A profligate purchase and an interesting stress test

After much deliberation about buying a heart rate monitor capable of recording Heart Rate Variability (HRV), I have decided to be profligate, and have bought a Polar RX800cx.  I had been vacillating between a Polar RS800cx and a Suunto t6r.  For the purpose of measuring HRV, the Polar RS800cx and the Suunto t6r have very similar capability.  Both devices record R-R interval (the interval between the ventricular contraction of consecutive heart beats), and either can be used to provide the raw data required by the Firstbeat software which I have discussed on several recent postings.  This software computes a stress/ recovery index which promises to provide a sensitive indicator of impending over-training.  This stress/recovery index is based on estimate of the balance between activity of the parasympathetic nervous system (which promotes recovery) and the sympathetic nervous system (which promotes fight or flight).

When it came to the final decision about which device to buy, I was swayed by the fact that RS800cx comes with a stride sensor that appears to provide a reliable measure of cadence.  I am very interested in assessing my cadence when running, but a discussion of the reasons for my interest will have to wait for later posting as my first objective was to see how useful R-R data might be for measuring stress levels.

 

The Polar Own Optimizer

Although I have focused mainly on First beat software in recent weeks, the software that comes with the Polar RS800cx does include a utility called Own Optimizer, which assess stress levels on wakening in the morning.  Unfortunately, the information provided in the manual provides little scientific justification for the Own Optimizer.  As far as I can gather, Own Optimizer is based largely on the changes in both heart rate and HRV in response to rising from sitting to standing.  In principle such measurements might provide a sensitive measure of the degree of withdrawal of parasympathetic nervous activity and increase in sympathetic activity associated with the mild challenge of standing-up. Therefore, I will be curious to experiment with Own Optimizer, but the output of Own Optimizer is difficult to interpret unless one has baseline data recorded while in a relaxed state. There is little point in me trying out Own Optimizer until I am fully recovered from my recent episode of illness.  Nonetheless, I hope eventually to compare Own Optimizer with the Firstbeat stress/recovery index.

 

Back to the Poincare scatter plot

Meanwhile, I exported the R-R data from my new RS800cx and subjected it to the same Poincare analysis which I had presented in my post on 26th June.  The Poincare analysis requires the production of a scatter plot which can be produced easliy using software such as Excel.  Interpreting  the scatter plot taxes the grey cells, but the effort is probably worthwhile.

The Poincare analysis is based on a scatter plot in which each heart beat is plotted on the x-y plane with the x-coordinate equal to the interval between the preceding beat and the beat of interest, while y coordinate is equal to the interval between the beat of interest and the following beat.   A heart beat for which the preceding inter-beat interval is equal to the subsequent inter-beat interval must lie on a line that represents the equation x=y.  This line slopes upwards and to the right at 45 degrees, as shown in the figure below.  Strong parasympathetic activity is associated with large beat by beat variability in heart rate, so consecutive interbeat intervals will differ substantially in duration and the  points representing the heart beats  will lie at a substantial distance for the 45 degree line. 

Conversely, if inter-beat interval varies slowly (over a time scale long compared with the average inter-beat interval) then each heart beat will be represented by a point near the 45 degree line, though the slow variation will cause the location of the points representing the heart beats to wander along the 45 degree line.  Slow fluctuations are largely due to sympathetic nervous activity, though there is evidence that parasympathetic activity can also contribute to slow variations – however for the present purpose, let us assume that sympathetic nervous activity is mainly responsible for slow variation, resulting in a spread of points along the 45 degree line, while parasympathetic activity is responsible for rapid variation that causes a spread of points away from the 45 degree line.

 As I described on 26th June, if there is a good balance between sympathetic and parasympathetic activity, the scatter plot should produce a cluster of points that looks like a swarm of bees the spreads out both along the 45 degree line and away from the 45 degree line. If we draw an ellipse that captures most of the points, this ellipse will be almost circular.  Conversely, if the person is stressed, so that there is an excess of sympathetic activity, the scatter plot will produce a cigar shaped cluster extending along the 45 degree line.  In the data which I had recorded using my ‘home-made’ ECG device before my recent illness, the scatter plot had exhibited a large spread away from the 45 degree line in addition to a large spread along the line.  The ellipse that embraced most of the points was pleasingly round.

