Fatigue or Cardiac Drift?

By October 23, 2016 September 21st, 2017 Athletic Performance, Endurance Sports, Exercise

Comparing power to heart rate provides insight to two very different models of exercise intensity as it relates to endurance.

Anyone who has ever exercised with a heart-rate monitor knows what happens over the course of a training session — maintaining a steady intensity level results in an increased heart rate (HR).

HR rises over the course of a workout because as the body tires it has to work harder to maintain the same power output. This is true of any endurance activity: walking, running, cycling, rowing, and others. Upon careful evaluation of our other day-to-day activities we would also find this same phenomenon occurring — which is why many people feel fatigued at day’s end.

This phenomenon can also be measured in terms of power output. Power-focused exercise is a traditional approach, one solidified during the running boom of the 1970s. A runner, for example, would focus on a 5-mile run at an 8:30 min/mile pace. Other power-based formats include, pace (minutes-per-mile or -kilometer), speed (miles or kilometers-per-hour), wattage output and others.

This approach has obvious benefits, such as facilitating the tracking of caloric and power output. However, it also has important drawbacks. In a workout in which power output remains steady, the observed rise in HR is known as cardiac drift (see Figure 1). This is easily measured during exercise, and well described in the scientific literature.(1-3) However, it is usually treated as a casual observation that holds little value, and at worst, as evidence that heart rate is an unhelpful biometric.

FIGURE 1. Cardiac Drift at a Constant Pace


The problem underlying this view is that caloric output and pace are not the best indicators of the body’s internal physiological performance. In a workout guided by power, many people have wide ranges of HRs and metabolic responses, sometimes from below aerobic thresholds to above anaerobic thresholds and higher. But all these responses can be effectively measured through HR. The information it provides — which could be used to help improve health, reduce injuries, regulate body fat and develop athletic performance — is easily overlooked when following the traditional training format of maintaining a specific power output while letting HR drift as it may.

Except for research purposes, measuring cardiac drift may be relatively unhelpful because it only reveals that the body is working harder (but not how hard). Consider the HR elevation in Figure 1. Over the course of the workout, fat-burning is reduced while glucose use rises, and muscle fatigue elevates.

These and other metrics cannot directly be measured by most people — but they can be indirectly measured through HR. An approach that focuses on maintaining the same HR guarantees that the same percentages of fuel use (and the same level of overall physiological commitment) is maintained throughout the workout.

In effect, this allows us to track how the body tires in response to the workout. We also can measure the rate of decrease in power (watts), speed (mph) or pace (min/mi) to quantify how quickly this occurs. We can refer to this metric by the same name we give to this physiological phenomenon: fatigue. Monitoring fatigue has much more potential value than observing cardiac drift.

FIGURE 2. Fatigue Increase at a Constant Heart Rate


The body’s fatigue — a crucial factor in sports — reduces pace, speed and, along with contributing to gait irregularity, lower movement economy and reduced performance. A decrease in running pace during a marathon, for example, is positively related to blood markers of muscle damage.(4) The better we can regularly measure fatigue, the more value it can have as a metric.

Fatigue occurs in our bodies at varying rates. In addition to being relative to a workout’s time and intensity, it also relates to the individual’s overall health and fitness status. External factors such as temperature and humidity also have an impact, as do other factors such as hills and altitude.

Assessing fatigue using changes in HR and power output, as indicated in Figure 2, even without the benefits of blood tests or laboratory evaluations, can provide a useful assessment tool that may be related to a number of important factors (5-8):

  • Substrate utilization (fat- and sugar-burning).
  • Stress hormone response.
  • The autonomic nervous system.
  • Recovery requirements.
  • Neuromuscular fatigue.
  • Gait and movement economy.
  • Other complex, interrelated adaptations by the brain and body.

All these factors are influenced by fatigue, can potentially reduce exercise benefits, and may even impair both health and fitness. Other HR evaluations have long been important assessment tools, including:

  • Resting HR.
  • HR recovery.
  • Heart rate variability (HRV).
  • The MAF Test (examples below).

Evaluating HR, instead of applying the traditional power-based approach during exercise, could play a vital role in helping individuals balance health and fitness, especially for those without access to laboratory testing. This is also useful for clinicians, coaches and scientists, and is applicable to all types of workouts, from submaximal exercise to high-intensity training and competition, as well as for rehabilitation and beginner exercisers.

Submax Fatigue

We generally associate fatigue with harder workouts, but it actually develops during all movement. The concept of submax fatigue is specifically important because submax exercise may provide the most health benefits with the lowest risk. In short, moderate fatigue may produce a more significant physiological response than more extreme fatigue. For competitive athletes, submax performance may be the best predictor of endurance performance, in part due to its relationship with fatigue.

Specifically, measuring changes in power in relation to exercise HR can help an athlete assess fatigue, especially during submax states, the type most people perform. This was demonstrated in a study by Wingo and Cureton who showed that during submax cycling for 45 minutes maintaining a steady HR, power output decreased 37 percent, VO2 decreased 24 percent and VO2max decreased 7.5 percent.(2)

The same idea is presented below during a field test by two runners. After an easy 15-minute jogging warmup, each runner performed a 10-mile submax run on a 400-meter track using a heart monitor to maintain the same HR of 146, with each one-mile split recorded.

