10 Eye-Opening Heart Rate Truths REVEALED
What this covers
Your running watch manufacturer isn't going to tell you this so we will. There are a number of things that impact your heart rate during training and racing. Today Lindsey Parry, Shona Hendricks, Ntutu Letseka, and Devlin Eyden share the 10 things that can wreak havoc on your heart rate numbers.
When you're ready, we'd love to help you become a better runner. Grab our full mobility flow here: https://coachparry.com/9m8u-Mobility-Flow Grab a strength plan for runners here: https://coachparry.com/9jhi-Strength-Training And a training plan here: https://coachparry.com/kk76-Training-Plans
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Heart rate during training does not reflect actual exercise intensity due to multiple physiological and environmental factors, and athletes must understand these 10 influences to interpret their monitor data correctly.
- Temperature, humidity, and cardiac drift systematically distort heart rate relative to true intensity
- Caffeine, sleep, stress, fatigue, illness, and age all independently affect what the monitor displays
- Heart rate monitoring is only reliable early in sessions and must be abandoned in later stages of long runs
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Heat elevates heart rate because increased core temperature changes pH levels in cells, causing cellular dysfunction, so the body increases heart rate to increase sweat rate and allow bodily cooling functions to occur.
“if the core temperature increases too much the pH levels in the cells change and essentially the cells die or they don't perform as they should and so the body is constantly trying to keep that balance of heat and so it does that by increasing your sweat rates and by increasing your heart rate to allow for these bodily functions to happen”
Heart rate becomes increasingly unreliable as an intensity metric the longer a workout continues, so athletes should use heart rate to set intensity early in runs (within the first hour) and abandon it as a guide beyond one to two hours due to cardiac drift.
“as you go beyond one hour and in particular two hours you become less and less reliant on heart rate leading your intensity”
Stress from any source (family, work, illness, accidents, or bad experiences) causes elevated heart rate during training, and the body cannot distinguish between psychological/social stress and exercise stress.
“stress has a major impact on heart rate and this happens particularly in races but also for everyday training sessions in a race you're already in a stressful environment your body's already excited and primed and therefore we have to take that stress into account when we set heart rate if it's a variable that you want to use when you are doing races”
When detecting fatigue via elevated heart rate (8-12 beats above normal), athletes should dial back intensity rather than continue hard training, potentially converting an interval session to an easy run or taking a rest day to allow recovery.
“instead of doing an interval session maybe turn that into an easy run or take a day off just allow the body to recover a little bit and you should be good to go from there”
Athletes should expect elevated heart rate when running in hot and humid conditions and must account for this when using heart rate to set training intensity.
“understanding that you are going to expect your heart rate to be elevated when you are running in Hot and Now humid conditions as well”
Humidity impairs evaporative cooling by reducing moisture evaporation at the skin level, creating a vicious cycle where elevated core temperature drives increased sweat rate, but insufficient evaporation prevents cooling, so heart rate continues to increase.
“humidity hampers that slightly there's so much moisture in the air so your core temperature rises you start to sweat a lot more and because there's so much humidity in the air there's not as much evaporation happening at the skin level so blood pumping through to the blood vessels in the skin hoping to try and cause more evaporation and more Cooling and because of that humidity level and high moisture level in the air there's not that cooling happening so it almost becomes this vicious cycle where because we're not cooling down core temperatures Rising again heart rate increases not evaporating and all the way back this constant vicious cycle that elevates your heart rate”
Cardiac drift is the phenomenon where heart rate increases over the course of prolonged exercise in response to blood volume loss from sweating, reducing the amount of blood available to pump oxygen and energy to muscles, forcing the heart to compensate by increasing rate.
“as we exercise our heart rate does drift out and it drifts out in response primarily to the fact that we are losing blood volume when we sweat so as we sweat to cool down that water has to come from somewhere and it typically comes from our Central blood volume that means we've got less blood to pump around the same amount of oxygen and the same amount of supply of energy to the muscles and the only way the heart can compensate is by increasing your heart rate”
Cold temperatures have a less drastic effect on heart rate than heat, and external gear (gloves, hats, tops) can mitigate the cold-induced heart rate elevation by maintaining core body temperature.
“as it gets colder it's not as drastic as the effects on heat so your heart rate does elevate slightly but we can also mitigate that with with external factors like the gear that you're using you can put gloves on you put hats on you put Tops on and obviously that can help keep your core body temperature warmer and so you don't have as much of an increased heart rate response in the cold”
Caffeine timing and dosage are critical for performance: peak caffeine concentration occurs approximately 45 minutes after ingestion, so athletes should consume caffeine 45 minutes before activity, and dose should scale with individual body size.