 Here is a comparison of the scatter-plot for data recorded today (using my new RS800cx)  with data recorded before my recent illness.  Both sets of data were recorded while sitting in a relaxed state, breathing slowly and deeply.  The pre-illness recording was in fact made at around 6pm at the end of a day at work, while today’s recording was made in the early afternoon after a relaxing Sunday morning.  Thus, if all other circumstances were the same, today’s plot might have been expected to show an even more favorable balance between parasympathetic activity and sympathetic activity.  However, all other circumstances were not the same.  Today, I am in the fourth day of convalescence after a peculiar and debilitating illness that lasted almost 4 weeks.

 

 

Poincare scatter plot pre- and post illness

Poincare scatter plot pre- and post illness

 

There are two striking differences.  First, the cluster of pink dots presenting today’s data has moved down and to the left, indicating that the average inter-beat interval was shorter.  A shorter inter-beat interval corresponds to a faster heart rate.  Before my illness, my average heart rate was around 46 beats per minute.  Today’s value was 59 beats per minute. Secondly, the ellipse that embraces the majority of the points is thinner – somewhat more like a cigar.  Today’s data reveals substantially reduced heart rate variability, especially a reduction of the parasympathetic activity that produces rapid changes in heart rate and scatters the points far from the 45 degree line.

 

Occasional parasympathetic surges

There are a few points that are outside the ellipse that embraces the general trend.  Several of these points represent heart beats for which a preceding short inter-beat interval was followed by a longer than usual inter-beat interval (causing a large displacement above the 45 degree line).  In fact these points represent a sudden drop in heart rate from around 58-62 beats per minute to 52-55 beats per minute. It appears that I was experiencing occasional surges of parasympathetic activity.  These surges were also apparent when I examined an even longer recording, so I suspect that they are not just random chance events but in fact represent some fairly consistent pattern of autonomic nervous activity.  I do not have an explanation for those occasional sudden surges of what appears to be parasympathetic activity.

 

The main conclusion

However, the main conclusion is very clear.  My recent illness has left me in a quite stressed state – at least compared to my relaxed pre-illness condition.  In fact, although today’s recording reveals a marked deterioration, it is not too bad for a 63 year old, so there is no reason for me to be too alarmed.   However, it would probably be unwise for me to resume vigorous training until the scatter plot of data recorded in a resting state returns to something more like the widely dispersed shape exhibited in my pre-illness data.

The apparent sensitivity of the Poincare scatter-plot to stress level makes me wonder how useful if might prove to be as a measure of training stress.  Once I have recovered fully, it will be interesting to compare the scatter plot following a hard training session (‘over-reaching’) compared with that following easier training sessions.  Maybe the Poincare scatter plot of data record during relaxed deep breathing will be as informative as either Polar’s Own Optimizer or the stress/recovery index computed by Firstbeat software.  Whatever the relative merits of the different ways of assessing autonomic imbalance from HRV data, it appears that HRV is a sensitive indicator of one’s internal milieu.

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14 Responses to “A profligate purchase and an interesting stress test”

  1. Ewen Says:

    That’s news! I thought you would have bought the Suunto. Hope you didn’t have to take out a bank loan to pay for it. The cadence results will be interesting.

    How difficult is it to manually do a plot from the data? Does it take long? I’m wondering if the Firstbeat software is totally necessary.

    Interesting result – easy to see the stress level and increased HR from the graph. If it continues to prove useful, I’ll probably buy one.

  2. canute1 Says:

    Ewen, I had been intially inclined towards Suunto. The main potential advantage of Suunto from my perspective is that some FirstBeat computations (EPOCH and training effect) are incorporated in the Suunto software. However, I am not convinced these two computations are really useful for runners as I think they undervalue the benefits and risks of slow paced running – perhaps these measures were intially tested on skiers. The Firstbeat computation of greatest value to me is the assessment of stress/recovery based on R-R data, but this is only available in Firstbeat Sports, or Firstbeat Health. These software packages can read R-R data from either Suunto t6r or Polar RS800cx .
    On the other hand, Polar RS800cx software provides Own Optimizer. which also assesses stress/recovery though I can find no good scientific data on this. The Polar software can produce Poincare plots, which I think are valuable. It also comes with a stride sensor that records cadence and distance, whereas, as far as I understand, the Suunto stride sensor only records distance. In addtion, the Polar facilities for preparing planned training programs to guide your training sessions appear to be better, though I am not sure how useful I will find these facilities. Both devices cost around £400.
    The Polar software will produce the Poincare plot – just search for ‘scatter plot’ in the help menu. However for the purpose of my recent post, I had exported the data from the Polar ProTrainer V5 software to a text file and then imported it into Excel, so that I could present both my pre-and post illness Poincare plots on the same chart for ease of comparison. I also wanted to confirm that the data could be exported easily, because there are other informative computations that can be applied to R-R data.

