Normalized Training Stress®

Normalized Training Stress® (NTS™) is the consummate method of quantifying the physiological stress from a training session. It considers the session’s discipline type, environment, intensity distribution, intensity levels, intensity durations, and your Training Stress Profile™ or your ability to absorb training stress.

Normalized Training Stress (NTS) Overview

Quantifying training stress is essential to effectively managing structured training to produce the best results in the least amount of time while minimizing injury risk.

In the mid 1970s, Calvert and Banister introduced the training impulse system (TRIMPS) which was a systematic approach to quantify training load that integrated both volume and intensity. However, it used heart rate (rather than pace or power) as the criterion measure which caused serious limitations. In the mid 2000s, Coggan addressed some of the limitations of TRIMPS when he developed the Training Stress Score® or TSS® for cycling that uses power, rather than heart rate, as the criterion measure of intensity. Yet many limitations with these traditional approaches still remain.

The Normalized Training Stress (NTS) method of quantifying training stress substantially resolves these limitations and provides a more accurate and actionable metric for training stress management in Optimized Training.

With traditional approaches, training load (or stress) is calculated as the duration of a session multiplied by an intensity measure for the session multiplied by an intensity-based weighting factor. Basically, as a session’s duration or intensity goes up, so does the training stress.

There are many other things that also have significant impact on training stress that are not considered when using these traditional approaches. Below are brief descriptions of some of the critical ones that NTS considers as it quantifies training stress.

Discipline Type

Training stress for a given period of time varies by discipline (swim, bike, run) due to the different muscle groups used, energy required, and the physicality of the movements. The stress from one hour of cycling is not as much as one hour of running due to factors such as the load bearing nature and repeated ground impact of running.

Based on this fact, NTS uses baseline stress values at Functional Threshold Power or Pace (FTP) intensity that are discipline dependent. Baseline stress values for cycling, running, and swimming are defined as follows:

Cycling: 60 minutes @ FTP = NTS 100
Running: 40 minutes @ FTP = NTS 100
Swimming: 40 minutes* @ FTP = NTS 100

(*The 40 minutes of swimming at FTP would need to be distributed over a longer training session such as one hour.)

NTS uses stress factors for intensities above and below FTP for each discipline which are calculated relative to these baseline values. (This is further described under “Intensity Levels” below.)


Environmental factors have a significant impact on athletic performance. For example, oxygen is required for aerobic activities, and there’s less oxygen available in the air at higher elevations. Therefore, your aerobic performance ability is diminished at higher elevations. Also, temperature and humidity affect your ability to dissipate heat to regulate body temperature. The more blood that flows to the skin for cooling means less blood is available to carry oxygen and energy to the muscles. The decrease in blood available to transport oxygen and energy will diminish your physical performance ability.

Traditional approaches use a static value for your FTP. Intensity metrics measured during the training session are evaluated against this static FTP value irrespective of the environment in which the training was conducted.

For example, if your running FTP was 7:30 per mile, conducting a 30-minute run session at a 7:35 per mile pace on a cool morning at 55° F and 40% humidity would produce significantly less training stress than performing that same session in the afternoon or on a warmer day at 90° F and 80% humidity.

Using traditional approaches, the training stress from these sessions would be quantified as being equal. This would significantly under represent the training stress from the session in the warmer environment which could result in increased injury risk and incorrect conclusions when analyzing performance and training effectiveness.

NTS uses EnviroNorm® technology to account for environmental conditions impacting both your FTP at the time of each training session and the raw training metrics measured by your training device during your training session.

Intensity Levels

As your training intensity increases, the stress associated with that intensity increases exponentially. With traditional measures of training stress, intensity-weighting factors are expressed for the session’s training intensity as a percent of FTP. (Example: If your FTP was 200 and you trained at 160, then the factor would be .80.) One major problem with this approach is that higher intensities are not weighted appropriately to account for the exponentially higher stress they induce relative to lower intensities.

