Training Load (TL) what's that?

Training load in endurance sports is an objective metric that quantifies the physiological strain of individual workouts independently of the training metric used (heart rate, power, or pace), making sessions comparable. It takes both intensity and duration into account to measure the total stress placed on the body. This objective measurement is also referred to as external load.

The formula for calculating training load is quite straightforward:

Training duration (t) in hours (h) is multiplied by the square of the intensity factor (I):

TL = 100 × t(h) × (I²)

In enduco, you regularly perform performance tests that determine your current fitness data. These tests measure key values such as your FTP (Functional Threshold Power), FTHR (Functional Threshold Heart Rate), or TPace (Threshold Pace).

These values form the basis for calculating your training load, which is why keeping them up to date is essential.

Every recorded intensity is compared against your threshold to determine the relative intensity of each training interval. The intensity factor (I) is 1.0 if you were to sustain your threshold intensity (FTP/FTHR/TPace) for 60 minutes. Based on this, the following approximate factors apply to different training zones:

Additionally, the intensity is also expressed as Normalized Power (NP) to better reflect the physiological strain of a session than average power alone. The goal of NP is to estimate the “perceived hardness” of a workout more accurately — especially in interval-heavy or variable-intensity sessions.

Normalized intensity is calculated using a sophisticated algorithm that considers variability in your power, heart rate, and pace. Here’s how it works:

Power data is recorded in short intervals (typically 25 seconds), then raised to the fourth power, averaged, and finally the fourth root is taken.

The result? Normalized power strongly weighs high-intensity segments and emphasizes the harder parts of the workout.

In Figure 1, you can see how normalized power (black line) differs from raw data (gray line), showing its ability to better reflect effort.

Figure 1: From Clarke & Skiba (2013). Rationale and resources for teaching the mathematical modeling of athletic training and performance.

This makes it possible to gauge how demanding a training session was for you. Your training can then be optimally tailored to your performance level. That’s why it’s critical to keep your performance data up to date. If your threshold values are significantly inaccurate, enduco may over- or underestimate your training — leading to either too much or too little assigned training load.

Training load is the foundation for assessing your fitness, form, and fatigue. These values help determine how much training volume you can currently handle and where your fitness stands. Ultimately, they are used to calculate how much training volume you’ll need to prepare for a specific race or event.

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