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.