Saturday, December 13, 2008

Measuring Rides

The most common question people ask about my bike rides are how long they are. Almost always, "how long" means "total distance". But distance is a very poor measure of the difficulty/effort of a ride. I am never happy just quoting the number of miles because that is only a partial description.

For example, I have done some 30 mile rides that took more effort than some 60 mile rides. Just by looking at those two numbers, it is natural to conclude that the 60 mile ride was twice the effort of the 30 mile ride. But that is usually not the case.

The primary reason for this is because most of my rides involve significant hills. The total amount of elevation gain and the steepness of the road (which are really two separate criteria) make a tremendous difference for the difficulty of a ride.

Two routes can be the same distance, but if one is flat and the other is hilly, the hilly one could easily require two or three times the energy to complete.

The real measure of effort it the total amount of calories burned. I estimate that I burned over 10,000 calories on some of my double centuries, which is a staggering effort. By contrast, my leisurely 20 minute morning commute burns only around 130 calories. However, I can only estimate the calories burned because I do not use a power meter to measure it directly.

There is an alternate measure that is almost as good a measure of effort as calories — time. This is because my rate of calories burned us usually consistent for all my training rides. Most research shows that "vigorous" cycling burns 600 to 800 calories per hour [see references here, here, here, here].

Because my level of effort is the same, note that speed (and thus distance) are not much of a factor. I will keep a fast speed on a flat course, and a relatively slow speed on an incline. For a given unit of time, I will use the same amount of energy, but the distance covered can vary widely with the hilliness/flatness.

For example, on a flat road, I can keep a sustained speed of 20 MPH which requires a significant level of effort. With the same level of effort climbing a 5% grade (a moderate hill), I would more likely average 9 MPH. For a steep hill (10% grade or higher), I am lucky to keep a 4 MPH average.

So if I report that I went on an 2.5 hour ride, that means I burned around 1500 calories (the average American eats 3790 calories per day according to one study). Of course this could mean either that I rode 50 miles on a flat road, or that I rode 25 miles in the mountains.

If I go on a 5 hour ride, it almost certainly would have required twice the effort of the 2.5 hour ride. However, the distances of the rides may or may not differ by the same factor of 2.

There are other factors (like wind, altitude, etc.) which influence the level of effort. But again these are all incorporated when measuring the ride by time, whereas they would make a distance measure that much more misleading.

So if you ask me how long I rode my bicycle on a particular day, expect me to reply with the amount of time, not the number of miles.

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