What is Cardiac Drift?

It's a natural evolution. Many people fall into running by surprise. Before you know it, you've run your first 5k and learned so much about yourself in the process.  Maybe you were pulled in by coworkers or friends. Maybe you finally decided to lose that extra 15lbs. Whatever the reason, one day you realize for the first time that running is not hard anymore: it's fun! 

As runners keep at it, 5ks eventually turn into 10ks, and sometimes half marathons and so on. The personal journey of a runner through progressively longer race distances entails an education in itself. These runners will begin to pick up on some 'tricks of trade' along the way. They start to realize that running long distance is a little different than middle distance. 

As race distances increase, it's more critical for runners to have a basic level of understanding of cardiac physiology: what's happening within the systems of the body during prolonged exercise. Cardiac drift is one of these concepts. Understanding cardiac drift is critical to avoiding dangerous mistakes on the long run, as well as optimizing race performance for personal bests. 

What is Cardiac Drift? 

Cardiac drift is a term that describes a gradual upward trend in heart rate that occurs over time during sustained efforts of cardiovascular activity.  Because causes are multi factorial, some drift can be seen in as little as 10 minutes, but in trained runners it is most easily observed over the course of 90+ minutes of activity. 

Put simply, cardiac drift describes how heart rate gradually increases over time, even in conditions wherein actual effort (more specifically the amount of work performed) remains the same. The physiological term 'work' is a concept that is calculated using multiple variables including pace, body weight, uphill/downhill changes, wind resistance, etc. For the simplicity of this post, we'll use the word 'pace', assuming all other variables are constant. 

Although it may change as fitness improves or dissolves, pace and heart-rate should be very closely in sync in the trained runner. This is equivalent to the odometer in a car. Cardiac drift would equate to gradually increasing RPM over a two hour drive, wherein your speed didn't change at all. In running we can think of heart rate as the odometer of the human body, giving vital information about how hard the 'engine' is working.

If the above happened to your car, you'd likely suspect a decline in performance, perhaps that something had changed. One week you're accustomed to 35 miles to the gallon and the next week it drops to 25 miles to the gallon. Gradual heart rate changes without a change in pace, can also be understood as a decline in performance. 

HR and pace data from 4 different runs for this runner reveal the characteristic cardiac drift even when pace is near equal or slowing at the end. On the second run (top right), it's clear how HR into the 170s begins to force pace to decline. Without surprise, these were all days with temperatures over 90 degrees. 

HR and pace data from 4 different runs for this runner reveal the characteristic cardiac drift even when pace is near equal or slowing at the end. On the second run (top right), it's clear how HR into the 170s begins to force pace to decline. Without surprise, these were all days with temperatures over 90 degrees. 

Most runners already understand that running harder makes heart rate (HR) increase. This relationship between effort and HR is different from cardiac drift. Through cardiac drift, the physiological effort does not change, although the increase in heart rate can make a runner feel as though psychological effort is increasing. This is a great example of how physiological limits are directly tied in to motivational limits. 

Why is Heart Rate so Important?

The most basic answer comes back to the previous analogy. If heart rate is comparable to a car's odometer, then like your car, there is a theoretical upper limit, a 'red line'. Similar to your car, we can estimate where the red line will be, but ultimately know at a certain point the engine will fail. Heart rate variability is so universal (in an uncomplicated population) that exercise physiologists might use heart rate as a blunt measure of work, effort, and health of the system overall, particularly as a quick screening tool. 

Heart rate in running has been studied extensively, and the insights are widespread. Heart rate strongly correlates to blood hormone profiles during exercise. It correlates to the amount of work the heart itself is doing. It can reveal whether your body is more likely to be utilizing glycogen or fat for fuel. For maximal performance in any sport, there is a theoretical sweet spot for heart rate. Going beyond that sweet spot theoretically may mean a drop in performance before the event is over. 

Optimal pacing in top endurance athletes means optimal control and awareness of heart rate. Although these athletes can condition their hearts to perform at an elite level, there is always an individual upper limit. A well controlled heart rate will mean less stress on the heart, optimal pacing, and avoidance of the proverbial 'wall' at the end of a marathon. Understanding the factors that affect cardiac drift can help you keep the engine fine tuned. 

Factor 1: Transition of Energy Systems 

From a complete stand still, to an all out sprint, the body uses several different types of fuel. Why? You only have one body, but it has to perform in a variety of ways. Survival of the species means that sometimes your body must function as a long haul truck, and at other times as a sports car. Think of a pack of wolves, that need the stamina to cover great distances and also the speed to take down their next meal. 

From the first minute of running to 90 minutes of continuous running, your body is slowly shifting gears between sugar burning to fat burning. Sugar is a finite resource in the body, copious to store and capable of sustaining only a couple hours of vigorous/intense activity before the tank is empty.

Short distance track events requiring explosive power cannot effectively use fat for fuel. 

Short distance track events requiring explosive power cannot effectively use fat for fuel. 

Fat on the other hand is a calorie dense fuel. It's readily stored in the body (big surprise there, right?), and can provide weeks of energy reserves at low intensity activity like sleeping or walking. Unfortunately, fat is not an ideal fuel for high intensity activities like sprinting.  

As mentioned above, the first few minutes of activity are nearly pure sugar burning, especially during hard efforts. The body gradually transitions from sugar to utilizing fat for 90% of energy needs at around 90 minutes of activity. Sugar is a precious resource, and this is essentially a means of survival through automatic self-regulation of a finely tuned system. At a consistent work rate, this transition is one factor that coincides with a gradual increase in HR. 

The biggest difference metabolically is that when the body burns fat, more oxygen is needed, hence the concept of 'aerobic' activity. For glucose (sugar) to be metabolized, no oxygen is required. Oxygen is transported to working muscle via the hemoglobin molecule, a complex protein in the blood. As the body shifts from sugar burning to fat burning, more oxygen is required in the muscle, which means more blood and the heart pumps faster.

