QUESTION FROM A READER: Why does my heart rate spike at the start of a run?

A lot of people who run with heart rate monitors often see their heart rate spike at the beginning of a run, only to subside after a mile or two. This kind of spike only happens if you didn’t warm up long enough.

  • If the body isn’t warmed up, there is little blood flow to the muscles (and therefore little oxygen).
  • For a short period of time, the muscles have to work anaerobically, increasing the heart rate.
  • The body rushes to shunt blood away from the organs and towards the muscles. This is a major stressor.
  • The spike in heart rate subsides when increased blood flow (and the oxygen that comes with it) allows the muscles to work aerobically.
  • Therefore, the spike itself is an indicator that your heart rate was inadequate.
  • It takes 12-15 minutes for the body to warm up properly.

Before starting a bout of exercise, our body’s internal machinery is largely inactive. The metabolism is working at a very low level, the big muscles are relatively quiet, and blood is moving largely within the core—cycling through the various organs, and back to the heart and lungs.

Muscles are fed by vast networks of capillaries—tiny blood vessels existing within the muscles themselves—which ensure that blood goes to and from every muscle cell. During rest, the majority of these capillaries are constricted. Very little blood goes in or out of the muscles.

This eases the demand on the heart during rest: constriction of the capillaries and peripheral blood vessels means that the overall volume of the cardiovascular system is greatly reduced. The heart doesn’t need to pump very hard to maintain blood pressure, which keeps the heart rate relatively low.

During exercise, muscles demand a huge volume of blood flow, and so the capillaries dilate to accommodate it. But the body isn’t designed in such a way that the capillaries can expand pre-emptively. They expand due to exercise itself. Asking the body is asked to exert itself from a cold start can be a major stressor: it has to drain blood from major organs abruptly and it has to shove them into muscles whose capillaries haven’t dilated yet—a process that can send the body into shock.

Because of this, a proper warm-up—a period of very low-intensity activity—is important for all exercise, but is critical for running: Every step we run, our legs have to break our fall. It takes a big use of the muscles to make this happen.

Without proper blood flow, the muscles are on their own. If the capillary networks haven’t yet expanded, very little blood is getting to the muscles for those first few minutes. This is a problem because blood carries oxygen. No blood, no oxygen. But even without oxygen, the muscles still need to find a way to perform the required activity. In this situation, the only way to accomplish this is by working anaerobically.

I’ve written before how anaerobic work is intricately tied to the stress response: when the body is under stress, it raises the heart rate and kicks up the functioning of the anaerobic system—which is able to provide energy at a massive rate—in order to deal with a presumed threat to its existence. The connection between stress, anaerobic activity, and a high heart rate runs deep: if any of the 3 increases, the other two will follow.

The observed spike in heart rate is a direct indicator of increased anaerobic activity.

It subsides after a mile or two is because it typically takes the aerobic system 12-15 minutes to activate completely. Blood finally pervades the muscles, bringing oxygen and allowing the aerobic muscle fibers to do their thing.

Heart rate drops to a manageable level once the aerobic system is in play—and to the degree that it comes into play.

Here’s the important part: A spike in heart rate doesn’t just tell us that our aerobic system wasn’t fully on yet. It also tells us that our warm-up was inadequate. The spike in anaerobic activity means that blood and oxygen was largely absent from the muscles. The body was forced to rush to bring blood to the muscles. Blood was hastily drained from the organs, and unceremoniously shoved through capillaries that hadn’t yet expanded to accommodate it.

That amounts to a lot of unnecessary stress.

The solution: warm-up for longer.

14 thoughts on “QUESTION FROM A READER: Why does my heart rate spike at the start of a run?”

    1. So the warm-up should be defined as a period where the heart rate slowly and steadily rises until it reaches a certain point. If you need to slowly move your limbs, and then walk, and then do something like jumping in place for a few times, and then move your limbs again, in order to NOT produce this spike, then that’s what your warm-up has to be like.


    1. Jherek:
      The specific person who asked the question had already tried that. Your response applies only to electrode-based heart rate monitors. The article topic applies to (optical) wrist and ear monitors as well (or any other sensor type you could dream up). This is because the article is talking about exercise phenomena that affect the physical rate of palpitations of the heart, rather than specifically what the sensor (whatever type it may be) is managing to pick up. The reason why the article starts with a mention of heart rate monitors is because it’s typically people who are wearing heart rate monitors that get to experience this physiological phenomenon through heart rate measurement in real time. If you took your pulse, you would be able to experience it as well.


  1. Two points (for me). 1) This only happens to me when it is cold outside, so I assume that plays a role as well. It doesn’t happen in the summer. 2) Even though my HR spikes to near my theoretical max heart rate, it doesn’t feel like my HR is high. I was shocked to discover that this wasn’t an issue with my heart rate monitor and that by measuring my pulse it was REAL. But if I don’t feel like I’m operating anaerobically, and it doesn’t seem to hurt my performance, is there any harm in just ignoring the spike for the first 8 minutes? (For me it never lasts longer than 8 minutes).


    1. Hi BWorkman,

      When it is cold outside, that is actually usually a hardware issue with the HRM. The HR conductivity is very different in the cold, so it basically goes awry and stays high until the HRM warms up from your body heat 10-15 minutes later. The specific issue I’m referring to is when your heart rate spikes 20-25 BPM and then slowly begins to subside over the next 10-15 minutes or so. More specifically, you see this the other way around: individuals start running at their MAF HR or higher, and then over the course of the next 10-15 minutes they get way faster at that same heart rate. But when this happens at a walk or very slow jog (that bespeaks a much lower heart rate) it’s usually a hardware problem.


  2. I find my heart rate spikes during my warm up. It only seems to happen on a morning run – maybe my day is my warm up and by the afternoon/evening everything is ready to go?? However, the spikes are exactly that, 110bpm to 158 and then down to 108 in the space of 30s. At first I thought it was the HRM so I got another and I use electrode gel and that shows similar spikes. I don’t feel like my heart rate is that high, I’m not breathless. It spiked 3 times during my warm up this morning between 04 and 0.6 miles. Is this normal and is the explanation the same? Thank you.


  3. Thanks for this. I noticed this effect (HR spiking) during my warm-ups, but it would subside in ~10min or so, like you’ve indicated. So I’ve been waiting until it lowered back down before I started my training workout in earnest, as I didn’t want the high HR to throw off my numbers and give me ‘false’ information about workout intensity and so forth. Now I a) know *why* it’s happening and, b) how to avoid it (warm-up even more slowly than I’ve been doing). Perfect, mahalo! 🙂


  4. I have noticed that about 50% of the time my heart rate spikes to 170-180 for about 5 minutes and then settles at 100-110 bpm. I am not a new runner, I have run 200 marathons but am 70 years old. I guess that I should ignore this spike in heart rate because it settles down soon after running.


  5. Hi John (and Joshua),

    What I’ve seen is that sometimes the “spike” is due to the electrodes of the heart rate monitor itself taking a while to warm up. So sometimes it’s a phantom measurement. But I’ve noticed that even in very cold weather, this spike rarely lasts beyond the first 10 minutes, which is well within the timeframe for a proper warm-up. By the time you start your workout, the heart rate monitor should be warmed up and ready to go.


  6. Since I wrote to you back in April, I purchased a Garmin Heart Rate Monitor strap to use with my Forerunner 235. Since I have been running wearing the strap, my heart rate when running is normal. My heart rate does not peak during the start of the run, it just increases normally at the start. I now wear the strap for all runs because I can’t trust the optical heart rate detected by the Forerunner to be correct.


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