The Basics…

So, I’m intending that this blog should be a basic course in the heart and how it adapts to exercise. It’s fair to say that this is something of a complex topic, and there is much controversy and debate. We have not got it all worked out by a long way, and I reserve the right to completely change my mind about things over time. That is the nature of medicine.


The heart is only one part of the cardiovascular system which helps you exercise. It includes the heart, the lungs, the blood, the muscles and of course the brain. My PhD touched on how the nerves and brain react to exercise. It was 3 years of pure frustration, as it turned out to be quite hard.

The first thing to understand are some basics of the heart itself. You can skip this if you already know all about it, but I will assume some sort of basic knowledge in future posts. You will have covered this in GCSE biology, or for those of us who are old and decrepit enough, O level biology.

Have a look at this picture.

Blood comes back from the body having been “used” into the right atrium via the superior vena cava (from the arms, chest and head) and the inferior vena cava (from the rest of the body). It’s important to realise that there is vast variation in the precise distribution. The right atrium is a thin walled chamber which discharges its blood into the right ventricle via the tricuspid valve. Valves are thin-walled structures that are elegantly complex pieces of engineering that keep blood going in one direction through the heart. People write entire books on valve structure and function. Valve disease can be important when considering exercise. 1-2% of the population have a “bicuspid” aortic valve, and exercise can accelerate it narrowing – it then usually requires replacement which is a major operation.

The right ventricle is the pumping chamber that forces blood into the lungs. The system is at low pressure, and the muscles of the right ventricle aren’t as well developed as those on the left side of the heart – the right ventricle is a bit thinner, and probably a bit more vulnerable to stress. This is thought to be important in the development of conditions such as ARVC (arrhythmogenic right ventricular cardiomyopathy) – more on this later. Blood flows out of the right ventricle and into the pulmonary arteries and the lungs themselves.

Once all the business of gas exchange has been done in the lungs (oxygen in, carbon dioxide out) then the blood comes back via the pulmonary veins into the left atrium. During most of my training the pulmonary veins were possibly one of the least exciting anatomical structures you had to learn about, but then it was discovered that AF starts there quite often, and now many people spend entire lives studying them.

The left ventricle is considered to be the main structure of the heart. Blood enters this from the left atrium via the mitral valve. It leaves the left ventricle via the aortic valve and passes into the aorta and from there around the body. Cardiology has traditionally focused on the left ventricle and its function.

One of the key concepts when understanding how the left ventricle works is the “ejection fraction”. This is the proportion of blood ejected from the ventricle each time it beats. Cardiologists, and patients, get very hung up on this measure, and it is important, but there are important limitations. The normal volume of the left ventricle is about 140ml in a typically sized person. Typically, around 2/3rds of the blood is ejected each time the heart beats (about 90ml). Anything over 55% or so is considered normal. But really, what the body needs is not for the left ventricle to have a particular ejection fraction, what the body needs is enough oxygen delivered to the tissues. That depends on how much blood is pumped around the system each minute and how much oxygen it is carrying. Doctors often forget this. I’ll talk more about this in another post. Disease of the heart muscle is termed cardiomyopathy. When people talk about “heart failure” they are usually referring to disease affecting the muscle of the left ventricle. Exercise, particularly endurance exercise can affect the heart muscle in both good and bad ways.

The aorta branches many times when passing blood around the body. The first are the coronary arteries. In most people, there is a right coronary artery and a left coronary artery – the first part of the left coronary artery (the left main stem) branches early into the left anterior descending artery and the circumflex. Narrowings in these arteries cause angina – a pressure or discomfort on exertion (usually) felt across the chest. A sudden blockage can cause a heart attack. There is a lot of debate about whether or not exercise causes changes in the arteries, and whether or not those changes are harmful. Again, a subject for a future post.


So that is the whirlwind tour of the heart structure. In future posts I will touch on valve disease, diseases of the heart muscle and coronary artery disease. Then having finished with the plumbing I will move onto the electrics. It will become clear that in fact as doctors we probably know less than you think. The world of sports medicine is in its infancy really.




Seasonal time in Zones

The Seasonal tab has now been re-introduced to the Navigator. Now it shows you your time in each heart rate zone for each of the last eight Quarters (subject to your Strava history going back that far). This enables you to see whether you really were base building over the winter and adding intensity in the early season – or doing whatever your training plan called for. It also makes it easy to compare your quarter-by-quarter efforts with those from a year ago.


The definition of the zones (Z1 to Z5c) are essentially the same as those defined by Joe Friel in articles that can be found online. The main difference from those and similar methodologies is that the Crickles estimate of your Lactate Threshold Heart Rate (LTHR) is calculated adaptively from your Strava data and applied consistently.

You’ll also notice that we have reverted to a light colour scheme for the Navigator and Crickles Charts. The Activities table on the Navigator works much better this way, and sometimes it’s just good to have a change.

3 quick ways to use Crickles

If you’re new to Crickles you can begin by using it for these three quick checks:

Check 1: Gauge your cardiac stress

On the landing page [Relative CSS] of the Navigator select your name in the Athlete: dropdown to get a screen like this:


The green bar shows you (me in this example) and where you rank amongst Crickles athletes for accrued cardiac stress over the past six weeks. This is based on activities you’ve entered on Strava with a heart rate monitor. If you’re way over on the right hand side you should be aware that you’re taking on a higher cardiac load than most other Crickles athletes. If you’re towards the left hand side you’re taking on less than the others (assuming that you regularly use a HR monitor and upload your activities).

Personally, I aim to be about where I am here: towards the right but not bang up at the end.

Check 2: Gauge your event readiness

If you have a race or special event coming up in the next few days, go to the Fit-Fat tab on the Navigator to see a page that looks like this:


Focus on the black graph: this shows Form. If you’ve been holding at or above about 10 for a few days and you’re now hitting 20-25 you’ll be in great shape for your event. If you’re in negative numbers you’re under-recovered. It’s normal to have negative Form numbers during training but tapering prior to an event enables the Fatigue (the red line) to fall away while Fitness (the green line) can remain high. Form is the difference between the two.

Obviously, if you’ve haven’t been doing any training a taper period won’t help!

Check 3: Compare a specific activity with a friend

If you have someone you exercise with who is also on Crickles, whenever you do the same activity together you can compare your efforts using Crickles Activity Charts.

Again, select yourself from the Athlete dropdown then select the Activity that you did with your training partner. Next, check the Compare another? box and choose your training partner and their record of the same activity. At this point the Metric dropdown will be populated with the set of metrics that you both have available.

chartsIn this example, Sean and I both used HR monitors and power meters so I could select altitude, cadence, kmh, heartrate or watts. Here, I’ve selected heart rate.

You may prefer to choose Distance rather than Time for the x-axis if you didn’t go round together. Here, I’ve checked the Smooth? box too to give a clearer picture.

This enables you to see where you and your partner were putting in relatively more/less effort. (In the example shown, I started off harder then my ride partner, Sean, put in a big effort on the hills in the middle of the ride while I took advantage of a pace car.)


If you’d like more info on any of these topics or if you have any questions or difficulties, please get in touch through the Contact page.

Crickles Activity Charts

It’s great to have charts of activity data on Strava and Garmin Connect but there are often times when you want extra capabilities. In particular, it can be useful to compare your efforts at the same event over time or to compare your performance at an event with others who did it with you. Crickles Activity Charts are specially developed for such comparisons. You can find them at and they work similarly to the Navigator.

First, choose yourself (or someone else) from the Athlete list and then choose the date range over which you want to search for activities here:


The Date range and other controls work as described on the How to notes for the Navigator. Once you’ve picked an Athlete and a Date range the Activity drop-down will be populated appropriately. Note that activities flagged as private will not appear for selection.

Once you’ve chosen an activity two new fields will appear:


One is a View on Strava link; if you choose this the activity selected will appear in Strava in a new window enabling you to confirm that it’s the one you meant to choose and to see context in Strava. The other new field is a drop down list called Metric showing you what graphable data is available for the activity. In this example you can choose from altitude (which is nearly always there) and is measured in metres, cadence (in RPM), heart rate (in BPM), speed (given as kph) and power (given as watts).

This list changes from activity to activity – for example, watts requires you to have used a power meter.

Once you’ve chosen a metric you can also show how you want it to be charted using the x-axis and the Smooth? controls. For comparisons, the x-axis settings of distance and histogram are typically most helpful.

charts_x.pngThe Smooth? control is especially useful for power data, which is noisy. It’s also useful for fields such as kmh and heartrate when a comparison is being made.

Note that Smooth? will transform a histogram into a density plot.

To compare activities you simply check the Compare another? box under the first activity. This then causes a second set of controls for Athlete, Date range and Activity to appear. If you want to compare two of your own activities from different days you just select yourself as both the Athlete and the Second athlete.

Here’s an example of my own, comparing a recent 10 mile effort round Regents Park with a similar effort from last summer. The first activity is shown in blue and the second is shown in pink.

Looking at smoothed power shows that my wattage is quite a bit down:


(If you’re familiar with power plots you’ll be struck by how little noise there is on this chart – normally power (viewed on a graph) continually oscillates a lot around what we might imagine to be the true signal.)

Second, we can look at a density plot of heart rate on the same two rides:


This shows that on the more recent ride my heart rate was centred on a 160-165 range whereas last year it was rising to the 170-175 bpm range for much of the ride.

