, treadmill access is a must-have if you live anywhere that gets extreme cold or extreme heat and want to train seriously year-round. Being from Minnesota (and having coached many runners in the Midwest), I’ve had plenty of experience modifying workouts and training sessions for the treadmill.
On top of that, I’ve just wrapped up a biomechanics study as a part of my PhD dissertation that involved 60 runners completing a treadmill run in a motion capture lab, so I have a lot of experience working with biomechanical data from treadmill running.
When you’re doing treadmill workouts, you can’t always just translate your outdoor workouts 1:1 and expect everything to go well. There are several important physiological, biomechanical, and psychological aspects of treadmill running that differ from outdoor running.
Moreover, I’ve found that a lot of runners have ideas about treadmill training that aren’t in alignment with the scientific research on treadmill running. So, this article is designed to refute some of these incorrect ideas, and provide some guidance on how to incorporate treadmill running, when necessary, into your own training.
Just want to know the most important info before you hit the treadmill? Click the link below to go directly to my seven scientifically-supported best practices for treadmill training.
Biomechanics and physiology of treadmill running
First, let's review some basic facts about treadmill running versus outdoor running:
Treadmill running is almost as physically demanding as running outside
One flawed intuition that many people have about treadmill running is that you’re “just staying in place,” so treadmill running can’t possibly be as physically demanding as running outside.
Treadmill running, the logic goes, just requires you to support yourself upright, while during running outdoors, you have to propel yourself forward. This reasoning is fundamentally wrong, and comes out of an inability to understand reference frames. Think about it like this: imagine you made a video of yourself running on a track, using a GoPro set up on a tripod. Now imagine taking that GoPro inside and duck-taping it to the treadmill belt (let’s assume it’s a very long treadmill!).
Would your running form in the videos look any different? No! From the perspective of the ground (outside) and the treadmill belt (inside), you are propelling yourself forward in both cases.
Not convinced? Here are two plots of the horizontal ground reaction force during running, both on a treadmill and over ground. Notice how both plots feature the same characteristic braking impulse (the negative part of the graph) during the first half of ground contact, and a propulsive phase (the positive part of the graph) in the second half, as you push yourself forward.
The same argument, by the way, applies when the treadmill is set to an incline: from GoPro-view, running up a 5% gradient hill outdoors looks the same as running on a treadmill set to 5% incline, in terms of vertical displacement of your body’s center of mass.
Moreover, under controlled conditions, physiological measures of effort, like oxygen consumption (“V-dot”), heart rate, and blood lactate levels, are almost the same for a given speed . We’ll get to the “almost” part later: part of it has to do with one big difference between treadmill running and outdoor running, which is air resistance.
Your biomechanics during treadmill running are almost the same as running outside
The overall features of running, like the support and propulsion of your body’s center of mass, are the same during treadmill running and outdoor running, but what about more subtle things, like your footstrike angle or the way your knee bends when you hit the ground?
A 2019 review study analyzing 33 different treadmill-vs-overground biomechanics studies on a total of almost 500 runners concluded that treadmill and overground running are pretty comparable :
>Spatiotemporal, kinematic, kinetic, muscle activity, and muscle–tendon outcome measures are largely comparable between motorized treadmill and overground running.
There were a few notable differences, though. Compared to running overground, when you run on a treadmill, you have:
- More of a forefoot strike, to the tune of about 10 degrees. Typical footstrike angles range from about -20 to +20 degrees .
- Less vertical oscillation (1.5 cm less). Typical values are 6.1 to 11.3 cm .
Those are the changes I find most salient, given their effect size relative to the range of typical values you’d see. There are other differences, but they have pretty trivially small effects. On the treadmill, you also have:
- A slightly more flexed knee at footstrike (2.3 degrees): typical values here range from 10 to 26 degrees , so this is really tiny.
- Slightly less knee flexion during stance (six degrees). This makes sense given the decreased vertical oscillation.
- Slightly longer ground contact time (5 ms). This one barely registers as statistically meaningful.
The footstrike difference is probably the most relevant difference when it comes to any changes in injury risk.
For example, one study found that Achilles tendon force is 12.5% greater on the treadmill, compared to overground running at the same speed .
To me, this is further evidence that runners (on average!) adopt a more “calf-driven” strategy on the treadmill. Overall, though, treadmill running is very similar, biomechanically speaking, compared to overground running.
