Training

TRIMP (Training Impulse)

A single number for quantifying training stress

Plain English

Training Impulse (TRIMP) is a method for quantifying how much physiological stress a training session produces, expressed as a single score. It combines session duration with heart rate intensity, giving more weight to time spent at higher intensities. The goal is to turn "how hard was your workout?" into a number you can track, compare, and manage across days and weeks.

The Mechanism

TRIMP was developed by Eric Banister (Simon Fraser University) in 1991 as a way to measure cumulative training load for endurance athletes. The original formula multiplies session duration by the ratio of exercise heart rate to maximum heart rate, then applies a weighting factor that exponentially increases the score for work done at higher heart rate zones. This exponential weighting reflects the non-linear physiological cost of intensity: a session at 90% max HR is not merely twice as stressful as 45% max HR, it is several times more demanding.

Modern wearables like Garmin and WHOOP use TRIMP-derived algorithms to generate their own training load scores. Garmin calls this Training Load; WHOOP calls it Strain. Both apply a similar principle: duration multiplied by heart rate intensity with exponential zone weighting. The specific algorithms differ across platforms, so scores are not directly comparable between devices.

In the fitness-fatigue model (Banister, 1991; Calvert, Banister, Savage, 1976), TRIMP scores accumulate to generate two quantities: fitness (the chronic training load, a slow-moving positive adaptation) and fatigue (the acute training load, a fast-accumulating negative). The difference between fitness and fatigue at any point is performance readiness. This is the theoretical basis for acute-to-chronic workload ratio monitoring.

Why It Matters

TRIMP translates any workout into a common currency for managing weekly stress.

TRIMP gives you a single comparable number for sessions that differ in modality, duration, and intensity, making it possible to manage training load across a week rather than just counting sessions. A 90-minute Zone 2 run and a 45-minute HIIT session may produce the same TRIMP score despite looking nothing alike on paper. More importantly, tracking TRIMP over time reveals whether acute load is outpacing chronic capacity, the key precondition for overuse injury and overtraining. The Acute:Chronic Workload Ratio (ACWR) is built on TRIMP or equivalent load scoring.

Common Misconception

Many athletes assume higher TRIMP scores are always better, treating each session as a target to beat. TRIMP is a load management tool, not an achievement score. The goal is not to maximize it but to keep acute load (recent sessions) proportional to chronic load (established baseline). Systematically chasing high TRIMP without recovery erodes HRV, raises resting heart rate, and increases injury risk, exactly what load quantification is designed to prevent.

Signs It Is Disrupted

Acute TRIMP (7-day total) is more than 1.5x chronic TRIMP (28-day average), the high-risk ACWR zone
Session RPE increases for the same TRIMP-producing workload across consecutive weeks
HRV declines week over week alongside rising TRIMP accumulation
Persistent soreness or minor joint complaints emerge after a spike in weekly training load

How to Improve It

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Track TRIMP weekly, not daily. Daily TRIMP swings are noisy; a 7-day rolling total compared to the prior 4-week average reveals whether acute load is escalating beyond chronic capacity.

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Limit weekly TRIMP increases to 10%. The 10% weekly load increase rule limits acute-to-chronic ratio escalation, the primary mechanism linking load spikes to soft tissue injury risk.

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Include rest days in your TRIMP view. Days with TRIMP near zero are part of the load management system; suppressing or skipping recovery days inflates acute load without building chronic fitness.

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Use TRIMP across modalities. Assigning TRIMP scores to strength, cardio, and conditioning sessions in the same log allows total body stress management rather than managing each modality in isolation.

3 Things to Remember

TRIMP converts any training session into a single stress score by multiplying duration by intensity with exponential weighting for high heart rate zones.

TRIMP is a load management tool, not an achievement metric: the goal is to keep acute load (last 7 days) proportional to chronic load (last 28 days), not to maximize each session.

Tracking weekly TRIMP and limiting increases to 10% per week is the primary mechanism for preventing the acute load spikes that predict overuse injury.

Appears In

→The Strength Protocol →The Cardio & Zone 2 Protocol →How to Tell If Your Training Is Actually Working

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