  3. Ewen Says:

    Thanks Canute. If the RS800cx does the Poincare plot, I’ll keep that in mind. My RS 200 has been reliable, even surviving a stack into gravel when I tripped over one run.

    The cadence facility will be useful too. A running friend of mine has the RS 200 with the footpod – he said the cadence count works well. The distance is not as accurate though, as even when calibrated for the user it can be out by up to 5% depending on terrain and speed.

  4. Carsten Says:

    Thanks for a great post. I am quite interested in the assessment of stress/recovery based on R-R data but unfortunately Firstbeat Sports and Firstbeat Health does not seem to be available to individuals. Do you know of any other software that combines R-R data and recovery?

  5. RICKS RUNNING Says:

    Help;
    As a man of great wisdom can you settle a pub debate between me and a friend.
    TO HEEL STRIKE OR MIDFOOT STRIKE!
    My friend is a good runner and works in a runners shoe sop, he says to land heel first is the most natural and best way to run and if it wasn’t the best way the shoe companies would not make 80-90 % of there shoes for heel strikes, he even told me the great haille had shoes made with extra padding in the heel for his world record at the marathon as he is a heel striker!
    THE CASE AGAINST HEEL STRIKING
    First watch this video

    It looks like haille is midfoot striking to me!
    If you ran bare foot you would not last long on your heels, as you are landing on bone and missing using the foot arch for absorbsion.
    The Africans, the best runners in the world land fore-mid foot.
    if you land heel first you are overstriding.
    the shoe companies got it wrong, aiming at joe the jogger who has poor running form. landing heel first.
    midfoot is the most natural best way to run!
    PLEASE BE JUDGE ON THIS CASE AND LET US KNOW WHO IS CORRECT.
    THANKS

  6. canute1 Says:

    Carsten, You are correct in pointing out the Firstbeat market both the Sport and Health software to professionals. I have a legitimate professional interest in Firstbeat Health, and I had not realized that it would be unavailable to non-professionals. At this stage, I am not sure what the best option for the non-professional is. I will be very interested to see how the Polar Own Optimizer works out. In principle I think that a calculation based on change on HR and HRV on standing shortly after awakening in the morning might be a sensitive measure. My experience so far with the Poincare plot is also promising, though I need to understand more about the apparent parasympathetic surges visible in my data. Also the assumption that scatter along the 45 degree line is purely due to sympathetic activity is certainly an over-simplification. Over the next few weeks I hope to explore various different options and will post the results on my blog.

    Rick, Thanks for your question about heel v mid-foot striking. I am flattered to be described as wise, but if I were truly wise I would avoid answering this question. Because I am prepared to run the risk of being foolish, I will give a simple answer: I consider mid-foot (or fore-foot) is better. Nonetheless, a large number of sub-elite athletes run very well with heel striking. The reasons why I consider mid or forefoot striking is better would take too long to present in this response, but I will post a detailed account of my views on this question on the weekend.

  7. RICKS RUNNING Says:

    Thanks Canute, I look forward to reading your views

  8. Carsten Says:

    I have sent FirstBeat an email asking about the options for non-professional for obtaining a license to Firstbeat Sport and I am still waiting for a reply. I own a Polar RS800 and I have seen the Own Optimizer feature but I was not able to find any information about how it works. I am not sure about the Poincare plot since I basically don’t understand this graph and I need to do more research.

