Friday, June 13, 2014

A Tale of Two Sprint Workouts

Thursday of both last week and this week I did a sprint workout. This involved cruising over the Sunnyvale Trader Joes, buying some rice tortillas, goat chedder, and organic blueberries, stuffing them in my pocket, then cruising residential Sunnyvale streets to the Steven's Creek trail. Along the way, on roads of relatively smooth pavement and with enough length between intersections, I do some sprints (4 each of the weeks). Then I cruise the Steven's Creek trail back across the freeway mess and do a few more (2 the first week, an extra the 2nd week when I sprinted for a green light).

I'm not sure how much these help the sort of riding I do. But I think improving the top end potentially offers benefits at lower powers, as well as if I need to put in hard efforts to close gaps. I've not been a position where I was sprinting to win races in quite a long time. But sprint repeats challenge multiple aspects of riding: muscle strength, neuoromuscular coordination, pedal smoothness. Plus, they're fun.

Last year I set a Strava goal of 700 watts for 5 seconds. This year I increased that to 705 watts. The best results come when the road is slightly downhill and transitions directly to a slight uphill. Then I can build up speed without working hard, transition to the uphill, and immediately punch it not to increase speed but just to hold the speed I have. Doing this I can apply my maximum power at a relatively optimal, unvarying cadence without shifting. For 5-second power, shifting is death. Consider pedaling 2 strokes per second, where a shift takes 1/8th of a pedal stroke. Then that's 1/8 of 10 pedal strokes in 5 seconds, or a 1.25% loss of power, around 10 watts. Indeed, I hit my 700 watt target last year on exactly this sort of terrain. From my present job good sprint spots are hard enough to find, sprints with the down-up transition are substantially rarer. So most of the time I just sprint on flat roads. My sprints discussed here were on flat roads.

After rides the first thing I typically do is check my maximal power curve. In this case my obvious interest is in 5 second power. 1 second power isn't very reliable, especially with a Powertap (I'm discussing Powertap power here: Vector power would be higher). Power numbers get more reliable the longer the time interval. 5 seconds is a good duration for sprint power.

So in this case the first week my maximum 5-second power was 691.4 watts on my 4th sprint. My 2nd week my maximum 5 second power was 688.2 watts on my 6th sprint. So based on this I was stronger the first week, right?

Well, no. Strava will calculate a maximal power curve for the whole ride but I'm nore interested in looking at each sprint separately. So I did this myself, using Perl scripts. Here's the result for the first week:


The 4th sprint was by far the best. The 5th was aborted when a Google employee, riding his Google bike toward me, decided to veer from the right side of the road to the left, directly in front of me, and I had to brake. My 6th sprint was then truncated by workers on the road. But I had no such excuse on 1-3. It had been a long time since my previous sprint workout and clearly my legs took a few attempts to remember how to do it.

Here's the result for the second week:


Here my sprints were much more consistent. The final, 7th sprint was just to catch a green light. It came relatively soon after the 6th and I only sprinted hard enough to catch the green, so it barely counts. The weakest sprints were 1 and 5. #1 was my first of the day and that's never my best. #5 came after the long gap from riding the Steven's Creek Trail, which is unsuitable for sprinting. #6 was my best.

These observations are reinforced in the following plot, which is the maximum 5-second power for each sprint workout:


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