The short answer

The TrImp (Training Impulse) method is a tool used to model your aerobic endurance capacity.  The time and intensity of your workouts are applied to a model of your aerobic capabilities in each sport.  This model, which is specific to each athlete, allows me to design workouts that optimize your improvement, in each sport, while preventing overuse injury. 

The long answer

Aerobic endurance development can be modeled through the use of TrImps as a measure of aerobic intensity and the Fitness/Fatigue model as a predictor of aerobic capacity. 

The TrImp Method

A TRaining IMPulse (TrImp) is defined as training volume x training intensity and is specific to endurance training because of its dependence on heart rate.  Researchers initially proposed simple methods of calculating the TRIMP of a workout session, such as the following:

     TrImp = training time (minutes) x average heart rate (bpm).

     For example, 30 minutes at 145 bpm. TrImp = 30 x 145 = 4350.

The initial calculations provided a great monitoring tool to analyse progress and adherence to training.  With the data, training schedules can easily be calculated.  Weekly TrImp totals could be used as targets for general cardiovascular fitness goals or to encourage increased activity.

The main disadvantage of this calculation is that it does not distinguish between different levels of training, which would be a problem for a more advanced athlete.  For example:

Case 1 - 35 minutes at 130 bpm gives a TrImp score of approximately 4,500

Case 2 - 25 minutes at 180 bpm also gives a TrImp score of approximately 4,500

So the TrImp training zone method was developed.  Using heart rate zones to weight intensity, Foster et al split intensity into five zones (expressed as a % of max HR).  The zone is used to quantify training intensity; where TrImps are calculated as the cumulative total of time spent in each training zone.  Another example is used to illustrate the above terms:

Case 1 - 30 minutes at 140 bpm.  %max HR = 130/185 x 100 = 79%. Therefor, training intensity = 3. TRIMP = training volume (time) x training intensity (HR zone). 30 x 3 = 90.
Case 2 - 25 minutes at 180 bpm.  %max HR = 180/185 x 100 = 97%. Therefor, training intensity = 5. TRIMP = training volume (time) x training intensity (HR zone). 25 x 5 = 125.

This method finds that 25 minutes of working at a very high heart rate results in a higher training load score than 30 minutes at a lower heart rate.

The Fitness & Fatigue Model

Periodized training is based Now that we have a solid method of measuring aerobic intensity, we can apply it to the Fitness/Fatigue model developed by Morton. 

This model is the foundation on which periodization theories are built.  It suggests that ....

Applying TrImps to the Model

Now that we have a solid method of measuring individual aerobic intensity and a sound practice for balancing fitness and fatigue, we can combine the two to create a training model.  Now TrImps can be applied to you.   For each sport you are training, a model will be developed and used to create a workout that pushes you to your limits, safely.  Optimizing TrImps helps to prevent over-use injury while ensuring development at a pace that is right for you.  

The limitation of the TrImp heart rate zone method is that it only quantifies aerobic training.  Strength, speed, and technical sessions, which do not involve high heart rates but are nonetheless intense, cannot be calculated accurately by these means.  This is where common sense coaching comes in.  A good coaching program uses the data that a TrImp model can provide, but does not rely solely on it.  Athlete feedback, training metrics, common sense, and experience all combine to play a role in development.

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