Toolbox: Optimizing Intervals Part 1

Interval training involves high-intensity periods of work followed by lower intensity periods of work or rest that are repeated for a specific number of repetitions, determined by the fitness level of the individual. What are the things we should be considering to optimize intervals to suit our needs?

Jelle Wallays TT training

Interval Basics
When designing intervals, we typically consider the following four variables:

1. The intensity of each interval.
2. The duration of each interval.
3. The rest between each interval.
4. The total number of intervals or sets.

Many people forget to target or plan the rest periods, but they are arguably just as important as the intervals themselves. The rest periods allow for a buffering of lactic acid from the blood and ensure a certain amount of recovery for the next high-intensity interval.

The application of these variables is determined by the training goal of the individual, and it is focused on the energy system that is to be challenged. The three energy systems that can be challenged during interval training are ATP-PC (sprints or shorter intervals), anaerobic (moderate distance or time), and aerobic (longer distance or time). It can also be effective to further break down both the anaerobic and aerobic energy production into secondary levels of power and endurance.

All three energy systems have different workloads and recovery times based on the intensity of the intervals and the time allowed for the buffering of lactic acid and replenishment of ATP so that the following interval can be completed at a high rate of work.

Designing Intervals
Designing interval training can be challenging. There are some clear physiological principles and guidelines, but even these leave significant room for interpretation. Many athletes and coaches experiment within the typical boundaries of training levels or zones by adjusting the length and intensity of the work interval, along with the length of the rest interval. They also experiment by increasing or decreasing the number of sets within an interval workout. Why? Intervals are hard work, and we want to make sure we’re maximizing the return on our painful effort; we want to find the best interval workout possible.

A common interval guidance chart looks something like this:

Energy System / Workload / Recovery (work:rest) / Reps

ATP-PC / 10 s / 30-60 s (1:3 – 1:6) / 6-10

Anaerobic / 1 min / 2 min (1:2) / 4-8

Aerobic / 3 min / 3 min (1:1) / 6-12

The Evolution of Intervals and the Role of Training Levels or Zones
The introduction of power meters in the late 1990s was a game changer for interval design and completion, as it supplied highly accurate data for both prescription and also precise measurement of the effort. Numerous coaches and athletes began using power to try different ways of targeting intervals with the goal of improving the efficacy of the dose-response relationship. This led to the introduction of some new systems that have since evolved over the years.

Coggan Classic Levels
One of the first power-based training targeting systems to emerge was Dr. Andrew Coggan’s Functional Threshold Power (FTP) and its associated training levels. Dr. Coggan designed these to be descriptive of workouts, but coaches quickly saw the benefits of using them to target and prescribe workouts. This system is now typically referred to as the Coggan Classic Levels. They were based on the establishment of a tested FTP with training levels assigned as a percentage of that FTP. This use of percentages created a watts-based range for intensity of efforts in desired training levels while also supplying a recommendation for time/length of intervals in those levels.

Table 1: Coggan Classic Levels

Table 2.1: Coggan Classic Levels Example

The Coggan Classic Levels gave excellent guidance to the general design of intervals and interval workouts, helping coaches and athletes target a desired physiological/energy system response. This system provided insight into the key challenges of intervals: intensity and time.

However, there were some limitations. Coggan Classic Levels was an excellent solution for training at or below FTP, but when training above FTP, the intensity-time relationship varies more between individuals, which was not accounted for. Here’s what Dr. Coggan wrote about this problem:

Although it is logical to define power-based training levels relative to FTP for predominantly aerobic intensities, this does not make sense at higher intensities and shorter durations of exercise, where factors other than metabolic fitness are the primary determinants of performance. Because of this, in the original system I created, level 6 (Anaerobic Capacity) is defined as simply being higher than level 5, whereas level 7 (Neuromuscular Power) is not linked to FTP at all. While appropriate and correct, these definitions are of somewhat limited use for prescribing the power outputs and durations of intervals intended to increase, such as sprinting ability or resistance to fatigue during very high intensity, unsustainable exercise.

The desire to improve upon this system was one of the driving factors behind the development of the Power Duration Curve and the resulting iLevels.

Coggan iLevels
The challenge of the Coggan Classic Levels was one of individualization and dealing with the variation between individuals at higher-intensity training levels (above FTP). This led to the development of Individualized Training Levels, or iLevels for short. iLevels are training levels based on each athlete’s unique physiology and power duration capability, designed to give the coach or athlete even better guidance for targeting intensity and time. This new system was an evolution of the classic seven training levels into nine more specific training levels. These levels were related but allowed not only more individualization but also more specificity in targeting energy systems.

Table 3. Coggan iLevels compared to Coggan Classic

iLevels are a perfect blend of the strengths of the classic system and the power of individualization via the WKO4 power duration model. This system supplies individual target intensity and time ranges for training above FTP based on the performing athlete’s actual unique physiology.

Table 4. Coggan iLevel Example

This new system, as you can see in the example above, supplies not only a more individualized intensity target for efforts over FTP, but also specific time ranges for best results. This evolution gave coaches and athletes actionable intelligence for continuing to improve their design of effective interval workouts. It was a significant advancement that opened the door for even further improvement.

Jasper De Buyst climb training

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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