Last Updated on November 1, 2020 by Heather Hart, ACSM EP
The human body is an incredibly complex, and fascinating machine that so many of us take at face value. For example, when it comes to preparing for a race, we know we need to train our bodies over time to be able to cover the distance we are demanding from it. But what exactly does that entail, besides safely building mileage as to not anger our iliotibial band or plantar fascia? It involves countless physiological responses and adaptations that likely never even cross most runners mind. But being aware of these responses can only help your training process. Such as today’s topic: cardiovascular drift and running. And more specifically, what runners need to know about heart rate drift in regards to their training and racing.
Bear with me as I nerd-out on the exercise science, but then try my best to break this down in easy to understand terms.
Cardiovascular Drift & Running:
Picture this: you’re a few miles into your workout or race, and you’ve settled into your long run pace. The easy, “I could do this all day!” effort – the one that ultrarunners specifically adore. Your heart rate remains steady…until all of a sudden it doesn’t. Your heart rate has increased, even though your pace and intensity have not. What exactly is going on here?
It’s the heart rate drift phenomenon. Also known as cardiovascular drift, cardiac drift, or my favorite, cardiac creep.
Speaking of…that would make an amazing Halloween costume. The “Cardiac Creep”. But I digress.
What is Cardiovascular Drift?
The simple answer: cardiac drift is the natural increase of your heart rate during an extended period of cardiovascular exercise (such as running), despite intensity remaining the same.
The more complex answer: Cardiac drift is the upward drift of heart rate over time, coupled with a progressive decline in stroke volume and the continued maintenance of cardiac output, occurring while exercise intensity remains constant.
Wait, Stroke Volume? Cardiac Output? What’s That?
Let’s break down the science-y talk with some definitions:
Heart rate: this one most of you likely already know. Heart rate is the number of times your heart beats per minute. Which is exactly why it’s measured in…you guessed it: beats per minute (often abbreviated “bpm”).
Stroke volume: is the volume of blood pumped from the left ventricle per beat. In other words, the amount of blood that leaves your heart to rush to the rest of your body (like those hard working legs and lungs) with each heart beat.
Cardiac output: The amount of blood the heart pumps through the circulatory system (from your heart, through your body, back to your heart again) in one minute. Cardiac output = Heart rate × Stroke volume.
Remember that all of your body’s functions require oxygen. Increased effort (like running) requires increased oxygen levels, and blood is the vehicle that carries oxygen to your cells. Not to mention, blood also carries fuel and nutrients to your cells, as well as carries away waste products. In fact, blood does a lot of amazing things within our body, but for the sake of this post know this: increased effort (running) requires increased blood flow.
What Causes Cardiovascular Drift?
When you first start running, a bunch of things happen within your body. First of all, your sympathetic nervous system kicks in, causing your heart rate to increase. It’s basically your nervous system saying “hey body, wake up! We’ve got work to do!” and kickstarts your cardiovascular system into overdrive. This is because you need all of that extra oxygen carrying blood to literally make your body move at a higher effort compared to it’s normal resting state.
When your heart rate increases, initially your stroke volume (the amount of blood the heart pumps with each beat) increases as well. This has to do with the increased need of blood being delivered throughout your body.
Eventually, after a couple of minutes, the parasympathetic nervous system works to bring your body back to homeostasis. Your body has figured out how much oxygen it needs to function at that effort. As a result, your heart rate and stroke volume will plateau as you maintain a steady effort.
But over time as you are running, you begin to experience an increase in core body temperature. It makes sense – there’s a lot going on within your body that results in energy, or, heat. In fact, approximately 20% of the energy produced in contracting muscles is used for muscle contraction; the remaining 80% is converted to heat energy, and therefore exercise causes an increase in muscle temperature. (Takeda R et al, 2018)
Stroke volume decreasing:
When continuous effort exercise continues over time, stroke volume gradually decreases (although it remains higher than it would at rest). The decrease in stroke volume is likely due to:
- thermoregulatory stress (your core body temperature heating up)
- plasma loss (water loss through sweating and other bodily functions)
Previous theories also include the flow of blood to the skin for cooling purposes (cutaneous blood flow) as a cause for decreasing stroke volume. However, emerging research shows that this theory may not be very credible. One study observed the same stroke volume during moderately intense exercise in the heat (35°C) and cold (8°C) in trained subjects, despite a four-fold difference in cutaneous blood flow, reinforcing the dissociation between SV and cutaneous blood flow. (Coyle et al, 2001)
Heart rate increasing:
Regardless, the increase in heart rate is proportional to the decrease in stroke volume, so cardiac output is maintained during exercise. In other words, your body’s oxygen requirements are the same, so the heart has to work harder to maintain that level of blood flow to your muscles (lungs, organs, etc.).
What did you just say to me? Laymen’s terms, please.
Here’s what runners need to know about cardiac drift:
- When you start running, your heart starts working harder to provide more oxygen, via blood, to your body .
- If you maintain a steady effort and pace, your heart rate & the amount of blood your heart pumps will initially stabilize.
