TRIMP, short for Training Impulse, is a heart rate–based score for how much stress a workout put on your body. It combines how long you exercised with how high your heart rate was, and an exponential weighting makes hard minutes count for much more than easy ones. Exercise physiologist Eric Banister developed it in 1975. Because it only needs heart rate, it works for any sport where you wear a heart rate monitor, which makes it the most broadly applicable training load metric across sports.
How TRIMP Is Calculated
TRIMP rests on the idea that training load depends on both how long and how hard you exercise, and that stress rises exponentially with intensity rather than in a straight line. An hour at 90% of maximum heart rate costs your body many times more than an hour at 45% max HR, because high-intensity exercise activates anaerobic pathways, causes greater muscle damage, and puts far more stress on your autonomic nervous system.
Intensity is measured by how hard your heart was working relative to your resting and maximum heart rate, and time at higher intensities counts disproportionately more. Say you run 45 minutes on Tuesday as a threshold run in Zone 4, then 45 minutes on Thursday at an easy Zone 2 pace. The threshold run earns more than twice the TRIMP of the easy one, even though both took the same time, because hard efforts accumulate stress much faster.
Not every version of TRIMP weights intensity this way. Simpler versions weight heart rate linearly, or assign a fixed multiplier to each zone (Edwards' zone-based TRIMP), which underweights high-intensity work. Banister's exponentially weighted TRIMP applies an exponential curve to heart rate intensity and better captures that cost. FITIV uses exponential TRIMP.
Practical TRIMP ranges:
- Easy 45-minute Zone 2 run: 25–40 TRIMP
- Moderate 60-minute Zone 2–3 run: 50–80 TRIMP
- Hard 60-minute threshold workout: 80–120 TRIMP
- Long 2-hour endurance ride: 100–160 TRIMP
- Very hard interval session: 80–130 TRIMP
Over a week, recreational athletes training 5–7 hours total 300–550 TRIMP, and competitive amateurs training 8–12 hours total 500–800.
How TRIMP Relates to Training
TRIMP is the heart rate counterpart to TSS and the foundation of the HR-based Fitness/Fatigue model. It feeds three numbers:
CTL (Chronic Training Load): A 42-day rolling weighted average of daily TRIMP. Reflects fitness level: a rising CTL means aerobic fitness is building.
ATL (Acute Training Load): A 7-day rolling weighted average of daily TRIMP. Reflects current fatigue. ATL rises quickly with hard training and falls quickly with rest.
TSB (Training Stress Balance) = CTL − ATL. The difference between long-term fitness and short-term fatigue, the "form" number that determines readiness to race or train hard.
TRIMP is especially useful for multi-sport athletes and for sports where power measurement is impractical or too expensive. A runner, swimmer, rower, or cross-trainer can use it across every sport for one unified view of training load.
How FITIV Uses TRIMP
FITIV Pulse automatically calculates exponential TRIMP for every workout with heart rate data, including running, cycling without a power meter, rowing, elliptical, swimming (with Apple Watch), and any other HR-monitored activity. It also calculates TSS: from power when you ride with a power meter and have an FTP set, or from heart rate (HR TSS) when you don't. You choose which method drives your training load view; the help center compares each training load method.
FITIV displays TRIMP in the individual workout summary and aggregates it into your short-term (7-day) and long-term (42-day) training load, with a Training Status that tells you whether your recent load is Undertraining, Optimal, Peaking, Overreaching, or At Risk. Because TRIMP comes from heart rate, runs, rides, rows, and gym sessions all count in the same currency.
Frequently Asked Questions
What is the difference between TRIMP and TSS?
Both measure training load (the physiological stress imposed by a workout) but use different input data. TRIMP uses heart rate data and can be applied to any HR-monitored sport. TSS uses power data and is specific to cycling (or running with a running power meter). TSS is generally more precise for cycling because power responds instantly to effort changes, while heart rate lags 30–60 seconds. For non-cycling sports, TRIMP is the standard choice. FITIV offers both, and its TSS works from power or heart rate data, so you can choose the method that fits your training.
Can TRIMP be used for strength training?
Heart rate during strength training does not accurately reflect the full physiological cost of the session: it underestimates the mechanical stress on muscles and connective tissue while also being complicated by the valsalva maneuver and breath-holding. That's why FITIV also offers Effort Minutes (your Perceived Effort rating × duration), an RPE-based load method validated in resistance training contexts, alongside heart rate-based TRIMP.
Does TRIMP account for training in heat?
TRIMP partially accounts for heat stress because heart rate is elevated in hot conditions (cardiovascular drift), which raises the HR fraction and therefore the TRIMP score. A 60-minute Zone 2 run in 35°C heat will produce a higher TRIMP than the same run in 15°C, which correctly reflects the greater physiological demand. This is one of TRIMP's strengths: it captures real-time physiological cost rather than external workload.
