Training Impulse (TRIMP)

Training Impulse (TRIMP) is a method proposed by Dr. Eric W. Banister in 1975 (refined in 1991) to quantify internal training load.

Where power and pace describe external work, TRIMP uses heart rate and duration to estimate the body’s internal response. It addresses a practical question: how do we combine heart rate and time into a single load score when power is unavailable or not appropriate?

Core Concept

The basic logic is: Load = Duration × Intensity.

In TRIMP, “intensity” is not just average heart rate; it reflects the non-linear response associated with higher heart rates.

• Going from 130 to 140 bpm adds relatively little stress.
• Going from 180 to 190 bpm (near maximal) usually produces a much larger internal response.

Banister introduced an exponential weighting so high-heart-rate work contributes more than low-heart-rate work of the same duration.

Formula (Banister TRIMP)

Trainingload.ai can use the classic Banister TRIMP (exponential model) when the required heart-rate inputs are available:

TRIMP = t_min * HR_ratio * 0.64 * exp(k * HR_ratio)

Where:

• $t_{min}$ = duration (minutes)

• $HR_{ratio}$ = heart rate reserve ratio (HRR ratio)

  HR_ratio = (HR_avg - HR_rest) / (HR_max - HR_rest)

• $e$ = Euler’s number (2.718…)

• $k$ = sex coefficient (male 1.92, female 1.67) reflecting different lactate-threshold responses at high HR.

Example

Assume a male athlete (HRmax 200, HRrest 50):

1. Easy run 1h @ 140 bpm (HR_ratio = 0.6):

   TRIMP ≈ 60 * 0.6 * 0.64 * exp(1.92 * 0.6) ≈ 72

2. Hard effort 1h @ 185 bpm (HR_ratio = 0.9):

   TRIMP ≈ 60 * 0.9 * 0.64 * exp(1.92 * 0.9) ≈ 194

Even at the same duration, TRIMP at high intensity can be ~3× the low-intensity value, showing the non-linear effect of intensity.

Variants and Evolution

Beyond Banister’s model, other variants exist. Trainingload.ai supports these depending on data availability:

Typical Use Cases

1. A data source for PMC

For sports where power is hard to obtain (swimming, trail running, gym work), TRIMP can be a practical alternative input for calculating ATL (Fatigue) and CTL (Fitness).

• TSS is typically used for cycling/road running.
• TRIMP can be used for any aerobic activity.

How Trainingload.ai uses TRIMP

Trainingload.ai treats TRIMP as an internal-load input. It is especially useful when power, reliable pace, or structured workout targets are missing. In plan review, TRIMP can help explain why a session felt costly even if external output was modest.

TRIMP is most useful when it is interpreted with context:

• compare it with RPE and notes from the athlete;
• watch heat, dehydration, and cardiac drift;
• avoid using it alone for short sprint or anaerobic interval sessions.

2. Review HR and RPE mismatch

If your TRIMP/RPE ratio looks abnormal:

• Normal: high TRIMP corresponds to high RPE.
• Abnormal: TRIMP is high but RPE is low (HR drift from dehydration or illness), or TRIMP is low but RPE is high (HR won’t rise—often seen in parasympathetic overreaching).

Limitations

1. Cardiac lag: in 30-second all-out sprints, power peaks immediately but HR may rise slowly and only catch up later. TRIMP can underestimate short anaerobic interval load.
2. Environmental noise: heat, dehydration, caffeine, and lack of sleep can elevate HR (cardiac drift), inflating TRIMP—your heart beats faster, but it doesn’t mean your muscles did more work.
3. Non-specificity: 100 TRIMP from swimming vs. running places very different musculoskeletal stress; TRIMP can’t distinguish mechanical load.

References

• Fellrnr: TRIMP Explained
• Banister, E.W. (1991). Modeling Elite Athletic Performance. In: MacDougall, J.D., Wenger, H.A., Green, H.J. (eds) Physiological Testing of the High-Performance Athlete.