- But as you continue running, the amount of blood that your heart pushes out with each beat decreases, due to various stresses.
- Your heart has to work harder to maintain the necessary output (oxygen) required, so your heart rate increases, even though intensity/pace remains the same.
How Does Cardiovascular Drift Affect Runners?
Here’s the part you need to care about as a runner. Because of the phenomenon of cardiac drift, maintaining a steady pace overtime does not mean you are maintaining a steady effort, on a physiological level. Your heart rate will increase, which means your oxygen needs will increase, your core temperature will increase…basically, your body is going to work harder over time even if your pace and intensity NEVER changes.
This is one big reason why many coaches and athletes choose to train via heart rate effort, rather than strictly pace. It is entirely possible that your “easy” pace can become a “hard” pace due to cardiac drift over the course of a workout, depending on a number of factors. This may be detrimental to the physiological responses you are looking for from a specific workout.
Can I Delay Cardiovascular Drift?
Here’s the good news! You can absolutely delay cardiac drift, and use that to your advantage. Science seems to universally agree that proper hydration status is the key to delaying cardiac drift. And here’s why:
Our body’s major cooling system is via sweat. Sweat evaporates off our skin which allows heat to escape the body. Consuming liquids replenishes the fluid lost during sweating, and allows the body’s natural process of cooling itself continue. If you don’t replace those fluids (or start your workout dehydrated) your body has a much harder time keeping itself cool.
In other words: dehydration = decreased sweat = increased core temperature = decreased stroke volume = increased heart rate.
Blood plasma is the liquid part of the blood that carries cells (and thus oxygen) and proteins throughout the body. Plasma contains 91% to 92% of water and 8% to 9% of solids (Matthew et al, 2020). If you’re dehydrated, plasma volume decreases. If plasma volume decreases, stroke volume decreases, as less blood is being pumped through the body .
And guess what happens then? Heart rate increases!
There are endless research studies (like this one and this one) showing that well hydrated athletes, as well as those who were able to replenish fluids during exercise, were able to keep their core temperature down and delay a drop in stroke volume, and thus, delay heart rate drift.
How Do I Account for Cardiac Drift While Training or Racing?
Now that you know what cardiac drift is, how do we utilize that knowledge and apply it to our training?
Understand the purpose of each workout.
Ideally, if you’re training for a race, you’re following some sort of training plan. And hopefully, this plan has a variety of workouts designed for different physiological responses and adaptations.
Some workouts are designed for recovery. Others, for building endurance, fat oxidization (using stored fat for fuel), and increasing capillary density. These two examples are more dependent on effort rather than a specific pace. In this case, I always recommend to my athletes that they start on the LOWER end of their prescribed heart rate zones, to account for cardiac drift over the workout, while still remaining in the “zone” we are aiming for. Further, it gives evidence to the argument of why you should ignore pace, and slow down if you need to, to stay in a specified heart rate zone.
Other workouts are designed to push your lactate threshold, increase turnover, or help you hit specific splits for a race time goal. In these examples, pace DOES matter. In these cases, understand that pace is your primary concern, and that the workout is going to feel harder over time as a natural response to cardiac drift.
You already know that hydration is important. But this gives you one more reason to drink up. Maintain proper hydration before, during, and after your workout, for all of the reasons mentioned above, to help delay the onset of cardiac drift.
Ultramarathons and Heart Rate Drift
Because I focus mainly on ultramarathons, I’m sure some of you are wondering how cardiac drift affects ultra runners. After all, we’re running for so long (upwards of 24, 30, 48 hours or more!) how does that affect heart rate? Is your heart rate going to continue to “creep” until it explodes? (OK, I’m dramatic here, but ultrarunners…am I right?).
One research study showed in a test of 24 hour treadmill runs, that heart rate increased due to cardiac drift for the first 6 hours, but then actually decreased from hour 6 to hour 24 (Gimenez et al, 2013)
Another study analyzed the heart rate of 10 male elite cyclists during the first Race across the Alps in 2001 (distance: 525 km -not running, but ultra endurance none the less). In all subjects, exercise intensity declined significantly during the race, as indicated by a decrease in HR(average)/HR(max) of 23% from 0.86 at the start to 0.66 at the end. (Neumayr G et al, 2004)
It appears that intensity, and thus heart rate, naturally decreases over an extended period of time during ultra endurance events. This is possibly due to a number of factors, such as decreasing glycogen stores (fueling), as well as mental and physical fatigue (sore muscles, etc.) forcing a runner to slow down, thus decreasing heart rate.
In short: cardiovascular drift is something that naturally happens to anyone who participates in extended cardiovascular exercise. But, being aware of it’s existence allows us to be mindful of how it can affect our training and racing efforts, as well as what we can do to help slow down cardiac drift.
Is your head spinning or did you learn something new? I am hopeful that this article gave you more insight into some of the cool physiological responses that happen to your body when you are running. If you have any questions about cardiac creep / how heart rate drift affects runners (or need clarification about something I discussed) please leave a comment below!