TRIMP (Training Impulse)
TRIMP (Training Impulse) is a heart rate–based training load metric that quantifies the physiological stress of a workout by combining exercise duration with heart rate intensity, weighted by an exponential factor that reflects the non-linear physiological cost of higher-intensity exercise. Originally developed by exercise physiologist Eric Banister in 1975, TRIMP is the most broadly applicable training load metric across sports — applicable to any activity where heart rate is monitored.
Deeper Explanation
The fundamental insight behind TRIMP is that training load is a function of both how long you exercise and how hard, and that the relationship between intensity and physiological stress is exponential rather than linear. An hour at 90% of maximum heart rate is not merely twice as stressful as an hour at 45% max HR — it is many times more stressful, because high-intensity exercise activates anaerobic pathways, causes greater muscle damage, and produces far greater autonomic nervous system stress.
The TRIMP calculation:
TRIMP = Duration (minutes) × HR Fraction × Zone Weighting Factor
Where:
- HR Fraction = (Average HR − Resting HR) / (Maximum HR − Resting HR). This normalizes heart rate between 0 (resting) and 1 (maximum).
- Zone Weighting Factor applies an exponential multiplier based on intensity, reflecting the disproportionate physiological cost of higher heart rates.
Example: An athlete with resting HR of 55 bpm and max HR of 188 bpm completes a 45-minute Zone 4 threshold run averaging 162 bpm:
- HR Fraction = (162 − 55) / (188 − 55) = 107 / 133 = 0.80
- Zone weighting at 0.80 HR fraction ≈ 2.3 (exponential TRIMP)
- TRIMP ≈ 45 × 0.80 × 2.3 ≈ 83 TRIMP points
The same athlete completing a 45-minute easy Zone 2 run averaging 130 bpm:
- HR Fraction = (130 − 55) / 133 = 0.56
- Zone weighting at 0.56 ≈ 1.1
- TRIMP ≈ 45 × 0.56 × 1.1 ≈ 28 TRIMP points
Three times the session load for the threshold run versus the easy run of equal duration — which matches the physiological reality.
Monotone vs. Exponential TRIMP: Banister's original 1975 formulation used a simpler monotone weighting. Modern implementations use exponential TRIMP (also called Lucia TRIMP), which applies an exponential curve to the zone weighting and more accurately captures the cost of high-intensity work. 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
Weekly TRIMP for recreational athletes training 5–7 hours: 300–550. Competitive amateurs training 8–12 hours: 500–800.
How TRIMP Relates to Training
TRIMP is the foundation of the HR-based Fitness/Fatigue model — the equivalent of TSS for non-power-based training. It feeds into:
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 particularly valuable for multi-sport athletes and those who train in sports where power measurement is impractical or cost-prohibitive. A runner, swimmer, rower, or cross-trainer can use TRIMP across all modalities to get a unified view of systemic training load.
How Fitiv Uses TRIMP
Fitiv Pulse automatically calculates exponential TRIMP for every workout where heart rate data is available. This includes running, cycling without a power meter, rowing, elliptical, swimming (with a waterproof HR monitor), and any other HR-monitored activity.
When a cyclist uses a power meter, Fitiv calculates TSS instead of TRIMP for cycling sessions, as power provides a more precise intensity measure than heart rate. For non-cycling activities, TRIMP is used. Both metrics contribute to a unified training load dashboard.
Fitiv displays TRIMP in the individual workout summary and aggregates it into the training load trends showing CTL (fitness), ATL (fatigue), and TSB (form) over time. The training load dashboard allows athletes to see how TRIMP from different sports contributes to their overall physiological load — particularly useful for triathletes or athletes who cross-train across multiple disciplines.
Frequently Asked Questions
Q: What is the difference between TRIMP and TSS? A: 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 calculates whichever is most appropriate for each session.
Q: Can TRIMP be used for strength training? A: 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. For strength sessions, Fitiv uses RPE-based load (Rate of Perceived Exertion × Duration) instead of TRIMP, which has been validated in resistance training contexts.
Q: Does TRIMP account for training in heat? A: 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.