A brief aside: Overground versus real outdoor running
You might have noticed that I’ve been using the phrase “overground” in the discussion above on biomechanical differences. That’s because virtually all biomechanics labs are not set up to study “real” continuous running outdoors on pavement, grass, or tracks.
These studies involve running down a short runway, then walking back to the start and doing it again. Labs vary on the length of their runway; at Indiana, for example, ours is really short–only a bit longer than ten meters. You basically only have a few steps where you’re running at a constant speed before you slow down again.
Plus, you’re basically running in an indoor office space, which is not at all the environment you’re used to running in.
Based on my own experiences running down these kinds of lab runways, I tend to be pretty skeptical that these overground studies are really reflective of how people run in the real world. So, when you hear “overground,” picture running a few steps down a carpeted hallway in an office building–not a snapshot of your gait in the middle of your usual 10-mile training loop.
Treadmill running feels faster than it really is
Despite trivial to nonexistent differences in the actual physical demands of the treadmill, many runners find that treadmill running feels faster than it really is.
This was well-illustrated by a clever study that had runners on a treadmill manipulate the speed of a virtual-reality scene (running down a simulated hallway) until they felt that the VR scene’s speed matched their running speed. Across three different speeds, runners consistently overestimated their own speed .
Indeed, if you hide the treadmill speed readout and ask runners to set the speed to their typical running pace, they set the treadmill much slower (though I’m a bit skeptical of these results because of the same overground vs. true outdoor running issue mentioned above) [8,9].
The differences between treadmill running and outdoor running
We’ve seen that the physiological and biomechanical demands of treadmill running are pretty comparable. However, there are some important differences both in the abstract and in practice that we need to take into account when adopting workouts for the treadmill.
Air resistance is significantly lower on the treadmill
When you run outside, even on a calm day, you have to overcome air resistance. At 6:00 mile pace, for example, you effectively have a 10 mph headwind. The difficulty of overcoming this resistance increases with the square of running speed–so at 5:00 mile pace, the air resistance you face is four times greater than at 10:00 mile pace.
As a consequence, differences in expected oxygen consumption (read: metabolic demand, i.e. difficulty) between the treadmill and running outdoors are negligible at slow speeds, but increase dramatically as speed increases. At 4:30 mile pace, for example, metabolic calculations suggest that oxygen consumption could be up to 8% lower on the treadmill vs. outdoors .
The lack of air resistance on the treadmill means your body heats up faster
A far less-discussed consequence of the lack of air resistance on the treadmill is the fact that your body’s ability to shed heat is significantly impaired.
This phenomenon is shockingly under-studied (perhaps because researchers don’t want to own up to how unrealistic their poorly-ventilated exercise labs are?), aside from a short exploratory study in a Brazilian sports science journal that shows–not surprisingly–that increasing airflow with a fan decreases skin temperature during treadmill running .
Anyone who has done a long run on a treadmill doesn’t need a study for proof–your sweat-soaked clothes at the end of a run are proof enough that the heat load during treadmill running is much higher than doing the same run outdoors (even if the ambient temperature were the same).
Most treadmills are not located in thermally optimal conditions
If I had my own physiology lab, one priority would be to have a top-of-the-line HVAC system with tightly-controlled heat and humidity settings.
Why? Because the human body generates an immense amount of heat during running, which gets dumped into the room that the treadmill is in. Combine that with a lack of airflow over your body, and you’ve got a recipe for overheating.
Rough figures on caloric expenditure during running indicate that a typical runner could generate over 1000 Watts of heat when running at 6:00 mile pace . And of course, the faster you run, the more heat you generate. Add in a few hundred watts from the treadmill itself, and you’re essentially running a 1500 watt space heater at full-blast. After several miles, the room you’re running in will get very hot and humid.
Even in a large gym, where the air temperature is constant, the air temperature is usually going to be in the 70s in Fahrenheit–still a bit hotter than you’d like for long, fast workouts.
And finally, for runners in cold climates, you aren’t going to be heat-adapted during the winter months, so you’ll be far more affected by warm temperatures.
All these factors conspire to make treadmill running more thermally demanding, in practice, than you might naively expect.
Treadmill running is much more tightly constrained than outdoor running
When running on the treadmill, you have to regulate your gait mechanics extremely tightly. Deviate even a bit from the belt speed and you’ll run up against the front of the treadmill, or fly off the back. Your lateral position can’t change much either.