Figure 3.: Trends of speed and pace of two runners, both at a constant submax HR of 146 BPM, as submax fatigue increases.

fig-3_submax-fatigue-trendsIn Figure 3, submax fatigue is measured as a slowing of pace while maintaining the same submax exercise HR. While Runner A has increased fitness due to the ability to run faster at the same HR, both fatigue at about the same relative rates as indicated by percent reduction in pace. This submax field test, also called the maximum aerobic function (MAF) test developed in the early 1980s by the author, is relatively accurate and simple enough for virtually all exercisers to perform regularly.(6)

An important goal of exercise is to reduce fatigue while maintaining a given work-rate, or to increase speed at the same HR, for example. This would be demonstrated a month later in the runners in Figure 3, when their submax speeds were faster at the same HR. In addition to reduced fatigue, these changes are also associated with increased fat-burning, aerobic development, improved competitive performance, and others.(5-7)

More on Fatigue

Fatigue is a double-edged sword: too little and the body is not stressed enough to obtain positive training benefits that improve health and fitness. Excess fatigue, which commonly occurs when following a power-based workout where HR rises (cardiac drift), can induce unhealthy stress. This level of fatigue requires longer recovery times, with the real potential for causing muscle weakness, gait irregularity, reduced economy, injury and overtraining, as part of an overall impairment of health. Dawson and colleagues had cyclists train for four hours starting at a submax intensity, with rising heart rates/cardiac drift.(3) Afterwards, through electrocardiographic assessment, the subjects showed evidence of cardiac dysfunction.

There is no doubt that parameters of heart function — cardiac output, stroke volume, and redistribution of blood flow to the skin — change measurably as part of the process of adapting to fatigue. But so does the metabolism: it manipulates the balance of fuels (fat and sugar). Muscles respond to increasing fatigue by enlisting more fiber contraction. In addition, reductions of VO2max also occur throughout the workout.(2) Without the ability to regularly measure heart rate during exercise, one may only be able to improve while risking injury, overtraining and reduced health (often, without knowledge of the risk involved).

Special thanks to Ivan Rivera for editing and graphics, and Hal Walter for editing.



  1. Wingo JE, Cureton KJ. Body cooling attenuates the decrease in maximal oxygen uptake associated with cardiovascular drift during heat stress. Eur J Appl Physiol. 2006 Sep;98(1):97-104.
  2. Wingo JE, Cureton KJ. Maximal oxygen uptake after attenuation of cardiovascular drift during heat stress. Aviat Space Environ Med. 2006 Jul;77(7):687-94.
  3. Dawson EA, Shave R, George K, Whyte G, Ball D, Gaze D, et al. Cardiac drift during prolonged exercise with echocardiographic evidence of reduced diastolic function of the heart. Eur J Appl Physiol. 2005 Jun;1(94):305-9.
  4. Del Coso J, Fernández D, Abián-Vicen J, Salinero JJ, González-Millán C, Areces F, et al. Running pace decrease during a marathon is positively related to blood markers of muscle damage. PloS one. 2013 Feb 27;8(2):e57602.
  5. Maffetone P. White Paper. An Introduction to MAF: Maximum Aerobic function. Independent. 2016. https://philmaffetone.com/white-paper-introduction-maf-maximum-aerobic-function/
  6. Maffetone P. White Paper.  MAF Exercise Heart Rate: How it can improve health and sports performance. Independent. 2016. https://philmaffetone.com/white-paper-maf-exercise-heart-rate-can-help-improve-health-sports-performance/
  7. Maffetone P. Marathon Pace Prediction. Indpendent. 2016. https://philmaffetone.com/original-research-marathon-pace-prediction/
  8. Maffetone PB, Laursen PB. Athletes: Fit but Unhealthy? Sports Medicine – Open. 2016;2(1):1-4. https://sportsmedicine-open.springeropen.com/articles/10.1186/s40798-016-0048-x


  • André Almeida says:

    Thanks for the interesting article.

    This sunday i’ve completed the Lisbon Half Marathon. Not being used to such distances (my training is twice a week around 7k), I felt some fatigue in the final kms. Then I looked at the data of my HR monitor:

    Although I kept my running pace almost constant (slight decreasing it in the final kms), my HR climbed at a constant rate with the kms, at the rate of 1,2 heart beat / km. This value shows muscle fatigue / subtraining, or heat-related cardiac drift?

  • jason hall says:


    im 34,male,running for 5 years and have been maf base building for 7 months or so as its reduced injury for me and had i some weird left heart area pains recently, had everything checked and xrays/ultrasound/stress test all passed but they did check for heart thickening but ultrasound said it was ok. my cardioligist said ” dont do 50k any more, even twice a year as it damages your heart long term”.

    i only run 40-60k per week with typically 2- 3 50k events in a year planned of which i run and the rest is all 5-15k Maf based road and trail runs and the odd holiday mountain runs of 20/30k at maf except when going up steep stuff. Basically over a whole year my average heart rate is 134bpm for running(maf 141-146)

    whats your take on long term heart damage when training at rates at maf or below with a few burst/mountain climbs and 50k ultras thrown in?

