A few months ago I acquired a Powertap, and began using power in my training.
For those of you who don't know, power is a measure of the rate at which you are exerting energy, and is measured in Watts (W). As a rough guide, most trained cyclists would do somewhere around 100W when riding slowly, 200W riding at a brisk pace, 300W when riding hard, and up to 1000-1500W when sprinting. Simply put, it's a measure of intensity, or how hard you are riding at any given moment.
The story I'm going to tell is an illustration of why measuring intensity is very important for cyclists, if we wish to measure our training load in any meaningful way.
On the weekend of the 9th and 10th of January, I did two rides. Both rides left at 7:00am, on the same bike, in similar weather conditions (bloody hot!).
On the Saturday I rode with a few mates who all race in B-grade, one grade higher than me. We did a very popular route through the hills - from Adelaide up to Norton Summit, then Lobethal, and home via Gorge Rd. The total distance was 69 kms, in a time of 2hrs 25 mins, at an average speed of 28.5 km/h. The other riders were stronger than me and I was on my limit at many times during the ride! My only respite came when one of the other guys got a hunger flat and we had to slow down while he consumed copious amounts of sugar.
On the Sunday I was after a more relaxing pace, so I went for a ride with another group, along the flat roads by the beach. We took it quite easy, and had a nice lengthy coffee stop. The total distance for the ride was also 69 kms, in a time of 2 hrs 20 mins, at an average speed of 29.5 km/h - almost exactly the same time and distance as the ride on Saturday.
Obviously, the two rides were quite different in terms of how physically demanding they were. And yet, if I were tracking my training by kilometres or hours ridden, they would be 'objectively' the same! So, just how different were they?
Fortunately I had recorded both rides on my Powertap. The program I use to analyse my training data is called Golden Cheetah, and it is able to calculate a metric called 'BikeScore'. BikeScore is similar to another metric called Training Stress Score (TSS), which is used by the very popular program called WKO+.
Both BikeScore and TSS are measures of the training load of a particular excercise session. You can sort of think of them as being duration multiplied by intensity, but a little more complicated. Under both of these metrics, a score of 100 is meant to be the training equivalent to riding a one hour time trial as hard as you can.
So, in terms of the overall training load, the best way to compare these two rides is to compare the BikeScore (or TSS) values for each. What were they?
Saturday: BikeScore = 170
Sunday: BikeScore = 80
As you can see, that's quite a difference! In fact, the Saturday ride had a BikeScore of more than double the Sunday ride! i.e. the training effect of the Saturday ride was twice as strong.
What is the moral of this story? If you are to measure your training in any significant way, you should not just record distance and time, but also intensity. This measure of intensity can be as simple as a percieved rating between 1 and 10, or as complex as logging every second of your journey with a power meter. Either way is far superior to distance and time alone!