Overall, I think this is a bad thing: each step on the treadmill is extremely similar to the previous one. This probably concentrates loading on your body, instead of dispersing it out across a wider range of tissues. This has some (theoretical) implications for injury risk, which we'll get to in a moment.
You can’t self-regulate your speed on the treadmill
In the real world, you can self-regulate workout difficulty by changing your speed within the reps. On the treadmill, you can only manually adjust the speed or incline. Given that real-world running and racing involves self-regulated pacing, we should consider treadmill running less “specific” of a training in this regard.
The real issue with an inability to self-regulate running speed is that workouts can get quickly out of hand. Suppose you think you’re in about 32:00 10k shape, and your coach prescribes 10x1k at 10k pace (3:12/km = 11.65 miles per hour).
If you did this workout on the track, and were having a bit of an off-day, you might find that your splits go something like 3:14, 3:12, 3:13, 3:15, 3:16, 3:18, 3:17 …–i.e., you can’t quite hit the right pace early on, then you find that your speed gradually drifts slower until you settle in to your actual 10k pace, for how you’re feeling on that day. This is fine: off-days happen, and your estimate of your fitness level is going to vary by 1-2% anyways.
What happens on the treadmill? Well, usually runners will set the belt to 11.7 (always gotta round up, you know!) and hang on until they completely tank on rep 7 or 8.
Very quickly, the backside of this workout can turn from what was intended to be a pretty controlled effort into an extremely challenging, knee-grabbing killer of a session.
Not all treadmills are calibrated correctly
You might take for granted that a treadmill set to 10 miles per hour is truly moving at that speed, but alas, that isn’t always the case. Two factors affect the accuracy of a treadmill: the first is whether the motor speed-to-belt speed relationship is calibrated correctly in the treadmill’s software, and the second is whether the treadmill actually maintains this speed when you are actually running on it.
That first point is not that hard to get right–it’s just a matter of getting the motor spinning the correct speed, given the length of the treadmill belt, the diameter of the rollers, and the desired speed. Things are trickier when a runner is actually on the belt–when you land on the treadmill, the reaction forces from the ground pinch the belt between the platform and your foot, slowing or even stopping it momentarily.
Moreover, the rearward braking and forward propulsion forces also alter the belt’s speed, and all three of these forces increase as a function of running speed and body weight. Poor maintenance, which can cause increased belt friction or slack in the belt itself, will also impede accurate belt speed, especially as the treadmill heats up over the duration of a run.
Factors that affect treadmill accuracy
In general, the following qualities will tend to make a treadmill more accurate:
- A newer treadmill (older models are more likely to have wear and tear that affects accuracy)
- Commercial-grade treadmills (which tend to be designed for long-term use)
- Treadmills that get regular maintenance
- Treadmills with more powerful motors and higher power draw (since they can more effectively counter belt pinching)
- Running at a slower speed
And in contrast, treadmill inaccuracy is most liable to be a problem for heavy runners, at high speeds, on an older, poorly-maintained, consumer-grade treadmill with a weak motor.
Treadmill calibration and foot pod accuracy
How do you know if your treadmill is accurate? That’s easy–just calibrate it. How? Well, I have a whole separate article (forthcoming) on how to calibrate a treadmill. It’s fairly straightforward, though it does require a friend to help you out. Another option is to wear a foot pod, like the Stryd pod or the COROS POD 2. But then, how do you know these devices are accurate? Without hard data, you can end up going down an accuracy rabbit hole.
As a part of my dissertation studies mentioned earlier, I do have comprehensive accuracy data on the Stryd pod. Unfortunately, I’m not able to share it just yet… I will update this post once I have defended my PhD and published the data (current estimate for that is late spring/early summer 2023).
Treadmill platform stiffness can affect the difficulty of running
One point of differentiation among different treadmill manufacturers is the construction and support of the treadmill “platform,” or “bed”–the solid plate that’s underneath the belt. On the kind of research-grade treadmills that you’d find in a biomechanics lab, this platform is a thick steel plate bolted directly to a concrete slab.
The amount of flex or bounce that you’d get out of it is pretty much zero. On the other hand, the kinds of treadmills you might find at a gym or buy for home use might include thinner metal plating, springs, or shock absorbers, among a range of contraptions designed to soften your landing.
