Training Impulse, or TRIMP, is a sports science metric designed to quantify training load. Instead of treating every minute as equal, TRIMP weights harder heart rate work more heavily. A short interval session can have the same or higher TRIMP than a longer easy run, because the physiological stress is higher even when the clock time is not.
Where TRIMP comes from
TRIMP is rooted in classic performance modeling. The fitness and fatigue model described in the training science literature treats training as a quantified impulse that drives two simultaneous responses: a positive fitness gain and a temporary fatigue cost. Tracking a consistent training impulse gives coaches and athletes a way to see how the body is adapting over time, not just what was done on a single day.
How TRIMP is calculated
The Banister TRIMP method uses heart rate reserve and an exponential weighting so that high intensity work counts disproportionately more. The goal is to capture the reality that a minute at the edge of your capability is far more taxing than a minute at an easy effort.
TRIMP_exp = sum(D * HR_r * 0.64 * exp(k * HR_r))
k = 1.92 (men) or 1.67 (women)
In this formulation, D is the time spent at a given heart rate, and HR_r is your heart rate as a fraction of your reserve. The exponential term applies a steeper curve to higher intensities, and the constants come from observed relationships between heart rate and lactate response.
Why this beats simple averages
Many platforms also offer simpler, zone based TRIMP estimates that multiply time in each heart rate zone by a linear weighting. While easier to compute, these zone methods can create jumpy results near zone boundaries. The exponential model smooths that effect and more closely mirrors how training stress rises at higher intensities.
From daily TRIMP to training trends
A single TRIMP score is useful, but trends are where the metric shines. By storing daily TRIMP, you can compute short term (acute) and longer term (chronic) training load. Many systems define acute load as the average of the last 7 days and chronic load as a longer window such as 42 days, which helps compare your recent fatigue to your longer term training base.
How TrainrAI calculates TRIMP
TrainrAI uses the Banister TRIMP method with your heart rate series, resting heart rate, and max heart rate. When max heart rate is not available, it is estimated from age and sex so the score remains consistent even with limited data. This keeps your TRIMP history comparable across weeks, not just within a single workout.
How TrainrAI uses TRIMP
TrainrAI stores daily TRIMP to track acute load (last 7 days) and chronic load (about 28 days). These trends inform readiness, training capacity, and recommended exertion ranges when enough history is available. In practice, TRIMP acts as the backbone for training progression and helps the coach decide when to push, when to hold, and when to recover.
Limitations and real world considerations
TRIMP is powerful, but it is not perfect. Strength sessions can produce high muscular stress with modest heart rate response, and heat, altitude, dehydration, or caffeine can elevate heart rate without a matching increase in training load. That is why TrainrAI blends TRIMP with recovery signals and training context rather than relying on one number alone.
Citations
- Fellrnr.com, "Quantifying training, TRIMP and TSS (Training Stress Score)." https://fellrnr.com/wiki/TRIMP
- Morton RH, Fitz-Clarke JR, Banister EW. "Modeling human performance in running." Journal of Applied Physiology (1985), 1990. https://pubmed.ncbi.nlm.nih.gov/2246166/
- PeakWatch Docs, "Training Impulse (TRIMP)." https://doc.peakwatch.co/en/trimp.html