Beyond FTP II: Understanding Critical Power and Watt Prime for Optimal Training

The previous article looked at power profiling as a way to look beyond FTP testing to gain further insight into a rider's physiology and capabilities

Critical Power (and Wprime)

FTP as a single number or output obtained from a 20min effort has many shortcomings in how to explain what adaptations have contributed to fitness (if any) and while the power profile attempts to show a greater dissection of the ability of a rider and where a rider's strengths and weaknesses are, the idea behind Critical Power (CP) seeks to provide more context on what the aerobic and anaerobic contributions are to a particular power output using linear regression. Essentially, two metrics emerge from this relationship, the CP and a finite amount of anaerobic power called W’(att) Prime measured in kilojoules (KJ). 

Critical power can be thought of as the point where a rider goes from steady state to non-steady (rapid fatigue) and represents aerobic ability of Type 1 and Type 2a fibers. 

W’ Prime can be thought of as the capacity above your aerobic sustainable power and is a set amount (finite) that depletes over time before fatigue. 

Critical power requires 3min, 5min, 12min, and 20min (20min is the shortest duration. It can go upwards of 40-75mins depending on training status) maximum efforts where those values are then plugged into an equation (or online calculator). 

Critical Power Curve with W'Prime

Advantages 

• Think of critical power as a layer above power profiling where it begins to provide data on your sustainable power (can also call this your FTP) as well as the capacity above your sustainable power (think of this as a battery). 

• It, therefore, has greater objectivity than using one data point that single FTP testing uses. In other words, with four maximal efforts, there is more clarity on your specific physiology. Programming therefore becomes more specific to address this unique physiology since it reveals a balance between aerobic and anaerobic capabilities. Further expanding, 20mins assumes a 5% fatigue from 20 to 60mins (since “threshold” power is considered 60mins). This is usually incorrect as most average riders experience significant power decrement after around 30-40mins. Using critical power helps to navigate this problem.

• Critical Power can be used to calculate power and the duration it can be held for for efforts between 3 and 20mins and that require power output above that Critical power number. This can be useful for calculating interval length or for race lengths between 3 and 20mins.

Disadvantages

• Critical power testing does require two maximal efforts (and four if more objective data is required). Like the traditional or "classic" method of 60 minutes, performing two (or four) maximal tests in a week can be psychologically challenging and a rider needs to ensure they are motivated and fueled. It can be challenging for the busy time-crunched cyclist.

• Critical power does give a greater amount of objectivity, especially with quantifying aerobic and anaerobic capacities but it cannot tell if this is from a greater ability anaerobically (within glycolysis) or if it is from a greater oxygen uptake and oxidation of energy (VO2 max). All that can be determined is capacity. Additional testing in a clinical setting like a sports lab would be needed to provide this information.

How to Test Critical Power

The below illustrates a typical testing session (helps if you have done a 20min FTP test in the past):

	Time	Description	% of FTP	% of FTHR
Warm Up	30 mins	Easy Riding	40-70	<70
	3x 1min	Fast Pedal 110 RPM	80-90
	3 min ALL OUT	Get out the saddle hard for the first 15-20s and then hang on for dear life	Full OUT
	30 mins	Easy Riding	40-70
Main Set	12 min ALL OUT	Try to keep a constant output with an increase for the last 30s	Full OUT
Cool down	15 min	Easy Riding

Calculating Critical Power (*see the previous blog for power profile chart for FTP/Sustainable power normative values)

1. Multiply average power during 12mins by 12

2. Multiply average power during 3mins by 3

3. Subtract the answer in number 2. from number 1.

4. Take the answer in number 3. by 9. Congratulations you have your CP

If using Critical Power number as a replacement for FTP number, use 94% of CP to replace that number.

Calculating W'Prime

1. Subtract average power in 12mins from average power in 3mins

2. Take the answer in number 1. and multiply by 0.24. Congratulations you have your W'

Anaerobic KJ sizes (*W'Prime):

Men	Woman
Average 9.0 to 35 KJ	6.2 to 24 KJ
High > 22.9 KJ	High>17.2 KJ
Medium 13.5 to 22.9 KJ	Medium 9.2 to 17.2 KJ
Low <13.5	Low <9.2 KJ

Note: The above table is measured in kilojoules and is taken from FRC values in WKO5. While contrast can be argued between W'Prime and Functional Reserve Capacity (FRC), take the idea of what the size is as an idea above sustainable power

In the next post, we look at Power Duration Curve (PDC).

Power Duration Curve (PDC)

Any help with further questions, discussions, consultations, or coaching. Please don't hesitate to reach out to darrin@thethreshold.coach

@darrinjordaan

@wattfarming

Train Hard and Prosper!

Darrin Jordaan

MSc (Med) Biokinetics WITS

HMS (Hons) Sports Science UP

BK 0016934

CSCS

UCI Level 1 Cycle Coach

IronMan certified coach