The purpose of this example is not to propose that others should choose to ride at lower heart rate and power but to illustrate how these charts can be used to gain insights on your data. You can equally use the same functionality to compare the speeds of two competitors over the same parcours, or even – by charting altitudes – to compare the barometric/mapping fidelity of two devices.

Although not an app, Crickles Charts work well on an iPhone in the same way as the Navigator, as described here.

Only activities from 1/1/2017 are currently available for these charts – please get in touch if you’d like to select from earlier activities.

Should I screen my kids?

Unlike many on Crickles I am a long way from being a professional athlete. I ran the London Marathon in 2001 and was overtaken by a tree. At last year’s Porlock Hill Climb (which I went to, but failed to enter in time) I would not have been last. But 31 minutes is not great. Most of my sporting activity is to help my kids. My youngest likes to run – typically about 10K. My middle one loves cycling and triathlons. As they are young (11&13) I do most of their training with them. It’s getting harder, and I can see a time in the not too distant future when I won’t be able to keep up.

My day job involves many things, but I specialise in sudden cardiac death. All too often I am dealing with families who have lost someone out of the blue. Sometimes they are athletes, probably more often than chance would predict, sometimes not.

Around 1 in 50,000 athletes <35 years old die suddenly each year.

So I wonder about whether or not I should get my kids screened. Many conditions which lead to sudden cardiac death can be picked up by quite simple tests, such as electrocardiograms (ECGs). It takes 5 minutes. In countries such as Italy and France there are widespread screening programmes. I sometimes see people from the UK who have abnormalities picked up when they have  been to Europe to participate in an event, and had their mandatory ECG.

There has been a limited study in the UK, run by Sanjay Sharma, probably the best known Sports Cardiologist in the UK. They screened almost 5000 athletes between 14-35. 1 in 300 had a potentially serious underlying conditions after more complete evaluation. ECG screening seems to work. A large study in Italy demonstrated that it reduced the risk of sudden death in athletes by 90%.

But it’s not quite so easy. Athlete’s ECGs are “abnormal” anyway, and knowing what is normal for an athlete takes training. Furthermore, many people with abnormal ECGs don’t have problems after more detailed testing.

As some will know, identifying something potentially serious doesn’t mean that something serious will happen. In fact for some diagnoses, such as long QT syndrome, it is often more likely that nothing serious will happen. And then they may be banned from sporting participation, and struggle to enter certain careers. A diagnosis, which may never cause harm, can be life changing. I have all too often been unable to help families, who want to know what the future holds. I see people desperately grasping for certainty where none exists.

On the basis that I haven’t dropped dead yet, and neither has my wife, and on the basis that my kids are still amateur and have no symptoms, I think I’ll leave it a bit longer. But as data mounts, and if they become more serious, I will probably err towards organising some testing for them. Like most parents, I worry far more about them than I do myself. In the meantime I’ll be cycling with one son on Sunday on Exmoor.

If you are more interested, try starting here:

Improvements to Navigator Activities

If you’ve looked at the Activities page on the Navigator recently you will have noticed some improvements.

Layout and organisation

The columns are centred now, where appropriate, making it easier to read, especially for numeric fields. Also, activities are, by default, sorted by Date with the most recent at the top. It’s also still possible to re-sort, as explained in Using the Crickles Navigator.

It’s now also easier to search through your activities using the Search box. For example, I find this helpful if I want to compare my “Park Loops” rides.

Normalised Power

Normalised Power is now shown for each of your cycling rides where you used a power meter.

View in Strava

You can now also open an activity in Strava from the link in the Strava column.


The old link to the Navigator (at has been online until just now but has not benefitted from any recent upgrades. I have now deleted it as an encouragement to switch to the current version at There was no functionality in the old version that is not also in the new one.

Getting the Navigator on your phone

I’ve found it very helpful to make the Navigator appear on my phone like an app. Here’s how you do it on an iPhone:

1. Open Safari and enter in the address bar to open the Navigator.

2. Hit the icon at the bottom of the screen that looks like this:

MaxHR by athlete

3. Choose Add to Home Screen.

You’ll need to select yourself – or whichever other athlete you want to look at – each time but that’s easy using the alpha list. On my iPhone 7 the images are readable as either portrait or landscape.


The same procedure works on the iPad but it’s not quite as neat since Safari retains desktop-like tabs on the iPad and is less app-like.

I don’t have an Android device to test on so I leave that as an exercise for the interested reader.


Those high heart rates you sometimes see…

The frequency of very high heart rates

A standard guideline for your likely maximum heart rate is 220 – your age. For example, someone aged 40 would be expected to have a maximum heart rate of about 180 bpm. Contrary to my belief before I started learning more about this, there is no evidence that maximum heart rate increases with fitness. It is therefore surprising to see how often heart rates much higher than this rule predicts appear on our Garmins.