(Though not directly related to my point here, it’s worth emphasizing that a bouncier or stiffer treadmill platform will have very little effect on the actual mechanical stress your body experiences–it’s not hitting the ground that causes injury; it’s pushing off the ground . And the muscle forces that propel you off the ground don’t appreciably change on different surfaces.)
Some research suggests that bouncier treadmills can make running easier. A 2017 study for example, found a 7% decrease in the energetic cost of running, and a 5% decrease in heart rate, when running on a bouncier treadmill compared to treadmill with a stiffer platform . Still, the mechanical properties of the treadmills you’d find at a gym are fairly similar, compared to the mechanical differences between treadmills and outdoor running surfaces .
Seven scientifically-supported best practices for treadmill running
Given everything we’ve seen so far about treadmill running, here’s how to put it all together in training.
1. It’s not worth setting the incline to 1% to “counteract” the lack of air resistance
A highly-cited study from 1996 argued that setting the treadmill incline to 1% is the best way to account for the lack of air resistance on the treadmill. By comparing oxygen consumption both outdoors and on a treadmill, the researchers showed that a 1% incline best matches the oxygen demands of outdoor running–but only to a statistically meaningful extent at speeds faster than 7:10 mile pace . Slower than that and it’s more or less a wash.
This is one of those research findings that has taken on a life of its own; some runners take this “1% rule” as practically gospel. I don’t recommend following it, even during workouts at fast speeds, for the following reasons:
First, in practice, the thermal stress on the treadmill usually more than offsets the lack of air resistance. I have yet to see a treadmill setup that’s adequately cooled or ventilated. Especially in long workouts, the heat buildup on the treadmill ends up making the treadmill session harder than it would’ve been outside, even without an incline.
One often-overlooked point about that 1996 study is that it assessed oxygen cost only during six-minute bouts of running (with a six-minute rest afterwards!)–not nearly long enough to see the effects of heat buildup, especially considering that the authors reported that the in-lab temperature ranged between 57 and 64 degrees F.
Second, though setting the incline might make the metabolic demands of the workout more specific to outdoor running, a 1% incline makes the biomechanical demands less specific. Most runners are training for races that take place mostly on flat ground, so you want your gait mechanics on the treadmill to be as close as possible to your gait mechanics during the race you’re training for.
(Incidentally, this is a big part of why I generally advise against using curved, nonmotorized treadmills–even though you can easily get the same metabolic demand as running outside, the mechanics are different, which is not what we want from a specificity-of-training perspective.)
Third, setting the incline increases mechanical stress on your calves and Achilles compared to running at the same speed on flat ground . Inclines aren’t as much of an issue for the knee or the shin [18,19], but given that treadmill running already increases calf and Achilles forces compared to overground running, I think it’s unwise to add to this already-elevated stress by working out on an incline.
Fourth, if you map this same concept on to other aspects of training it doesn’t make any sense at all. Would you do a tempo run on a 1% uphill grade if you were wearing supershoes, or if you knew you’d have a tailwind? Certainly not.
On balance, I don’t think the tradeoffs with setting the treadmill incline make sense. If your workout feels too easy, you can always just set the speed faster–that way, you’re still getting a more road-specific stimulus.
2. Drink water or sports drink during treadmill workouts
As discussed at length above, treadmill running usually puts a lot of heat stress on your body, and because of the lack of airflow, sweat isn’t going to evaporate as effectively either.
You can partly counterbalance the deleterious effects of this heat stress by keeping a water bottle handy and drinking from it when you’re thirsty. As I’ve said before, drinking “ad libitum” – meaning “drink water when you're thirsty” – is a superior strategy to any pre-planned hydration schedule.
If you’re doing a workout, it’s a good idea to put a sports drink in your water bottle. I recommend this even for non-marathon workouts on a treadmill, more for the “mouth rinse effect” than for any benefit from the carbs.
Drinking or even swishing a sports drink in your mouth lights up motivational and reward circuits in your brain, which improves endurance (especially in treadmill time-to-exhaustion tests, where performance is heavily dependent on your motivation level–which is pretty similar to the mental state you’re in when you are trying to get through a tough workout on a treadmill) .
Because the actual amount of carbs isn’t that important for non-marathon workouts, you can water down your sports drink if you’d like. As long as the drink tastes somewhat sweet, you should get a performance benefit above and beyond the hydration benefits of water.