  9. RICKS RUNNING Says:

    The painful truth about trainers: Are running shoes a waste of money?

    http://refreshingnews9.blogspot.com/2009/04/painful-truth-about-trainers-are.html

  10. canute1 Says:

    Rick, I found the tone the article irritatingly sensationalist, but nonetheless, I agree with the message. Heavy cushioning and/or pronation control are almost certainly bad because that impede the development of good muscular control of foot fall. As a youngster I spent the summers barefoot, and as a marathon runner in my twenties, my favorite shoes were Onitsuka Tigers – which 40 years ago were very thin soled and lighter than plimsolls. Now that the fat pads have disappeared from under the balls of my feet, and my downward protruding metatarsal heads have become more prominent, I prefer a slightly thicker sole. I still bounce barefoot on trampoline to keep the intrinsic muscle of my feet in reasonably supple state, but I do not run barefoot on roads or cross county to minimize abrasion and bruising of my feet. If one accepts that forefoot landing is better than heel-strike, shoes with padded heels are simply an obstruction that interferes with elastic recoil of the muscles and tendons. What is your current view about shoes with inner-springs?

  11. Stavros Says:

    Hello,

    Actually you are reading the data in the poincare diagram wrongly.

    The pink series of data represent a healty state of the heart, while the blue are not.

    Have a look at:

    http://www.physionet.org/events/hrv-2006/yang.pdf

    Thanks!!

  12. canute1 Says:

    Stravros, Thank you. I think we are both being over simplistic is interpreting one Poincare plot as healthy without taking context into account.

    As I have described in my blog, in general the spread at right angles to the 45 degree line is indicative of parasympathetic activity and the spread along the line is more strongly influenced by sympathetic activity. In many circumstances, parasympathetic activity is healthy as it tends top be associated with rest and recovery. In fact the degree of spread at right angle to the line in my pre-illness plot is greater than is typically seen in an untrained person, and reflects the fact that when I am not ill, I have greater level of parasympathic activity than the average person. This is associated with a low resting HR and, I think, indicates fitness.

    One circumstance where spread away from the 45 degree line is probably unhealthy is when the points are located far off the line in the manner indicative of ectopic beats illustrated by Dr Yang in the reference you quote. I do not believe that the spread in my ‘blue; chart is characteristic of ectopic beats, though I am well aware that at my age ectopics are a potential concern, though even in this is debated. The majority of the evidence indicates that occasiuonal ectopics (or supraventricular tachycardia) are not unhealthy if there is no other evidence of heart disease. But the clear evidence of consistent p waves in the ECG trace, and absence of a compensatory pause after the shorter R-R intervals for the data shown in the blue plot shown on this posting suggests that there were no ectopics in that record.

    The other thing to say is that too much parasympathetic activity or an absence of sympathetic activity can also be a sign of over-training. Although the pink plot of data after my illness suggests diminished ratio of parasympathetic to sympathetic activity, if you read subsequent blog entries you will see that I moved from the relative sympathetic excess shown in the pink plot to a state of parasympathetic success which I considered was an overprotective response to possibly excessive training.

  13. jprestidge Says:

    Regarding the parasympathetic surges, I have noticed similar on my poincare plots using the polar data; the only difference is that mine seem to occur during recovery from exercise or when my heart is around 100bpm, and are more marked – in some cases the change in rr interval is as much as 500ms between two beats (ie 30bpm).
    There is no particular pattern to this and it doesn’t always happen after exercise. I am at a loss to explain this – it’s not ectopic beats. Not sure if you got to the bottom of your issue.
    Cheers,
    JP

    • canute1 Says:

      JP, thanks for your comment.
      I did not identify the source of apparent parasympathetic surges while sitting in a relaxed state, in my own data, though I wonder whether perhaps they are associated with non-conscious relaxation of the gastro-oesophageal sphincter i.e minor burps to expel inhaled air from the stomach.

      It should also be noted that asymmetric Poincare plots are frequently observed in athletes. See http://www.researchgate.net/publication/228727268_Mechanisms_of_asymmetric_Poincar_plots_obtained_by_means_of_24-hour_Holter_monitoring_in_athletes . The cause is uncertain. However increased asymmetry due to sudden lengthening of the R-R interval is associated with a general increase in HRV and appears to be correlated with increased fitness.

      With regard to surges of parasympathetic activity during cool-down after exercise, I observe that about 45 sec after I stop exercise at a rate that was enough to make me breathe deeply, I experience a sudden large increase in beat to beat variability. For me this typically occurs at HR a little lower than 90 b/min. You can see a clear example in the figure 1 of my post on 25th June 2013, and a less clear illustration in fig 1 of my post on 17th July 2013. I believe this is the normal re-establishment of parasympathetic dominance during recovery, at a time when my breathing is deep and hence exaggerates the sinus arrhythmia arising from increased return of venous blood to the heart during inspiration.

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