For example, if you were to complete a 60-minute session at your FTP, this would traditionally score a 100. If you were to attempt to complete only 20-minutes of training at 125% of your FTP, you would likely not be able to physically complete it despite it being only 25% more intensity than your FTP. Using a traditional model, this 20-minute session would only score a 52.

A significantly stressful but more realistic session with intensity at 125% of FTP (zone 5) might include closer to 8 minutes at 125% and another 30 minutes at 68% of FTP (zone 2). Using a traditional approach, this session would only score a 41.

As a comparison, if these three sessions were three cycling sessions, NTS would score the 60-minute FTP session as 100, the “undoable” 20-minute session at 125% of FTP as 220, and the significantly stressful but realistic session including eight minutes at 125% of FTP as 91.

NTS uses dynamic intensity-weighting factors that match the exponential increases in training stress associated with increasing training intensity.

Intensity Distribution

When accurately quantifying the physiological stress from a training session, the distribution of the stress within the session plays a major factor. Using averages or normalized intensity values for the entire session as with traditional measures of training stress, leads to highly inaccurate results. For example, consider these two sessions:

Session 1
60 minutes steady @ 68% of FTP (Zone 2)

Session 2
10 minutes @ 68% of FTP (Zone 2)
4 minutes @ 115% of FTP (Zone 5)
2 minutes @ 68% of FTP (Zone 2)
4 minutes @ 115% of FTP (Zone 5)
2 minutes @ 68% of FTP (Zone 2)
4 minutes @ 115% of FTP (Zone 5)
16 minutes @ 68% of FTP (Zone 2)

The 42-minute session with twelve minutes at 115% of FTP (Zone 5) would clearly produce more training stress than the 60-minute steady state session at 68% of FTP (Zone 2). However, scored traditionally based on average normalized intensity across the entire session multiplied by the duration, both sessions score the same value of 46.

Using NTS, these sessions score 29 and 70, respectively, which is a much more accurate representation of the actual training stress produced.

Intensity Duration

Similar to the way training stress increases as intensity increases, it also increases the longer you are performing at a specific intensity.

For example, if you complete a training session including forty minutes at your FTP, the stress associated with the first ten minutes of the forty minutes is not the same as the stress associated with the final ten minutes.

The longer you are training at an intensity level during a session, the more stressful it becomes. The final minutes are significantly more stressful and impactful on your training results than the initial minutes.

The NTS algorithm considers the progressive increase in training stress as you continue to train at specific intensities for longer durations.

Your Training Stress Profile®

The training stresses produced from various types of training sessions is relative to your ability to absorb them as is reflected in your Training Stress Profile. One type of training session may be more stressful to you than someone else while another type of training session may be less stressful to you than that same other person. Your ability to absorb training stress is unique to you and is influenced by many factors beyond your current performance ability and training load.

For example, an older athlete may not be able to handle significant durations of intensities above their FTP but is still able to handle moderate durations at FTP and high durations of below-FTP intensities. These differences are reflected in your Training Stress Profile. Other major factors impacting your Training Stress Profile include age, body composition, sport age, performance ability, training load, and your genetics (PhysiogenomiX™ technology).

NTS uses these factors, if available, to further refine your NTS values relative to your ability to absorb each specific training session.


By accounting for the impact of a session’s discipline type, environment, intensity distribution, intensity levels, intensity durations, and your Training Stress Profile, Normalized Training Stress is able to more effectively quantify the physiological stress from your training sessions. This results in the increased ability to deliver your best training results in less time with fewer injuries.

Viewing Your NTS

Your planned and actual NTS values are displayed for each session so you can see the training stress for each of your workouts. When preparing for a planned session, you’ll also see an intensity icon to the right of the planned NTS indicating the preferred primary intensity metric (power, pace, or heart rate) to use for the session to achieve optimal results.

Objective feedback and insights from both your NTS and TrainX™ Score help to ensure you’re doing the RIGHT training RIGHT.

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* Training Stress Score and TSS are trademarks of Peaksware, LLC.**

Normalized Training Stress, NTS, EnviroNorm, Training Stress Profile, and PhysiogenomiX are trademarks of Predictive Fitness, Inc., the developer of TriDot and RunDot.