3. Listen to music, especially during treadmill workouts
The mismatch between your perceived speed on the treadmill and the actual belt speed can make treadmill workouts especially difficult. One simple fix is to listen to music–especially fast, upbeat music that you like. This is probably in the category of “did they really need to do a study to prove it?”, but playing music during intense treadmill running decreases perceived effort and increases time to exhaustion, even among fairly well-trained athletes [21,22].
Some people advocate trying to match the tempo of your music to your cadence during your run–I’m sympathetic, in theory, to this idea, but in practice it ends up being too complicated. Your cadence is a function of your running speed, and is idiosyncratic to you as a runner.
Your speed-cadence relationship differs from other runners, so you’d need a tool that reads in data from previous workouts, plus your planned workout for the day, to predict your target cadence throughout the workout. For now, just pick upbeat, fast-tempo music that you like–that’s what the evidence supports using anyways [23,24].
4. Rotate through different shoes if you train on the treadmill a lot
Because treadmill running is so constrained compared to outdoor running, your running gait is going to be a lot less variable–meaning each step is going to be very similar to the last. This means that the same tissues in your body are going to get loaded the same way, over and over, to a greater extent on the treadmill than outdoors.
Computational models suggest that mechanical stress is not evenly distributed during running, even in a given tissue: the particulars of exactly what muscles are producing exactly how much force can cause localized concentrations of mechanical stress in specific areas–e.g., the lower inside corner of your Achilles tendon, or the medial border of your tibia halfway up your ankle.
Check out the image below from a finite element model for an example:
Notice how the areas of high bone stress are highly localized. In theory at least, it’s better to spread out stress across the whole tissue to reduce injury risk, so treadmill running seems like bad news in that regard.
One partial fix? Rotate through a couple different pairs of shoes for the treadmill. Ideally, these shoes would be pretty different from one another–probably different brands, if you can swing it–though I’d avoid shoes that might aggravate any historical injuries. For example, I wouldn’t recommend zero-drop shoes if you have a history of Achilles or calf issues.
The different midsole constructions and shoe geometries should help spread out some of the tissue stress day-to-day, ameliorating the repeated loading on the treadmill to some degree.
There’s some weak, circumstantial evidence that shoe rotation is a good idea for outdoor running too , but the argument here is more theoretically-driven than empirically-driven.
5. Start treadmill workouts conservatively, progress the pace gradually, and don’t be afraid to back off
Because so many factors affect how difficult treadmill running is going to be, it’s worth starting workouts more conservatively than you would outside, then progressing the pace down gradually according to how you feel.
This is doubly true if you’re running on an “unfamiliar” treadmill–e.g. at a hotel gym when traveling. You don’t know if the treadmill is accurate, how hot the room is going to get as you get into the workout, and how the stiffness of the treadmill platform might affect your running economy.
Especially for long workouts, you may find that you need to dial the pace back in the final few miles if you are getting serious heat buildup.
6. Modify the workout if your effort level is getting out of control
Compared to outdoor workouts, you should be far more eager to dial back the pace or end the workout early, because of your inability to self-regulate its pacing like you can outside.
When runners are craving a very tough training session, I like workouts with “self-limiting” properties, like long, strong runs (especially through hills), and long repeats, like 5x2km or 3x3km at 10k to 8k pace. Unlike more traditional hard sessions like 10x400m or 8x800m, it’s hard to completely destroy yourself by running far beyond your fitness level, since the workout is too aerobic and too long to “fake.” If you fly too close to the sun, your pace will usually fade gracefully, instead of exploding spectacularly.
The problem with the treadmill is that you can destroy yourself with these kinds of sessions, because you lose that self-limiting property: no matter how bad you feel, the belt speed isn’t going to slow down!
Before starting your workout, you should have an idea of how difficult it should be–say, 8/10 effort, if 10/10 is an all-out race. As you get into the second half of the workout, monitor your effort level and modify the pace or the number of repeats to keep things under control.
7. Aim to do at least a little race-specific running on land before a major race
There’s no question you can get extremely fit training mostly or exclusively on the treadmill–2:13 marathoner Justin Young famously ran up to 140 miles a week, almost all of it on a treadmill.