  • David Simmons says:

    Thank you Ivan, that makes a lot of sense. And it helps to explain something, namely, that if i run five hours faster at say maf+15, or five hours still too fast at maf +8, i’m still looking at a bonk death march no matter how many 5 hour training runs i log, IF, i can’t maintain a sub-maf pace on the flats in the race due to an undertrained aerobic system, correct? Go figure. Thanks for the tremendous amount of work you put in on this site for us out here.

  • Shay says:

    Hello Dr. Phil,

    Thank you for another informative article.
    How does this training method relate to racing?
    For example if I keep training at a sub maximal HR for every workout, do I try and keep the same HR during a race or is it okay to go over or much higher?

    Thank you,
    Shay A.

  • David Simmons says:

    Ivan, knowing that you’ve run ultramarathon distance, and taking into consideration the graphs above, what is your opinion on the best way to approach a race…maintain a constant heartrate and get all you can on the front end with slowing pace on the backend, or increase the heartrate throughout the race to maintain constant pace? Does your answer change based on front pack runner vs. back of pack. Thanks.

    • David:

      I’d do a combination of both. For an ultramarathon, it’s quite necessary for you to run the majority of the race at or sub-MAF: if you run the significant majority of the race above MAF, you’ll end up bonking at some point OR you’ll have to dramatically increase your carbohydrate intake throughout the race, which wouldn’t bode well for recovery.

      So, if you maintain a sub-MAF pace (about 0-10 BPM below your MAF HR), you’ll find that it takes much longer for your speed to decline. So if you run flats like that, you’ll find that you can take hills above your MAF HR. If you fuel relatively consistently throughout the course of the run, you’ll find that you have enough fuel in storage to let your heart rate drift up for the last 10 or so miles of the race.

      For trail ultras, a hack that I use is to fuel on the uphills while moving at my fastest walking hike. This reduces my heart rate for the first few minutes after I fueled, my body can move blood into my stomach, and get the digestion done very quickly (within a matter of minutes). The reason I do this on the uphills is because on the uphills you need to expend much more energy to go faster, than you do on the flats and downhills. So if you hike the uphills while fueling, you get the advantages of (a) putting away some calories (b) without adding stress to your body (c) in the parts of the race where a greater energy expenditure nets you the least amount of gains.

  • Jerry Bradley says:

    How do you recommend running a 5. K run. Slower splits at start say for 1 k. —3ks of

    Focused. Pace faster than. First k. And the see what left for last k Have been running first k

    To fast and having to back off as race goes on. Thanks. JB

  • Steve says:

    If when competing one should aim for a constant pace during the whole event, that would imply heart rate would start low and gradually climb until the finish line as fatigue sets in. Sticking to a target heart rate during a race would lead to a gradually decreasing speed.

    The difficulty is estimating what that target pace should be for different distances (5km, 10km or half marathon), especially when all training is done at sub max.

    • Steve:

      A half-marathon pace is usually 30 seconds faster than your first mile MAF Test. We don’t yet have guidelines for shorter distances. However, guidelines such as these might not be optimal for all athletes. It’s better to test them out in training before using them in a race.

      • James says:

        I think for races of 2-3 hours (or less) racing on instinct works best and not pace or HR. You brain and body knows how to race, its the survival mechanism after all.

        • James:

          It’s still a question of physiological limits, and your instincts are only as good as how they point to those physiological limits. Given the amount of people that get dehydrated, stress-fractured, heat exhausted, etc. in 3 hour races, I wouldn’t put as much stock as you do in that survival mechanism. Your racing instinct is actually based on experience, not “instinct.” If you’ve only experienced 1 or 2 3-hour races, your “instinct” has a very poor notion of the task it’s supposed to be managing. So, the more experience, the better the instinct.

          Until you get the experience, the numbers will be better than your instinct. And when you get the experience, you’ll find that your instinct follows those same numbers rather well.

        • Steve says:

          Thanks, my MAF pace as a 44yr old (136) has improved but is still quite slow after 12 months of MAF. But I have a high max heart rate (198) and can run much faster than ’30 seconds quicker than my MAF’. I know it means I need to work on my aerobic fitness to close the gap but for now I have to go on an expected pace I think I can maintain. I run between 175-190 bpm in the event.

  • Slomoshun says:

    I am 2weeks into another base 8 week program. A stationary bike with HR and power measurement is used to do this as after a 15 minute warm up a steady continuos HR can be maintained for the hour workout.
    As the workout progresses HR rises and fatigue takes effect so the effort required to maintain HR at my MAF of 115 bpm at or 5 beats below is dialled back incrementally .
    Over the workout Power drops off 24%.
    I also swim, ride and run out doors maintaining as best As manageable my MAF HR or 115bpm but find the stat bike the best for doing the MAF test
    Personal stats are 65yrs on the dial weigh 67 kg 125/80bp and nohealth issues and no medication

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