# Predicting and Measuring Kilojoules to Determine Athlete Durability

BY Matti Rowe

Learn the significance of kJs in endurance training and how they relate to energy expenditure, durability and performance enhancement.

The interplay between nutrition and workouts or racing is becoming increasingly significant as athletes begin to adopt nutrition periodization or apps or coaches prescribe nutrition strategies. Making sure an athlete is fueled properly before and after activity is crucial to avoiding pitfalls of under-fueling as well as powering effective training stimulus. By measuring and predicting the energy outputs of an athlete, a coach can better plan this interplay through kilojoules (kJ).

## What Are Kilojoules?

Kjs are a unit of energy. To move a bike forward, a cyclist has to pedal, which requires energy or ‘work.’ A power meter can measure the rate of energy transfer (work/time or watts) from the rider to the bike. After a completed ride, software or a device can calculate the total work (energy) expended by the rider based on the file produced from power meter recordings expressed in kilojoules.

To convert the total work from kilojoules to calories, a standard nutritional unit of energy, divide the total work done by the conversion factor from kilojoules (J) to calories (kilocalories), 4.184.

However, the resulting energy expenditure converted to calories is incomplete because the number assumes 100% energy transfer efficiency from cyclist to bicycle. In practice, cyclists are 20-25% efficient, losing much of their energy to heat and metabolic processes.

Consequently, the resulting calorie expenditure calculated by converting from kilojoules to calories must be divided by the cyclist’s efficiency factor (~22.5%, the midpoint of 20-25% efficiency), producing an accurate conversion of energy from kilojoules to calories, with the resulting calorie expenditure nearly identical to initial energy expenditure in kilojoules.

In other words, there’s a 1:1 relationship between kJ and calories (kcal) because the conversion factors from kJ to Calories (4.184) and human efficiency (20-25%, or ~.225) cancel each other out.

Example:

1. Total work (energy) from activity: 500 kJs.
2. Converted to Calories – 119.5 Calories (kcal).
3. Adjusted for rider efficiency (22.5%): 531 Calories.
4. Comparing kJs to Calories: 500 ~ 531; ~1:1 ratio.

To review the total work generated from a workout within TrainingPeaks, open the workout from the calendar view and review the completed metrics in the summary view.

## Using the Workout Builder Estimated kJs

Provided the TrainingPeaks structured workout is based upon % of FTP, TrainingPeaks will predict the kJ expenditure by taking the predicted average watts from each interval block and adding them together.

To view the predicted kJ expenditure, click the workout once saved.* The number will appear in the Quick View as a planned value.

*If you’re using a Structured Workout you’ve created previously, and you don’t see predicted kJs in the planned workout summary, click on the workout, open the workout builder, make a minor adjustment (e.g., add a minute, then change it back) to trigger the kJ calculation.

## What is Durability?

Durability is the time and degree of performance deterioration caused by fatigue during prolonged exercise.

For example, imagine a rider completes a four-hour ride, averaging 200 watts, requiring a given oxygen consumption to produce the energy (kJ) to move the bike. Initially, with fresh legs and full glycogen stores, this consumption is stable.

However, hours into the ride, maintaining 200 watts requires higher oxygen consumption and, thus, more energy (kJ).

The increase in oxygen (and hence energy) required to maintain the same power as fatigue accumulates decreases the rider’s performance ability from a rested state.

Crucially, a rider’s durability in longer endurance events (~90 min+) is a key performance indicator, distinguishing higher-performing athletes from their less-successful peers.

In sum, a cyclist with high durability can expend a lot of energy (kJ) without a sharp drop off in performance from their rested values.

## How to Measure Durability

Athlete testing measures durability. Though there isn’t a clear consensus on a durability testing protocol, in general, various tests can be performed in two ways:

1. In a lab – Laboratory testing results of VO2 max, gross efficiency, and fat oxidation can be used to predict durability.
2. Field tests – Athletes test their ‘fresh’ (few kJs burned) abilities across race-relevant durations, perform work (burn kJs) to accumulate fatigue, and retest the exact race-relevant durations. The performance ‘drop-off’ from ‘fresh’ to ‘fatigued’ state determines their durability.

For example, a cyclist could perform a 5-minute maximal interval after a brief warm-up, ride until they burn 2,000 kJs, and then perform a 5-minute interval again and note the drop-off in performance. For example:

• First 5-minute Average Watts – 300
• Second 5-minute Average Watts – 270
• Result – 10% reduction in 5-minute power after 2,000 kJ

## Durability and Training

The durability literature is relatively limited, but in general, researchers have found that durability can be improved through training and that it improves as younger athletes progress into older athletes and accumulate more training.

Researchers have found some relationship between durability and time spent at specific intensities (below aerobic threshold (VT1/LT1)) and training distribution method (polarized); however, the strongest predictor is how much and how long the athlete has been training over the years, which would explain why older athletes tend to have better durability than younger athletes—they’ve trained more.

## What Other Factors Influence Durability?

Nutrition plays a crucial role in durability.

Prolonged endurance exercise requires oxidizing substrates to fuel working muscles, primarily fats and carbohydrates. The athlete’s exercise intensity and metabolic fitness determine the burn rate of fats and carbohydrates. The more intense the exercise, the higher the proportion of carbohydrates consumed, and vice versa.

Physiologically, the human body has infinite (practically speaking) fat reserves (even in the leanest athletes) and limited carbohydrate reserves.

Consequently, an athlete with high durability will be able to:

• Use a higher proportion of fat as fuel at higher exercise intensities.
• Ingest (and absorb) a lot of carbohydrates (30-120g/hr depending on intensity, duration, and workout/race goal).

An athlete with these characteristics can perform a lot of work at a relatively low metabolic cost, which means that they’re less fatigued towards the end of a race and can still produce high-intensity efforts in decisive moments in the race.

## Monitoring Durability in TrainingPeaks

### Workout to Workout

While durability is a relatively new performance indicator, an athlete or coach can test and evaluate durability over time with a little work. Here’s an example of how I do it:

Steps:

1. Using the Workout Builder, create a workout that tests a rider’s power over a relevant event-specific performance duration (e.g., 1 min, 5 min, 10, etc.) fresh, add Workout Builder interval blocks that necessitate significant energy expenditure (1,000 kJ+), and then repeat the testing block toward the end of the workout.
2. Once the athlete completes it, create laps to analyze their fresh versus fatigued performance and note that in the workout laps or workout comment. Then, add a tag to the workout so it’s easy to search for.
3. Periodically prescribe the workout as a test and then use the search function to quickly surface the workout to assess durability progression.

## Trends – kJs Dashboard Charts

While indirect, overall kJ expenditure loosely predicts durability. TrainingPeaks has a few dashboard charts that surface kJ usage at a glance.

### Dashboard Chart – Kilojoules by Week

The Kilojoules by week dashboard chart is available with the chart library in the Dashboard View:

1. Drag and drop it from the library into the calendar.
2. Click the hamburger icon to adjust grouping, date range, etc.

### Dashboard Chart – Calories by Month or Week

The Calories by Time Period chart is available in the library in the Dashboard View:

1. Drag and drop it from the library into the dashboards.
2. Click the hamburger icon to adjust grouping, date range, etc.

### Miscellaneous – Search Function

Click the hamburger icon to add a ‘Work’ column with the search function to quickly surface and sort workouts by kJs.

For athletes with lots of historical data, approximating the kJ demands of different events can inform training prescriptions to ready the athlete for peak performance.

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