In the Crickles cohort, which currently includes six women and 34 men, the maximum heart rates each of us has recorded on a Strava activity are as follows:

MaxHR by athlete

Since none of us (to my knowledge) is younger than 20, it is surprising that the majority of us have recorded heart rates of over 200 and for many of us our Garmins have at least once shown a heart rate much higher than that.

Should we be worried? The first thing to say is that the vast majority of these high heart rate readings are, we believe, strap errors. We now have access to well over 10,000 Strava activities on which the athlete used a heart rate monitor. Almost exactly 10% of these activities include a heart rate of 200 or greater. However, when we filter this to activities in which the high heart rate is reasonably sustained, it boils down to only 65 (at the time of writing). If the heart rate monitor always gave true readings, at least most of these 65 activities would warrant further examination. One third of the Crickles cohort has at least one of them to their name, and it’s fair to suppose that the Crickles group is reasonably representative of the community of very active amateur cyclists and runners.

Strap errors?

However, our sports heart rate monitors do not always give true readings! So how do we know whether a reported heart rate of, say, 225 bpm reported on our Garmin corresponded to our true heart rate or was an error from the strap? This is a question to which Mark, the Crickles cardiologist, and I have paid much attention. Let’s look at three examples…

First, here’s a recent activity of my own. Paula and I were cycling up and down the central mountain of La Gomera in the Canary Islands and my Garmin showed this:


On the ascent, it shows my heart rate as rock steady on 215 bpm. It stays at that level for the start of the descent, while we freewheeled to a cafe a little way down the hill. There, with my Garmin still showing 215 bpm, Paula took my heart rate at my wrist and found it to be 70 bpm. After our coffee stop, on the remaining descent the heart rate shown on my Garmin rose further and then fell but was still displaying over 170 bpm when we got to sea level. There, I swapped my strap for Paula’s identical Wahoo Tickr, which reported my heart rate to be half of what my own strap showed. This is a clear and complete strap failure!

Here’s our second example:


We see that the rider began on an easy downhill and yet his heart rate is very quickly being reported at around 250 bpm. Just after 10 minutes there’s another spike but still the cyclist has put in little effort and again it’s on a downhill stretch; also this one is short-lived. We see many spikes like this and ascribe them to probable contact errors with the strap: the athlete may well not have built up a sweat yet and there could well be a fluttering effect on the descent. We see a third spike at just over 30 minutes. This occurs at a time of greater wattage but is also short-lived.

Here’s our final example:


This time the athlete is progressing up a renowned arduous climb when at almost two hours into the ride his heart rate rapidly rises and stays in a range of around 200-220 bpm before rapidly falling again. While nothing is certain with sports equipment, this does not have the appearance of a strap error and merits attention from the athlete.

Going robo

When we first started Crickles we classified heart rate spikes by coding rules that encapsulated the patterns that we saw. Now that we have much more data we can use techniques of data science. The original rules have been decommissioned and now heart rate spikes are classified fully algorithmically. Does it work? It’s a developing art but already the algorithm can pick out the most concerning spikes, group together the spikes that look like contact errors and identify that the complete strap failure of our first example stands apart from all the other spikes. It also groups together other patterns that I haven’t covered here reasonably well. In short, it can analyse well over 10,000 activities, find the sustained heart rate spikes and group them as well as a lay human eventually could – all in a couple of seconds.

The work currently in hand is to improve the machine classification of spikes to capture more of Mark’s expert insight and fully utilises all of the data available in the Strava records. If everyone was a cyclist with a power meter and a reliable heart rate strap that would be much easier, but we cater for runners and rowers too and sports straps just don’t come with medical-grade quality.  We can already see that algorithms are better able to disentangle these factors than humans, especially as the volume of data increases.

It would be great to have more athlete data. For example, we so far have only one complete strap failure like the first case above. While the algorithm can easily identify it, if we had five times the amount of data and five complete strap failures we could be more confidant that the machine would correctly identify them every time. The best way you can help us is to encourage more people to sign up, either from the sidebar on this site or directly via

When high heart rates matter

A different kind of question is how we should alert Crickles athletes to spikes that merit further attention. Currently I do this informally – but I don’t even have contact details for everyone who has signed up to Crickles. I cannot foresee that we will post such information online, at least while we have a fully open platform. It’s an important question because this analysis may potentially flag the occasional issue at a stage when it can be addressed through a reasonable change in the athlete’s exercise programme but that may later require a more dramatic decrease in exercise and/or a medical solution.

Until we have a way to communicate information about spikes, please feel free to get in touch through the Contacts page (or directly) if you have any concerns about your own high heart rate readings.

For the large majority of athletes we’re likely to find no cause for concern in most high reported heart rates. Health issues aside, continually improving the Crickles data cleaning logic will help us to keep producing better quality training metrics than are available from all the platforms that overlook the problem.