Our primary sponsor (Cadence Cycling) did a good job of defining terms with regard to athletic power. Here is an excerpt from their original post.
Speed = a measure of the velocity of the bike, measured in mph or kph. Speed is not a good measure of exertion or intensity while cycling because it is greatly affected by opposing forces such as wind, grade, road surface, drafting, gravity (body & bike weight), aerodynamic profile and friction (drivetrain efficiency) as well as power. It is important to remember though, that races are won by the fastest rider, not the rider with the most power. Speed = Power – Opposing Forces (aerodynamic, gravity, rolling resistance) so in the end a rider must optimize this equation by increasing power and decreasing opposing forces as much as possible to produce maximum speed.
Intensity Factor (IF) = the normalized power for a ride with respect to the functional threshold of the rider = NP/FT. Therefore an effort at 100% of threshold should equal an IF of 1.0. If the rider has an IF of over 1.05 for over an hour, their functional threshold may have increased since the last test (or their power meter needs to be calibrated)
Functional Threshold (FT) = the maximum power a rider can produce for a period of 60 minutes. This can be estimated by completing a 60 minute time trial, a 60 minute “race-type effort” with a high normalized power (commonly a difficult criterium or fast group ride), by taking 95% of the power produced in a 20 minute time trial, 90% of the power produced in an 8 minute time trial or by completing a lactate threshold test in the lab.
Normalized Power (NP) = calculated power over a given duration that better takes into account non-steady state efforts. Average power will decrease if there are significant recovery periods during warmup, cooldown or in between efforts but the stress of the ride does not necessarily decrease (think of driving a carâ€¦ you can average under the speed limit but it doesnâ€™t mean you won’t get a ticket). Therefore, average power is not a good measure of exertion for non steady state efforts such as races, hilly rides and many group rides. Normalized power should reflect the actual intensity of the effort. It is calculated by taking a 30 second rolling average of the power values, taking these values to the 4th power, averaging these values and taking the 4th route of this number. Therefore, when the power spikes very high, these spikes will be given exponential weighting. For example, a criterium may produce an average power of only 160 watts (due to the regular periods of coasting) but the same race might yield a normalized power of 280 watts (due to the many accelerations). Though normalized power is a very good measure of true exertion, because NP works on a 30 second rolling average, rides with power spikes of less than 30 seconds may not be weighted as highly as expected and likewise other rides that contain maximal efforts of 30-60 seconds may be weighted more highly than expected.