How to quantify “Exercise dose”

Exercise has unparalleled efficacy in the prevention of a broad range of diseases and ailments associated with aging. There is, though, increasing concern that our generation of endurance athletes is the first in which so many people have systematically engaged in such punishing exercise activity so frequently over so many years. The consequence of this appears to be an increasing prevalence of arrhythmias in middle age: if exercise is a drug, our dosages may be dangerously high.

Cardiac Stress Score (CSS) is a metric that quantifies exercise dose. As well as looking at the absolute level of CSS, it is worthwhile to look at its components: Heart Rate Intensity and Exercise Time. Typically, in the off season we aim to spend a relatively high amount of time at a relatively low level of intensity. For example, here’s how the intensity and CSS of my own rides (I was only cycling) changed in the months from March to July in 2015:


On the chart each vertical oblong represents a month, given in the header; each green point represents one activity; point size reflects the CSS; and position on the HR Intensity/Hours plane shows the components of CSS.

In March all but one of the rides is at low intensity (<0.6) although the exercise time ranges up to 5.5 hours. Over the next few months the intensity and number of rides both increase until by July the intensity is consistently over 0.8, even for longer rides.

This illustrates how CSS and its components of HR Intensity and Exercise Time can be used to plan and track cardiac stress, both in absolute level (CSS) and intensity (HR Intensity squared). The same information is also available (not shown) as a listing, giving the CSS and other attributes for each activity recorded on Strava.

Here’s how the year to date activities map out for several of our test cohort (each athlete is shown in a different colour in their own mini-chart):


As expected, no one has gone mad yet: last year we collectively managed almost 20 activities with CSS of over 500, with the highest at almost 800. This year we have so far only accrued six activities with CSS over 300 and the highest level yet is under 400.

The activity profiles of the athletes vary. For example, Simon L is riding at a consistent intensity of around 0.9 over a wide range of times (one to four hours), while Paula has notched up many rides and runs within a narrower time band at a wider range of intensities.

Speaking for myself, the high-intensity bike rides reflect hard efforts rather than high power. In fact, it feels as though I’ve been over-reaching since Christmas and for the next few weeks I’ll be backing off a little, keeping my Garmin on but not looking at the screen. I’m happy to let the CSS rise but through longer, easier rides. On the chart, these may be of reasonable size but displaced to the left.

Z2 Turbo Session Challenge

It’s hard to be motivated for a low intensity base-building turbo session. Here’s a challenge to add some interest and focus…

Pick a day between now and next Sunday for a turbo/spin session; with the temp still ~0 it’s a good time for it. Set a personal target HR. For a Z2 effort I’d suggest 85% of your current LTHR (I can give you my estimate of your current LTHR). Do an hour on the turbo and try to stick to the target HR. When you’re done let me know and I’ll analyse your ride. The aim is to get the tightest HR distribution around your target level. In the calculation, I’ll discard the first 10 minutes of the hour so you can set your Garmin going straight away.

Visually, you’re looking for the histogram, which will look something like this, to get as narrow as possible:


Top tip: Although this isn’t a power challenge, using a power meter will cause your Garmin to sample more frequently so you’ll get a more sensitive, less blocky HR reading.

Welcome to Crickles

Keen athletes like to train hard, and frequent exercise is great for your health. However, it is necessary to balance training with recovery, and the harder the training the more recovery is needed. Recovery gives our bodies time to adapt to training and thus to become stronger and fitter. For endurance exercise, recovery also works for the heart. There is increasing evidence (see The Haywire Heart box) that endurance athletes are prone to arrhythmias that may be incapacitating. Building recovery into your training regime is a sensible precaution to reduce such risks.

Through this website we provide tools to measure the cardiac stress from exercise in a consistent way. One aspect of this analysis is the systematic analysis of heart rate spikes. This attempts to screen out known failings in popular heart rate monitors that often cause incorrect high heart rate values. Conversely, the analysis also aims to flag spikes that may provide an early indication of potential atrial fibrillation.

Another tool that we use is the Cardiac Stress Score (CSS). Unlike all other such measures  of which we are aware, CSS:

  1. accounts for spikes and other anomalies in the heart rate data;
  2. does not depend upon user input values – such as maximum heart rate, Lactate Threshold Heart Rate and Functional Power Threshold – that rely upon measurement protocols and change over time. Instead, we estimate these values from your Strava data;
  3. is directly comparable across activities, exercise types and athletes. This enables you and your Strava training partners to compare your efforts;
  4. can – for cyclists using a power meter as well as a heart rate monitor – be used to track fitness from the relationship of patterns in heart rate and power data.

At the time of writing, this analysis has been calibrated on a year’s worth of activity from a group of 14 keen athletes, all of whom cycle.

Content and tools will be added to the website over the coming months.