“the thing with caffeine is that you need to time when you take it and how much you take so the dose it depends obviously on the size of the individual the small individuals might require less caffeine than bigger individuals and in terms of timing caffeine reaches Peak Peak concentration after about 45 minutes you want to take the caffeine about 45 minutes before you do any activity”
Pre-race nervousness and adrenaline (arousal) elevate heart rate before the race even begins, so heart rate will already be elevated at the start line and will continue to be elevated in response to psychological stress even before physical effort begins.
“I'm sure you felt this you're standing on the start line of a race that that nervousness the butterflies that are happening that adrenaline that then goes that's a heart rate response and so we call that a stress or an arousal and so when you're standing at that race that's that's adrenaline that's happening and so what's going to happen is your heart rate is going to be elevated”
After the initial acute phase, caffeine produces a secondary effect where it stimulates nitric oxide release, causing vasodilation and opening of blood vessels, which allows better oxygen-rich blood delivery to muscles and may improve performance.
“after a while caffeine seems to have the opposite effect where it actually results in what's called vasodilation and it does this by stimulating the release of nitric oxide nitric oxide is a very is very well known for being it used to calm people so it opens up your blood vessels and this occurs in the smooth muscles in the vasculature of the smooth muscles so in activity um caffeine actually helps your body deliver oxygen-rich blood to muscles and that probably helps in how it improves activity”
Athletes should not use heart rate as a guide during racing because the body perceives race arousal as stress similar to a predator threat (like a zebra fleeing a lion), so pre-race heart rate is already elevated and unreliable for intensity guidance.
“it's very much why we we don't uh suggest that you use heart rates during racing because you're going to start in an already elevated heart rate states and this is a normal response right so we just see it as positive because we love running and and uh and racing is great and fun but the body sees it as a stress in the same way I'm sure that a zebra is standing and watching a lion staring at it uh it's heart rate response is also going to be up”
At extremely cold temperatures, mitigation via clothing becomes insufficient, and the body increases heart rate and blood flow to the skin to maintain core body temperature through thermoregulation, causing elevated heart rate even with proper gear.
“as it continues to get colder though we can't mitigate that by using clothes and gear that then does have an effect on your heart rate and your heart rate is going to be elevated because again the body is trying to remain in a state of homeostasis so your heart rate will increase so that the body can increase the blood flow to the skin to try to keep your body your skin warm and your core body temperature up”
Infection causes localized inflammation and blood vessel dilation around the infected area to deliver white blood cells and immune cells, which increases blood flow and heart rate as part of the immune response.
“what happens when you're an infection is that you normally get localized inflammation around where the infection is and what happens then is that your body will will dilate the blood vessels around that area and also make the blood vessels more permeable so this will then increase blood flow into that area it'll also increase your heart rate and this also allows white blood cells and other cells the immune system to get to the site that you are that is being affected”
Sleep deprivation causes the body to interpret its state as stress, elevating all physiological stress markers including heart rate, because sleep is how humans self-regulate stress.
“if you're not getting enough enough sleep your body thinks that it's in a state of stress and so all of the factors are going to be a little bit heightened and the first of that is your is your heart rate”
The optimal temperature range for training and racing is 11 to 13 degrees Celsius (51 to 55 degrees Fahrenheit), where it is easier to manage performance and heart rate.
“the optimal sort of temperatures for for training or racing in is between 11 to 13 degrees Celsius or 51 to 55 degrees Fahrenheit so it's a lot easier to sort of manage your performance and your heart rate in those cooler temperatures”
Caffeine activates the sympathetic nervous system by stimulating norepinephrine release, which triggers the body's fight-or-flight response, constricting blood vessels, increasing blood pressure, and raising heart rate in the short term (first 5 to 60 minutes).
“what caffeine does is that it basically activates the body sympathetic nervous system by activating the release of norepinephrine no epinephrine is responsible for numb for what's what is called your body's fatal fight response so this is basically priming your body for action and what this does is that in the short term it constricts blood vessels when this increases your blood pressure and also raises your heart rate”
Fatigue elevates sub-maximal heart rate at any given pace, so a 3-4% to 5% elevation in heart rate (8 to 12 beats per minute higher than normal at a set pace) indicates accumulating fatigue or early illness.
