Dynamic Functional Reserve Capacity (dFRC) in WKO5

Overview

Over the past ten to fifteen years, Functional Threshold Power (FTP) has been the focal point of performance coaching and analysis. Training levels and zones, structured workout targeting, and training response analysis have all been anchored to FTP or the similar Lactate Threshold (LT) markers, but what happens when we go above FTP? How do we measure our ability to do the hard efforts that can make all the difference in races and events?

To answer that question, let’s start with Anaerobic Capacity. Anaerobic Capacity can be defined as the maximal work performed during maximum-intensity, short-term (typically 30-90 seconds) physical effort, and it reflects the energy output capacity of anaerobic glycolysis. Simply put, it is the output of your anaerobic (without oxygen) energy system. The challenge of Anaerobic Capacity is that it can be difficult to measure and track, so we needed a more functional solution. Enter Functional Reserve Capacity.

Using the Power-Duration Model in WKO, we now have the ability to quantify your anaerobic capacity as Functional Reserve Capacity (FRC). FRC is similar to anaerobic capacity, only more functional, because it takes into account the small amount your aerobic system contributes (you are still breathing in those hard efforts, after all) to your total power output. Your FRC is basically the total amount of work that can be done during continuous exercise above your Functional Threshold Power (FTP) before fatigue occurs.

In other words, your FRC is your anaerobic battery. Once you go over threshold, you start draining that battery, and it can’t be recharged until your effort goes below threshold again. The further you go over FTP, the faster that battery drains. Both genetics and training affect the size of your battery, and understanding how training increases or decreases the size of your battery (as reflected in your FRC score) will help you improve your training efficacy and performance. 

But what if we can also use this measurement in a more dynamic way? What if we can track how we use our anaerobic “battery” in workouts and races to better understand on-the-road performance? To accomplish this, we are pleased to introduce Dynamic Functional Reserve Capacity (dFRC).

What is dFRC?

Dynamic Functional Reserve Capacity (dFRC) is defined as the real-time measurement of FRC utilization in performance. Simply put, it is how much we drained, and restored, the battery during a workout or race. This measurement is in kilojoules or joules as it represents the energy utilized. 

The concept might be best demonstrated in the above screenshot of a 6 x 8-minute supra-FTP workout performed by Jane Rider. The yellow line is Jane’s power output, and we can see that she hit the intervals pretty well and was actually increasing power on each one. The purple line is her dFRC, showing the amount of anaerobic energy drained in each interval. As each interval progresses, she goes a little harder, draining more battery but never fully depleting it. This is a good workout, but it could have been done a little harder as she never really drained the battery fully. As a coach, I would use this analysis to suggest next time she goes a little harder in the earlier intervals, she was holding back too much. 

What does it mean if my dFRC goes below zero?

You may have noticed that dFRC can sometimes fall slightly below zero. dFRC modeling is a difficult challenge; we can accurately estimate the burn rate of energy above FTP, but there are large variations in the recovery rate of that energy. In WKO5 your FRC and dFRC are based on your individual power duration curve as estimations of your recovery rate and fatigue.


If your dFRC is going significantly negative, however, this is typically an indicator of an inaccurate power duration model due to lack of testing or maximal efforts. Click here to watch Tim Cusick's webinar on how to maintain your model.

Key Benefits of Measuring dFRC


Now that we can measure dFRC and track how an athlete uses the anaerobic battery in workouts and racing, let’s talk about some of the key benefits of doing so.


  • Tracking dFRC allows us to understand how deep the athlete went during workouts and efforts above FTP.
  • Reviewing and tracking dFRC in an athlete’s workouts will give insight into specific workout types and targeted intensities that will lead to the optimal workout for that athlete.
  • Utilizing dFRC to review races, event performances, and hard efforts will demonstrate how well athletes use their FRC to improve results.
  • A better understanding of FRC as reflected in dFRC allows for better race performance. For example, it can help tell me when I should sprint, how hard I can go to establish a break, and if I can power climb that technical hill.
  • Team time trial planning can be better timed by using dFRC to determine pull time and intensity. 

 

dFRC and Anaerobic Work Capacity


The idea of dFRC is a new concept, but it’s also an improvement on an existing one. In 2012, Dr. Phil Skiba introduced the idea of W and W’balance. These concepts were based on the Critical Power model designed by Monod and Scherrer in 1965. The idea of W’balance and its uses was an exciting concept and an excellent starting point. WKO uses the Power-Duration Model and some additional elements to improve upon this idea, but credit should still be given to Dr. Skiba for the original idea.


For a more detailed overview of dFRC, watch Tim Cusick's educational workshop webinar below or on YouTube.

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