Still, I think it’s worth going out of your way to do at least one or two race-specific sessions overground in the final few weeks before a major race. The reasons for this mostly have to do with race-specificity: you want to spend at least a little bit of time running at race pace, on your goal race surface (and ideally in your racing shoes). Getting a bit of running in at race pace also helps you dial in your internal sense for what the appropriate pace actually feels like, which is immensely useful during a race.
If the conditions outside are very bad, you might have to get creative. Indoor tracks, inflatable sports domes, football stadiums, and covered parking garages are all places you might be able to sneak in a workout. It doesn’t need to be much–even just a handful of 500s at marathon pace would probably be worth doing as a mini-workout in the last week or two before your race to get a feel for race pace.
Switching your run or workout to the treadmill is a good idea if the heat, cold, or footing won’t allow a good-quality outdoor training session. For the most part, treadmill running is comparable to outdoor running, both biomechanically and physiologically.
There are some minor differences, but in practice, the major points to consider when adapting workouts to the treadmill have more to do with three practical concerns:
- Managing the increased heat load on your body
- Keeping your motivation and focus high
- Preventing workouts from getting out of control.
A water bottle is a must-have for treadmill running, as is a playlist with fast, upbeat music. Starting runs and workouts more conservatively and progressing the pace gradually can help keep workouts in the appropriate pace range.
If you run on the treadmill a lot, rotating shoes might help vary the stress on your body a bit, too.
I don’t recommend setting the incline to “counteract” the lack of air resistance–remember, after considering the heat, the lack of airflow cooling, the effects of treadmill platform stiffness, and treadmill calibration issues, it’s very difficult to tell a priori whether a given pace is going to be harder or easier than that same pace outside in the real world.
Better to start conservative and pay attention to your effort level throughout the session.
Finally, if possible, try to get a little bit of real-world running in at race pace before any major competitions: getting a sense for what your goal pace feels like over real ground will help you develop a connection between your effort level and the actual pace you see on a watch.
Treadmill running gear I recommend
Enjoy this article? If you need some gear for treadmill running, check out the links below. I don’t run ads to support my site–instead, I recommend running gear that I personally use and can vouch for. I earn a small commission from the affiliate links below.
Other ways you can support Running Writings include signing up for my email newsletter, checking out my book on training, and sharing this article to your friends!
Water bottle: Camelbak Podium Bottle
A good water bottle for treadmill running really only needs two requirements: (1) it should be easy to drink from when you’re running, and (2) it should not leak.
The Camelbak Podium fits the bill perfectly. Regular readers will notice that it’s a different bottle than the one I recommend for bottle service for elite marathoners–the Podium is a bit too big for that use-case, but for treadmill running it’s perfect. The jet valve is a particularly nice innovation that’s easy to twist open and doesn’t catch on itself when you’re drinking out of it like other water bottle valves sometimes do.
Sports drink mix: Tailwind Endurance Fuel
Tailwind is my personal favorite when it comes to a sports drink for treadmill workouts–the taste is milder than aggressively sour mainstream sports drinks like Gatorade and Powerade, which helps it sit better in your stomach.
The formulation is also free of the synthetic colors and flavors that you’ll find in other sports drinks, yet it isn’t completely flavorless. It’s equally well-suited for mouth rinse-style use and planned carbohydrate intake for longer marathon-style workouts.
If you want to hear more about the science of fueling, check out this podcast from earlier this fall where we go over all the details..
Earbuds: Boean Bluetooth Headphones
For headphones, you can go one of two paths: either get a high-quality, reliable set of earbuds like AirPods and take good care of them, or get cheap, semi-disposable earbuds, knowing that you’ll probably lose or break them in a year or so.
Since I typically only do treadmill workouts in the heat of summer and the dead of winter, I’ve gone with the latter strategy. Inexpensive Bluetooth headphones like this pair from Boean (and other brand names selling basically identical products) are marketed as sweatproof, but in my experience they do eventually succumb to regular use in hot, sweaty conditions. A pair of these lasts me 1-2 years.
A word of caution: don’t use wired headphones of any kind–one odd aspect of treadmill running is that friction between your shoe and the belt can build up static electricity, which will discharge periodically through any conducting connection–such as the metal wire connecting your ears to your phone. Unless you want to get zapped every 30 seconds or so, wear wireless headphones or earbuds instead. That static electricity probably isn't’ good for your phone either.