“when we are fatigued that heart rate will actually be elevated so that will really be a sign for you to say hang on I might need to dial back a little bit because at a heart rate or rather at a pace that I'm comfortable at running usually where normal heart rate is an X now it is suddenly quite elevated and usually you start to see an elevation or when you start seeing your heart rate being about 8 to 12 beats higher than normal that is an indication to say hang on I think I could be either getting sick or just a really good sign of I just need to dial things back a little bit”
Age-based formulas for predicting maximum heart rate (such as 220-age) are often inaccurate and fail to account for individual variability in age-related changes.
“there are lots of age-related formulas that try and help you compensate for this decreasing heart rate as you get older but most of them don't work”
Viral infection causes fever (elevated body temperature), which increases heart rate because the body attempts to cool the core temperature through increased blood flow to the skin for sweating.
“when you get sick particularly the viral infections one of the things that happens is that you get a fever and there's this rise in body temperature and so that rise in body temperature is going to cause your heart rate to rise because your body's going to want to cool down the core temperature and it does this by increasing blood flow to the skin for you to sweat”
Age is one of the biggest factors affecting heart rate, particularly because maximum heart rate decreases with age as connective tissues become less flexible and the body's ability to vasoconstrict and vasodilate blood vessels is reduced.
“age is one of the biggest factors that impacts heart rate as we get older in particular maximum heart rate goes down this is in response to the fact that our connective tissues become less and less flexible as we get older so heart rate comes down and the body's ability to shunt blood around by vasoconstricting and vasodilating our blood vessels and arteries is reduced”
Fitness level affects sub-maximal heart rate: as athletes become fitter and improve their aerobic function, their sub-maximal heart rate at a given pace gradually decreases and then stabilizes.
“what we expect from heart rate when you are fit or you are getting fitter is that your sub-maximal heart rate at a set pace so let's for arguments sake just use a six minutes a kilometer or 12 minute mile set pace and you will see your heart rate or sub maximal heart rate usually pretty consistent the fitter you get or the better your aerobic function over time you will generally start to see that sub maximal heart rate start to drop slightly and then stabilize”
Dehydration accompanying infection reduces blood pressure, so heart rate must increase to compensate and maintain adequate blood flow throughout the body.
“one of the outcomes of all of this the increase in blood in body temperature and increase in blood flow and increase in sweat is dehydration and again when you get dehydrated your blood your blood pressure drops and your heart rate Rises to compensate for this to ensure that you still get adequate blood flow”
Heart rate response to heat increases proportionally with each degree of temperature increase.
“with every sort of degree that increases there is a different sort of or an increased response in heart rates as well”
Caffeine increases alertness, reduces perceived fatigue and effort, and helps the body mobilize fatty acids for metabolism, thereby improving athletic performance.
“caffeine improves alertness it reduces fatigue it reduces your perception of effort and also it's believed that it helps the body mobilize fatty acids for metabolism so it can improve performance in that way”
Illness typically causes heart rate elevation of 5 to 15 beats per minute above baseline.
“all in all when you do get sick you will naturally see a rise in heart rate and it can be as much as 5 to 15 beats a minute”
Athletes must adjust their baseline heart rate settings (resting heart rate, recovery heart rate) and easy-run intensity targets when experiencing acute stress, and should track life stress separately to distinguish between overtraining effects and temporary stress-induced elevation.
“when we are in highly stressful situations tracking things like resting heart rate recovery heart rate and using heart rate to set our Baseline in particular for easy runs needs to be adjusted and stress has to be accounted for in your everyday training program and take a note of so that you can tell the difference between increased heart rates that are being caused because you are moving into over training or because you're going through some short room acute stress in your life”
Sleep is the mechanism by which the body's physiological functions are coordinated to happen, so sleep deprivation increases heart rate because heart rate is the 'port of call' that enables all other physiological functions.
“because the heart rate is essentially the Port of Call to then get all the other physiological functions happening so if you're not getting enough sleep it's very likely you're going to have an increased heart rate in your training session”
Athletes can train to reduce pre-race heart rate elevation through calming strategies practiced before races, but this requires practice and time.
“there are different strategies you can use to help that heart rate response to calm down before a race or before an event but those take practice in time”
Caffeine improves performance in running, strength training, and both high-intensity and low-intensity work.
“this performance boost helps for running for strength activity for high intensity work as well as low intensity work”