Threshold power has been the foundation of most training and racing methods for over two decades, and for good reason: threshold, either measured in a lab or tested in the field, defines the ceiling of sustainable aerobic output and anchors everything from zone models to performance evaluation. But bike racing is rarely a steady-state exercise. Attacks (making or chasing them), short climbs, and sprint finishes all draw on a separate energy system with its own finite capacity, and that can be a limiting weakness or developed into a race-winning strength. Functional Reserve Capacity (FRC) quantifies that capacity as the total above-threshold work an athlete can do before being forced back below threshold. How the power duration model derives it from the power curve is covered in The Power Duration Curve: A Deep Dive Into Modeled Power. Dynamic FRC (dFRC) goes further, tracking how that reserve depletes and recovers in real time throughout a ride. FRC shapes how you evaluate an athlete’s profile and build their training; dFRC shapes the tactical and pacing decisions that determine how that capacity actually gets used in the field and can help you customize workouts to ensure your athlete gets the most benefit from their training time.
The Anaerobic Battery
The easiest way to understand FRC is to think of it as a battery. FRC defines how large the battery is, measured in kilojoules, and that capacity represents the total above-threshold work an athlete can perform before having to reduce their effort back below threshold.¹ dFRC tracks the state of that battery throughout a ride, showing both the absolute value remaining (kJ) or the percentage of full capacity. When an athlete pushes above threshold, the battery drains. When they recover below threshold, it recharges.
That capacity number has direct performance implications. Take an athlete with a threshold power of 300 watts. With an FRC of 8 kJ, they can sustain approximately 567 watts for 30 seconds before having to reduce their effort back below threshold. With an FRC of 16 kJ, that ceiling for the same 30-second effort rises to around 833 watts.²
One modeling nuance: because modeled power and FRC are derived empirically across a range of durations, it functions more like an average of capacity across that range than a true maximum at any specific point. At some intensity and duration combinations, an athlete’s actual capacity exceeds that average, they effectively “go negative” relative to their FRC ceiling. At others, they won’t fully deplete their reserves before the effort ends. In other words, it’s normal to occasionally see negative values for dFRC in TrainingPeaks, even though that doesn’t fit perfectly with a battery metaphor.
Applying dFRC
There are many different ways to use dFRC as a coach or athlete, such as tactical race review, pacing evaluation, workout optimization, and identifying limiting weaknesses. Below are three examples:
Road racing
This road race was animated from the start, with several hard attacks early on that required this rider to make several anaerobic surges just to stay in the group. During the middle of the race, they chased down several smaller attacks and tried to initiate breakaways a few times. About 2/3rds of the way through a big surge from the group right after one of those attacks caught the rider out before their anaerobic battery was recharged, after a few minutes of valiant chasing to get back into the group they were isolated, their battery was drained, and they were forced to ride threshold or lower for the rest of the race to get to the finish, a few minutes behind the main group. The story told by dFRC from this race can be used by the athlete and their coach during a race review to determine if there were tactical decisions to be made, if the ride could improve their pack riding to expend less effort in the group, or if there is room to improve the FRC associated physiology with targeted training.
Time trials
In this mostly flat, non-technical time trial, this athlete went out a little too hard from the start and after the turnaround, as shown by their dFRC dropping to around 75% of their capacity. This isn’t catastrophically poor pacing, but does show some room for refinement. A well-paced time trial would show very few drops in dFRC until the very end, unless some very steep hills dictated a different pacing strategy, but even a very hilly, very technical TT shouldn’t lead to dFRC deplection of much beyond 50% for most of the race. Longer self-paced efforts like 70.3 or full distance triathlon bike legs should have almost no dFRC depletion, since those should mostly be raced under threshold power.
Structured workouts
dFRC can be used to refine structured workout intervals, including the intensity and duration of the work interval, as well as the recovery interval. In the first example below, this athlete had struggled to complete some of the standard VO2max intervals their coach prescribed. With the help of dFRC the coach determined that the athlete was sufficiently recharging their anaerobic battery before starting the next interval. Instead of lowering the power targets or making the work intervals shorter, they had the athlete test out different recovery durations between intervals (about 4 minutes, then about 6 minutes in the example). After this test, the coach modified this athlete’s interval sessions to have 6-minute recoveries instead of 4, and the athlete was able to successfully complete the workouts going forward. The second example is a well-balanced VO2max workout. The anaerobic battery still drains during the work intervals, and doesn’t necessarily recharge fully during the rest, but it doesn’t deplete so severely so early on in the workout to prevent the athlete from finishing the entire set.
What dFRC makes possible
FRC answers the question of what an athlete is capable of. dFRC answers the question of how much of that capability remains at any given moment. In racing, that distinction determines whether a move is well-timed or premature; in training, it reveals whether an athlete is executing intervals as designed or needs adjustments to workouts. Together, FRC and dFRC give coaches a more complete picture of how anaerobic capacity functions in practice, not as a fixed profile number, but as a resource that depletes and recovers continuously throughout every ride.
¹ Coaches familiar with the critical power model will recognize FRC as conceptually similar to W’. Both metrics quantify the finite work capacity above a threshold intensity (FTP or Critical Power). The key difference is that W’ is derived from a small number of maximal efforts at selected durations, typically two or three test points, while FRC draws on the athlete’s full power curve. That broader basis makes FRC more representative of an individual’s actual performance profile and less sensitive to the outcome of any single test effort.
² Doubling FRC doubles the above-threshold component of power, the 267 watts of capacity above threshold becomes 533, but threshold itself stays fixed, so total power rises by less than a factor of two.
