Thursday, October 11, 2018

Heart Rate Characteristics During HIIT

At the core of a runner's body is a variable pump that manages to spectacularly manipulate it's blood flow output. At rest, the heart pumps roughly 250 ml/min of blood (the idling state), but this can increase two orders of magnitude to upto 22000 ml/min during maximal exercise (redline). In highly trained athletes, maximal flow volume is of the order of 40000 ml/min. If you look at the ratio, 40000/250 = 160 times the value at rest.

Cardiac output is the product of heart rate (HR, beats per minute) and stroke volume (SV, ml per beat) expressed as :

Output = HR x SV 

The amount of oxygen a runner's body can be removed from circulating blood and used by the working tissues in a given time period is VO2 and all individuals have a maximum at any given point of time repreated by VO2max. When the running intensities exceed this capacity, the larger is their anaerobic constribution to energy production.

Mathematically, VO2 is the product of cardiac output and the amount of oxygen extracted from the blood. The difference between the amount of oxygen within the arterial blood and that within the venous blood returning to the heart is termed the arteriovenous O2 difference (a-VO2diff). This constitutes the extraction capacity of blood. Plugging and chugging :

VO2max = HRmax  x  SVmaxa-VO2diffmax    ---- EQUATION 1

From this simple relation, HR is then a surrogate measure of the dynamics of oxygen consumption by the runner. By utilizing a simple HR monitor, you can detect roughly when your body hits that maximum consumption region.

Apply this to a high intensity interval training (HIIT) run. Subject (me) used a powermeter, heart rate monitor and GPS as a secondary mode of monitoring pace (primary mode = time per lap).

The protocol was 4 x 5 minute intervals at 1:44 per lap. The dynamics of HR is shown below in relation to running speed and mass specific external running power :



One of the aims of this workout was to stay within 90-95% VO2 and that aim has been fulfilled in the first 3 of the 4 intervals. At the 4th interval, HRmax has been essentially reached. This picture completely agrees with VO2max tests that were recorded, therefore at virtually no point does the athlete hit his max aerobic ceiling until perhaps the very last interval. At this point, the 5 minute session was called off.  

Therefore, if the aim of the workout was to increase time spent at VO2max, this session wouldn't exactly provide the requirement. However, if the aim was to maximize time spent at between 90-95% of VO2max within the limited time allottment, then it met the aim. 

In my experience, some runners, especially the older individuals, get alarmed upon receiving notification that their maximum HR has been reached. Here are some thoughts on this observation :

1. Has the maximum HR been plugged into the watch correctly? Typically, a smart watch these days can automatically input maximum HR from stressful workouts into the settings. But normally, this is left to the user to input. Therefore, my question still stands. Has the user entered the correct "field-based" maximum HR into the watch's settings?

2. If we assume point 1 is met, then going by the idea that HR is a surrogate for VO2, a slowly climbing HR is a good sign the workout is delivering oxygen to working muscles in the way it's supposed to.

3. If maximum HR is reached pre-maturely, it indicates that the speed/pace set maybe too high. When the running speed is too high, the runner's body dissipates heat at a faster rate than if the speed were slower. When ambient humidity is high and sweat evaporation is reduced, the other significant way of body cooling is through heat convection from dilated skin surface blood vessels. For this to happen, the blood has to be shunted away from working muscles to the skin. Without adequate muscular blood supply, the runner may go anaerobic and must soon reduce or stop due to inability to manage heat and/or supply the muscles with oxygen.  There is a mismatch between the aim of the workout and the selected running speed. The runner must now correct themselves by re-calibrating their speed.

4. Research indicates that despite increasing age, maximum HR is more or less stable. Therefore, little benefit can be obtained by heart rate increase and any endurance training benefits is derived from the second and third terms in EQUATION 1. In other words, the more stroke volume your heart has and the more oxygen extraction is possible between arterial and venous blood return, the more the oxygen consumption for a given running speed.

5. Considering these points, reaching maximum HR at the desired point in the workout should be no cause of worry as long as this does not accompany other ill-effects such as dizziness, nausea or heat related sickness. The runner can ensure cerebral perfusion by bending forward in order to reduce the distance from the head to the heart.


Another use of HR during an interval session is to monitor the recovery dynamics. With the same amount of rest in between each interval, one finds that the baseline HR reached at the end of each recovery is progressively higher. This would indicate that in each subsequent interval, it would take lesser time for HR to climb to the values necessitated by the workout.

In the following diagram (click to zoom), the black lines indicate the slope of HR rise, the blue line would indicate the slope of HR fall and the thick red lines is the baseline HR reached at each recovery.



Looking at the data, the slope of HR rise during the first and last 5 minute interval were +0.06 beats/second and +0.04 beats/second. This indicates that the slope tends to the flatten out at the higher HRs. Secondly, the recovery slope is more or less the same, roughly -0.3 beats/second in the first recovery span and -0.28 beats/second after the final interval.

However the body's need to supply oxygen keeps HR elevated so within a given recovery time, the baseline recovery HR continues to climb. This, together with the signals from the running mechanics, might indicate to the runner that they would need to stop at some point. 

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