CP is effectively the amount of power you can sustain during an all-out, 1-hour running effort. Because repeatedly testing an all-out, 1-hour running effort is hard (increasing likelihood of sub-maximal performance and/or injury), Stryd has developed a protocol that uses shorter maximal efforts to calculate your CP. On a track you run 2 laps easy, rest, run 3 laps at maximum effort, rest, and then run 6 laps at maximum rest. This creates a power curve that can be used to predict with relatively high accuracy your CP.
Back in February I completed my first CP test. Then I trained for 12 weeks, ran a key race, and completed another CP test.
February CP test:
3 laps: 4:24, 401W, Max HR 185
6 laps: 9:30, 376W, Max HR 187
Calculated CP: 347W
April CP test:
3 laps: 4:16, 436W, Max HR 183
6 laps: 9:18, 409W, Max HR 186
Calculated CP: 370W
My 3-lap power:
My 6-lap power:
A few notes on protocol: these are max effort runs for me, leaving me exhausted. I used 45 minutes between runs (and ingested coconut water) instead of the prescribed 30 to ensure that I could give the 6-lap run the effort it deserved. During each run I tried to run relatively even splits but still gave an all-out sprint at the finish. These were all run on the UNC track, which is soft enough that I can run comfortably barefoot - but I ran these with minimalist footwear anyway since that is what I use for off-track running, where I spend most of training and racing time.
By any metric it seems that I improved. My power numbers for the same effort level were consistently ~9% higher than my power numbers in February. This led to a CP calculation 7% higher than in February and, perhaps most importantly, the increased power translated to faster speed, shaving 8 seconds off the 3-lap and 12 seconds off the 6-lap run. I don't believe that 100% of the performance improvement can be attributed to fitness improvement; as Andy Coggan has noted, experience with the protocol will improve performance slightly even if fitness remains constant. Still, I believe this shows that fitness/performance has indeed improved since February.
Based on this new CP, my new training zones are:
As I look back on my previous training program, I may have been overtraining the longer, sustained runs. My last long slow distance run averaged 296W, which is slightly over my threshold between Endurance and Tempo. Similarly, my Tempo runs were sustaining 360W, which is up in my Threshold zone near CP - that may be OK but I need to clarify if those runs were meant to peak in the Tempo zone or Threshold zone. My track intervals, on the other hand, seem dead on, with my last session averaging 371W; it seems I even have room to push it up a notch there.
Wednesday night was our first track meet of the summer and I set a new mile PR (5:45) by 6 seconds. Considering that I haven't been training for the mile race at all, I have to attribute this performance improvement to the success of my power-based training.
My mile PR power: