Training impulse is a model for evaluating training load. It is based on heart rate and attempts to quantify the stress of a given workout. In essence answering the question of the impact of a 1 hour jog versus a 30 minute track workout? I first became interested in quantifying my running after researching training loads based on training stress scores (TSS) which are advocated by the WKO+ software package. Bicycling calculations are based on power measurements. Swimming calculations expand the theory based on functional threshold (FT) and using the workout inputs of distance and time. However I will be the first to admit that 5x200 with 2 sets of drills is much more strenuous than a 1x1200 set.
In researching running calculations, I came across an excellent review by Raphael Brandon who documented the research using both heart rate (HR) and rating of perceived exertion (RPE). The training impulse is a summation of the number of minutes at a given level x scaling factor. For example 10 minutes in zone 1 (factor 1) + 20 minutes in zone 2 (factor 1.2) would equal 10 + 24 or 34. Kristian Lassan automated this as a plug-in module for SportTracks. The following table sets the baseline assumptions for the factors and time constants of 11 and 45 for the acute training load (ATL) and chronic training load (CTL) respectively. The time constants in this case equate to the past 45 days of TRIMP scores will be used to calculate CTL and the past 11 days for ATL. I have seen 42 (7 weeks) and 7 (1 week) used by programs as well.
These factors can be modified to suit your needs and training philosophy.
My initial deep dive into the calculations came about as I was seeing “wrong” TRIMP scores for a couple of workouts. As I looked further, the errant data was coming from my Garmin 305 Heart Rate Monitor which was report HRs of 200+ which are obviously wrong. I decided to work around this by setting the factor for 180+ to zero and this fell in line with my perceived exertion.
Currently training loads are a clever way for me to keep score. Ideally they can be actively in order to peak and taper for events. At this point I am not concerned with resetting the zones and factors, but with true FT testing and sport specific factors, I believe the tool could have even more value.