How is 4 min watts/kg actually calculated?

This, I thought, would be obvious but it’s clearly not done in a critical power fashion in terms of 4 consecutive minutes - e.g. for my best ride it says 5.7w/kg for 4 min (I agree - one of my best races - hence my particular intrigue!) but actually the critical power (from Strava) for 4 mins in this race was ~5.1w/kg and I have another ride/race higher at 5.2w/kg which shows a lower value for 4 min power in Xert.

Be sure to read our blogs, particularly the one on Fitness Signatures. Also review the glossary entry for Power Curve. It explains the problems with using CPx/MMP data to determine fitness.

Thanks. For a long time I’ve struggled to “believe” a lot of Coggan type fitness measurements as they apply to road racing. FTP does obviously have a correlation and 20min/1hr power means something (particularly for TT’ing & long climbs) but what’s more important is not what my 1 or 4 min peak power actually is, but how many times in a ride/hour I can hit that value (or just below that value realistically) (repeated “best efforts”) because that’s really what determines whether you are there for the finish of a race or dropped. It has bothered me for a while that there isn’t a good way to measure this (even if there are known ways to improve it )i.e. vo2max style intervals). Sometimes cyclists refer to this as the number of “bullets” you have. I’m not sure MPA/HIE really address that either but instead just expresses critical power points differently - i.e. between two different points on the power curve - maybe I’ve misunderstood. Perhaps “normalised power” does to an extent since it brings the standard deviation from the mean into account. Anyway - really like the site & the calculations - and love the graphs! I’ll definitely be using it for a while!

What you are referring to is what is called “Difficulty Score” on the system. Workout and Activity ratings are based on this value.

Great thanks - I’ll play with it some more.

I really love the MPA/peak power analysis of rides. I think it conceptualizes maximal efforts and really how you feel and cope in a race. Although I’m not really sure how to use it for training/improvement other than to be repeated intervals at those levels (which I sort of knew). I like Xert a lot, it definitely is a next step in training analysis, but I think it’s a bit overpriced (if you compare to Strava and Zwift which I’m already paying for).

So I’ve been paying for a couple of month because I can really see how the breakthroughs in 4min w/kg signature really correlates with my hardest races. But although I agree that CPx curve to measure fitness doesn’t always work, I still, despte reading the Fitness Signature blog, understand how this 4min w/kg is calculated. Could help a bit more?? I also really like that it accurately estimates my FTP/threshold from a hard crit/Zwift race and I’ve confirmed this by 20 min test. Maybe I’ll never do one again! That’s worth the monthly fee right there! One thing that I sort don’t like/agree with is how the MPA doesn’t take into account fatigue - i.e. I can ride just below FTP for 2-3 hrs (feeling I’m pretty empty) but at that point an over FTP/sprint is essentially counted the same as when fresh. Anyway - that’s enough points! Thanks for helping.

MPA is the main concept you’ll need to understand. It represents how much power you have. It is calculated by using your fitness signature and your power data. If your 4 minute power is say 300W and your Peak Power is 900W, it takes precisely 4 minutes for your MPA to go from 900W to 300W, while pedaling a 300W. As you can imagine, this unique situation where you start totally fresh and pedal precisely at 300W for 4 minutes at which point you are unable to sustain 300W anymore is a unique occurence and not one that doesn’t readily happens in regular ride data. Hence, although tracking your 4 minute power seems to be a way to quantify your fitness, it is nonetheless a specific application of MPA. Much more importantly, MPA defines how much power you have at every instant when you’re riding. We know your fitness signature is correct, when the MPA derived identifies your points of failure with precision. Once this happens, then, in principle, we know your MPA at an y point in time during any effort. Your PD relationship is then just a graph of the unique situations where steady-state efforts are calculated from fresh to failure using MPA.

Ok thanks - I think I get it now!