Sunday, November 12, 2017

W prime (W')

Since I'm using Golden Cheetah software more often these days, I came across a metric called W prime, written W'.  I think it's a really neat/cool metric that wasn't and probably still isn't available in Training Peaks Software WKO+ 4.  It's new to me, that's for sure.  Before I tell you what W' is all about, you first must understand what CP (Critical Power) and what FTP (Functional Threshold Power) are.  Let's start with the latter.  FTP is your "hour power".  That is, it's your maximum sustained average power output in 1 hour in watts.  It's your average sustained power during a 40k TT.  Another way of determining your FTP is to do a 20 minute TT and find your maximum sustained average power.  Then, take 95% of that value and that's your FTP.  Critical Power (CP) is synonymous with FTP.  To find your CP do a 3 minute, 5 minute and 10 minute Time Trial with a Power Meter.  Or, do a 3 minute, 8 minute and 14 minute Time Trial which I heard is a little better for plotting.  Plot the power (3 pts.) versus duration (time).  Your CP, in watts, is where the line levels off.  You'll need a curve fitting software program to properly plot the graph and find your CP.  i.e. hyperbolic curve fit.  Want an easier way to compute your CP?  Download Golden Cheetah software and use their CP and W' Estimator.  See graph below:


Now, the amount of work that you can do above your CP, shown by the red hatched lines above, is your W'.  It's a fixed amount.  If you're a math geek, and you want to calculate the actual W' value, all you have to do is find the equation of the CP line and integrate (using calculus).  How quickly that fixed amount lasts is dependent on how hard you go above your CP and if you get any rest or breaks.  Go hard above your CP and you're not going to last long.  Go easy above your CP (say 250w in the graph above) and you'll last longer than if you went hard (say 300w in the graph above).  If you get a chance to recover during your ride, or take a break, it will replenish the W'.  How much it replenishes depends on many things such as the amount of time you recover, the power you recover to, etc.  Sooner or later though, whether you go hard or easy above your CP you're going to bonk/crash/run out of gas.  The value of W' where you start to crash and burn is around zero (0).  The units for W', or sometimes labeled W'bal, (balance) is in kiloJoules (kJ).   How long you can keep up the power at W' varies.  How negative a value you can have for W' varies too.

Here's a stress graph (from Golden Cheetah) showing W' for my latest ride today:


Not sure why Golden Cheetah gave me a W'bal of 20 kJ to start with.  I suppose it's based on my FTP of 220 that I entered into Golden Cheetah.  The segment of my ride highlighted in red is a 3 mile Time Trial on my Mountain Bike on the Tow Path.  You can see I started out a little hot, close to 300w, and backed it down to about 250w.  I should have done this TT fresh, if I wanted a personal best time, but I had just ridden 8 miles prior.  Nonetheless, you can see my W'bal plummet because 250w is definitely above my FTP or CP of 220w.  In fact, W'bal went negative at the end of my TT.  Was I exhausted at W'bal=0, hell yeah, but you could see I still had about 1.5 miles to go.  So, at 250w (30w above my FTP) it only took about 5-6 minutes before I was near 0 W'bal.  It doesn't take long.  Lessons learned, if you want to do a TT do it fresh and don't start out too hot (rookie mistake).  You can also see that my W'bal didn't start to recover until I walked over the bridge from Bulls Island, NJ to the PA side.  The PowerCal HR monitor/meter showed 100w when in reality I wasn't even on the bike.  I was walking my bike across the bridge because of the heavy pedestrian traffic.  Anyway, that's W'bal and I think it's a neat/cool metric for post-ride analysis.  Probably even better for post-race analysis.

Power ON!  Coach Rob

No comments: