Jason and Will, the good coaches at Tri Hard, forwarded this interesting study on power output for time trials and the difference between constant and varied power output. A link to the study and Will and Jason's helpful abstract/summary is below:
Tri-Hard’s Scientific Study of the Month
Acceptability of Power Variation during a Simulated Hilly Time Trial. International
Journal of Sports Medicine, 2007, 28(2), 157-163.
Insight from the Study
The researchers of this study helped to verify findings from previous research
showing that the fastest way to cover a fixed distance while cycling over a hilly
course is to vary your power output on the hills. In this study, participants
completed two time trials over a hilly course. These time trials took the cyclists
about an hour to complete. In one trial, the cyclists were instructed to maintain a
constant power output. In another trial, the cyclists were instructed to vary their
power output on the hills. When riding uphill, the cyclists were instructed to ride
at a power output five percent greater than the power output that they hoped to
average. When riding downhill, the cyclists were instructed to ride at a power
output five percent lower than the power output they hoped to average. The
second approach of varying power output on the hills resulted in significantly
faster times compared to riding the same course at a constant power output.
Varying your power output when climbing and descending is a very effective
strategy for maximizing your speed over a hilly course. We recommend you
increase your power output about five percent when climbing. When
descending, we recommend you allow your power output to wane. Shooting to
keep your power output about five percent lower when descending is a good
goal. However, it’s actually often very hard to let your power output only
decrease by five percent when descending. You will often run out of gearing and
the ability to further raise your cadence. On descents, we recommend that you
increase your gearing and continue to pedal, aiming to keeping your power
output up near five percent below your projected average power output for the
ride, until you reach 30 to 35 miles per hour. At that point, coast. The additional
energy you would use to go faster than that while descending does not yield
significant time savings and is not worth it. Assuming you feel safe doing so, by
all means go faster than 30 to 35 miles per hour if you can get there by coasting.
But don’t exert energy pedaling to go faster than 30 to 35 miles per hour when
These same concepts apply even if you are not riding with a power meter. By
heart rate, stay in the higher end of your range for that ride when climbing and
allow your heart rate to drop into the lower end of your range when descending.
By good-old-fashioned perceived exertion, raise your effort level a notch when
climbing and let it go down a notch when descending.