With race distances over 2 hours in duration, a higher carbohydrate diet can help the body store additional glycogen in the liver and muscles. 

With race distances over 2 hours in duration, a higher carbohydrate diet can help the body store additional glycogen in the liver and muscles. 

Practice makes perfect, and with lots of time running  greater than 90 minutes, the body adapts over time. This is the physiological foundation for cardiovascular training. The volume of blood in the body increase over weeks, new capillaries grow, increasing blood perfusion of the tissue, and other cellular adaptations occur as well. Regular distance training (long runs) help optimize this system and can decrease the amplitude of cardiac drift during events 1-2 hours long.   

These adaptations take time to develop, weeks of training, and are completely lost after just two weeks of inactivity. This is why even seasoned marathoners must still put in the training time for each event they expect to complete. To perform at a longer distance, the whole system must be adapted from regular specific physical activity of long duration. 

Factor 2: deHydration increases cardiac drift 

Your heart beats at a particular rate to meet oxygen needs of tissue in your body. The more blood you have in your body (which is quite variable), the more the heart can squeeze out with each pump, which means it doesn't have to do so quite as often. Part of this is due to elastic energy of the heart muscle. 

When returning blood arrives, it stretches the heart like a balloon, just before it has a chance to contract. This 'preload' of blood takes advantage of the elasticity of the heart, meaning less force is require to eject it into the muscles. With sustained efforts, this advantage is lost due to relative and absolute loss of blood volume.

Relative blood loss means that it is being redirected elsewhere more important, so it can no longer contribute to fueling muscle. If you eat a giant sandwich while running, your body responds by sending more blood to the muscle for digestive function. This means less blood is available for performing work. Similarly, absolute blood loss creates a similar problem. 

As you lose fluid through sweating, blood volume decreases. This decrease in preload means the elastic recoil decreases, so the heart must rely on the squeeze alone to get the job done. No matter how hard it squeezes, it simply cannot pump out the same volume of blood per beat, so the heart rate increases gradually to make up for it. 

Sweat is a filtrate of blood, hence fluid and electrolytes lost must be replaced. 

Sweat is a filtrate of blood, hence fluid and electrolytes lost must be replaced. 

In severe settings, where heavy volumes of fluid are sweated out, dangerous cardiac conditions may emerge, but before this happens, your best performance is lost. Even just a mild amount of dehydration can cause a drop in performance. As novice runners are hydrating to avoid a medical emergency, seasoned runners are hydrating to maintain optimal performance. 

Smart hydration strategies, include taking in fluids regularly during a run. Waiting 60-90 minutes in warm weather will likely be too long to be able to catch up with drinking, as stomach capacity and emptying also have an upper limit. Being a proactive hydrater can help sustain optimal cardiac function, and reduce cardiac drift exacerbated by sweating related blood volume decreases.

Factor 3: Temperature Increases Cardiac Drift

Contracting muscles generate heat as a byproduct, just one of the perks of being a mammal. While your blood is responsible for a constant flow of oxygen to working muscle, it also has another vital job: preventing overheating.

Temperature regulation is a complex process, but the primary means during exercise is achieved through redistribution of blood to the surface of skin. This is where redness of the skin might begin to seen, but also bear in mind this is where sweat comes from. This system of cooling is well adapted in humans, but unfortunately, blood is a limited resource. Every ounce used for cooling, is one ounce no longer available for working muscle. 

The allure of running in exotic destinations must be met with adequate preparation. 

The allure of running in exotic destinations must be met with adequate preparation. 

With decreased blood available for working muscle, the heart is less efficient and is forced to increase the rate of pumping. This can easily be seen if you've ever watched your heart rate while running on a sunny Summer's day. On a long enough time-line, this system spirals out of control, either through heat induced injury, or dehydration. 

Cardiac drift of this nature will cause an increase in perceived effort at a given pace, but the heat can cause secondary problems. Eventually, excessively high core temperatures will send signals to the brain telling you to cease and desist. Those temperature regulators are there to keep you alive, and they work well. You are far more likely to stop running than to run until a boil. 

Ambient temperature outside, as well as humidity, increase cardiac drift perhaps moreso than the previous two factors discussed. For runners new to the marathon distance, this might be the first time they have done runs long enough to really experience the diminishing effects of cardiac drift. Over the course of a 3-4 hour training run, the temperature may increase 20-30 degrees Fahrenheit.

It's no wonder that researchers have consistently found low 40s to be the optimal temperature for the marathon distance. Although there is a high degree of personal variation, heat will ultimately detract from your optimal performance. This also can explain why in the US, many big marathons in the Winter take place in locations farther south like Arizona, Texas, and Florida.

In recent years, the Chicago Marathon has experienced unusually high temperatures despite the October date, which has lead to many heat related injuries and even deaths. Runners new to the distance must be aware of smart strategies to combat the effects of high temperatures during multi hour events. 

If you are looking for your first marathon, I would recommend avoiding any races anticipated to approach the 70s at your anticipated finish time. For experienced runners anticipating warmer race temperatures, training must match the event. Adequate time should be spent running in those conditions to allow the body to gradually adapt over time. This gives ample opportunity to practice optimal clothing and hydration strategies as well.  

be a better runner

As those new to running, begin to expand their profile to longer distances, the need to understand the physiological toll is critical. Safe and successful running of half marathons to full marathons and beyond means asking your body to function to its maximal capacity. It is an art of sustainability, of juggling finite resources, and learning to prepare your body for extremes. Learning to anticipate the needs of your body will ultimately make you a better runner. 

Until next time, don't stop moving!