Toolbox: Optimized Interval Design

In WKO4, iLevels are individualized training intensities and durations at levels above Functional Threshold Power. These are based on a rider’s own prior performance and capacity. How might we turn them into optimized intervals?

Power Duration Curve
Using the power duration curve, we can further break down iLevels into specific time and intensity targets to give the coach or athlete even greater actionable intelligence to identify the optimal interval point and achieve the desired response. Take a look at the report below to understand the output.

Power Duration Exertion Points

This system targets a specific point in your unique power-duration relationship that has the greatest “weight” or “exertion” on the targeted energy system. It then uses an intersection system to determine both target time and intensity. Sets, reps, and recovery are simply suggested at this time, but we are researching ways to further use information from the power duration curve to optimize such information. This is not the perfect interval target, but an optimal one.

The time and intensity targets are the optimal target within the iLevel; they help identify the intersection of time and intensity that elicits the highest response based on the power duration curve, which is based on each athlete’s mean max power. Therefore the iLevels system uses actual performance history, the model, and advanced algorithms to supply the estimated point or intersection for the optimal response to training stimuli.

Optimized Interval Examples

Building Interval Workouts with Optimized Interval Targets

First, this is an evolving approach and has a level of specificity that most coaches and athletes are not used to. It is important that we remember these are optimized targets to be utilized as guidance rather than absolutes. Coaches and athletes should explore and test to build confidence. That being said, the time and intensity recommendations are the key points of exertion in the power duration model, and this insight is invaluable and should not be ignored when designing interval workouts.

Second, optimized interval targets depend on an accurate power duration model. To ensure accuracy, an athlete must have clean data and a group of maximal efforts that occurred in the last 30-90 days. If an athlete has been just riding along for the last 90 days with no harder efforts, his/her iLevels will likely not be accurate prescription tools.

Role of Specificity
Specificity plays a dual role when building interval workouts. To explain this, let’s create a simple example of Jane Rider. Jane loves to ride the local Wednesday Night Worlds throwdown and can keep up until the group hits one particular hill. This hill is where everyone attacks at max power and separations occur. It isn’t long, and it typically takes her about 90-100 seconds to get over it, but that isn’t enough; she gets left behind by the break.

This tells us something about Jane’s training needs: she needs to improve her 90- to 100-second power to make the break. The principle of specificity would suggest that Jane do some 90- to 100-second intervals to improve, but let’s think a little deeper.

The short hill is generally a near maximal effort for the duration, and we need to assume that Jane’s anaerobic system is fueling the effort. In WKO4, the power duration metric that gives insight into an athlete’s anaerobic capacity (AC) or anaerobic work capacity (AWC) is Functional Reserve Capacity (FRC). FRC measures the total amount of work that can be done during continuous exercise above FTP before fatigue occurs. We could assume that increasing Jane’s FRC would lead to better success on the Wednesday Night Worlds challenge climb. This introduces a second specific need: improving the physiology behind her ability to climb this hill.

This results in two ways of viewing specificity. First, there is the specific time/hill (90-100 seconds of power climbing) required to achieve success, and second, there is the needed improvement in FRC to drive the energy system to help her achieve success. You might think at first that they are the same, but let’s look at Jane’s optimized targets in the figure above.

The optimal time target to increase Jane’s FRC is 45 seconds at 574 watts for extensive FRC building and 27 seconds at 689 watts for intensive FRC building. This is a significantly different demand (or interval) than an interval of 90-100 seconds. She is thus faced with an interval choice: should she focus on the FRC interval (which is shorter and more intense) or should she focus on the specific time demand of the course hill?

The Art of Coaching
The answer to Jane’s question is something that a lot of coaches and athletes would have an opinion on, and it is part of the art of coaching. The dilemma is a chicken-or-the-egg situation, but I would suggest targeting the underlying energy system, building FRC to a point of diminishing returns, and then applying that higher FRC to the course-specific hill. This can be thought of as a periodized approach toward an intervals strategy as part of an annual plan.

About Tim:
Tim Cusick is the TrainingPeaks WKO4 Product Development Leader, specializing in data analytics and performance metrics for endurance athletes. In addition to his role with TrainingPeaks, Tim is a USAC coach with over 10 years experience working with both road and mountain bike professionals around the world. You can reach Tim for comments at To learn more about TrainingPeaks and WKO4 visit us at

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