Thursday, October 11, 2018

Heart Rate Characteristics and Applications for Running

Inroduction

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 that can be removed from circulating blood and used by the working tissues in a given time period is called "VO2" and all individuals have a maximum at any given point of time represented 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 results in the famous Fick's equation :

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

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 is VO2max which can then be used to extract more speed out of your running. 

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 gets into that maximum oxygen consumption region.


Monitoring Heart Rate During Running

Input output characteristics are well studied with step inputs. A high intensity interval training session (HIIT) does exactly this - it involves step increases in pace or external power and holding said pace for a prescribed interval. This offers a good chance to study heart rate behavior.

Attached below is data from a subject (me) obtained from a HIIT training session where power, heart rate monitor and GPS as a secondary mode of monitoring pace (primary mode = time per lap) were all used in conjuction with each other.

The protocol was 4 x 5 minute intervals at a pedestrian 1:44 per 400m. The dynamics of HR is shown below in relation to running speed and mass specific external running power :

Fig 1 : Stacked data showing running power, heart rate and running speed with time from an interval session.

The most prominent observation is that in response to step increaes in power requirement (W/kg) or pace, heart rate takes several seconds to increase. This is called cardiac lag. This lag is simply a manifestation of an organic pump in our body that can only increase its beating frequency in a finite time as opposed to an instantaneous response. 


One of the aims of this workout was to stay within 90-95% VO2. Since heart rate is a surrogate of VO2, I might conclude that the aim has been fulfilled in the first 3 of the 4 intervals. At the 4th interval, HRmax has been essentially reached. This value of HRmax completely agrees with value of HRmax reached during laboratory VO2max tests done earlier in the year. Therefore, I might conclude that at virtually no point did I cross my maximum aerobic ceiling until perhaps the very last interval. And it was at this point that I called off the 5 minute session.   

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 I conclude it met the aim mostly.

Another use of HR during such 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 gets progressively higher. Conversely, this means that in each subsequent interval, it would take lesser time for HR to climb to the maximal 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 are the baseline HR reached at each recovery.

Fig 2: Heart rate dynamics during an interval running session

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 respectively. 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 effort signals sensed by the nervous system might indicate to the runner that they would need to stop at some point. 


Other Uses of Heart Rate 

Some other uses of heart rate have very good application for overtraining prevention and are described below :

Surrogate metric for operational efficiency : The comparison of HR along with corresponding running pace or power can be used to understand if the running is efficient with respect to the same course and temperature conditions. In an earlier post, I looked at the ratio of running power to heart rate as a potential application of this technique. So I won't expand on this here, but suffice to say it is an interesting area for personal exploration.

Dehydration : With dehydration, blood volume decreases leading to less blood pumped with each heart-beat. Earlier, a study published in the Journal of Applied Physiology found that heart rate increased 7 beats per minute for each 1% loss in bodyweight from dehydration. In other words, for a 68 kg runner, a loss of 1-2% of bodyweight which would increase heart rate by about 7-14 beats per minute.  This cardiac drift phenomenon increases heart rate with distance and duration.

A runner has two ways to compensate for dehydration related heart rate increase.

A) Account partially for this and allow heart rate to increase to about +7 beats more than the prescribed maximum by the end of the run. But that means more stress on the heart so it is termed an 'aggressive HR strategy'.

B) Account for this 7 beat increase by starting , let's say a 1 hour tempo run, at 7-10 beats lower than prescribed. But this means that the runner will not get the pace simulation for muscle loading and adaptation. This is a 'conservative HR strategy'.

Checking recovery :  A potentially good benefit of monitoring heart rate is to help avoid overtraining. If a morning heart rate reading is higher than baseline readings during recovery days, it might be an indication of predominance in nervous system sympathetic activity. In simple words, this means the body is either trying hard to cope with hard training or it is stressed out and needs a break.

Monitoring waking HR rates is a simple way to check for onset of fatigue or even illness. How valid is it? Only you can collect your own data and decide for yourself. A snapshot of 41 days of supine resting HR from my own data collection indicates that there are days when HR is up and days when HR is down. Over 41 days, the long term trend is one showing a decrease in resting HR which might mean the heart is either adapting and pumping more blood per beat or that my training sessions are not as stressful as they were earlier.

Fig 3 : Supine resting HR for 41 days from the author using an ECG holter. 


Concerns Regarding Heart Rate

In my experience coaching, 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. I would ask if 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. However, if the run power or pace is not as demanding and you continue to see a rising HR, that might point to a dehydration situation. 

3. Thirdly, if maximum HR is reached pre-maturely, it can also indicate that the speed/pace set maybe too high. That's the easiest explanation.

When the running speed is too high, the runner's body dissipates heat at a faster rate than if the speed were slower. At the same time, if ambient humidity is also high then sweat evaporation is reduced, and the only 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.

Outside of correcting for these factors, if a high HR is still observed, it is prudent to check with a medical practicioner and get an ECG based reading for cardiac health. Just remember, everyone is different. Your neighbour may have a maximum heart rate of 170 but you might be crossing 200. That speaks nothing of either of your athletic capabilities. It is as statistical as one person having bigger feet than another. 


Conclusion 

The above are some of the practical applications of heart rate monitoring. It is a guage to a variable pump in our body. By monitoring it, you stay honest with the body both during the workout and post-runs during diagnostics. Ofcourse, prior to such monitoring, it is important to know what are the maximal limitations of the heart and whether all monitoring equipment is working fine. 

Unfortunately, the narrative one hears often with HR is it's value as a zone-based metric. Yes, you can sort of get an idea of HR associated with lactate threshold intensities and use it to build zones. But  ultimately they are guidelines, not concrete pillars that cannot move. Anything that is organic is subject to change. 

Over the years, I've taken a step back at looking HR for only this zone training value. I think that the narrative is limited in view and those who only see it for this value miss the other half of the conversation.  I see HR as a purely organic signal into the workings of the body as a system. Pace and power are algorithmic signals that may not necessarily correlate with "internal state of affairs". Thats the wisest way to look at things.

P.S: I do not believe in the reliability of wrist based HR monitors and have exclusively used chest strap systems from Polar for many years. 

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