Training Load Explained: TRIMP, TSS, and RPE for Athletes
Training load is a quantified measure of the physiological stress imposed by exercise — the product of training intensity and duration. Rather than tracking workouts as simply "did it" or "didn't do it," training load metrics express the cumulative stress your body has absorbed and the fitness it has built, enabling evidence-based decisions about when to push harder, when to back off, and when you are approaching your peak. This guide explains TRIMP, TSS, and RPE-based load, and how chronic and acute load measures predict both fitness and fatigue.
What Is Training Load?
Training load is the quantified stress of exercise. Stress in this context is not pejorative — it is the necessary stimulus for adaptation. Without sufficient training load, no adaptation occurs. With too much training load relative to recovery capacity, overreaching and overtraining follow.
The fundamental equation:
Training Load = Intensity × Duration
A 30-minute Zone 4 run and a 2-hour Zone 2 ride might impose similar training loads by this calculation, even though they feel and look entirely different. This is the insight that load metrics formalize: volume alone does not capture training stress, and intensity alone ignores the fact that a 10-minute hard interval is less stressful than a 60-minute threshold effort at the same intensity.
TRIMP: Training Impulse (Heart Rate–Based Load)
TRIMP (Training Impulse) was developed by exercise physiologist Eric Banister in 1975 and remains the most widely used HR-based training load metric. It calculates load from heart rate during a workout, accounting for the exponential relationship between exercise intensity and physiological stress.
The TRIMP Formula
TRIMP = Duration (min) × Average HR Fraction × Zone Weighting Factor
Where HR Fraction = (Average HR – Resting HR) / (Max HR – Resting HR)
Zone weighting factors reflect that exercise at higher heart rate fractions produces disproportionately greater physiological stress:
- Zone 1–2 work: weighting factor ~1.0
- Zone 3 work: weighting factor ~1.5–2.0
- Zone 4–5 work: weighting factor ~2.5–4.0
Example calculation: An athlete with resting HR of 50 bpm and max HR of 185 bpm completes a 60-minute Zone 3 run with average HR of 155 bpm.
- HR Fraction = (155 – 50) / (185 – 50) = 105 / 135 = 0.78
- Zone weighting ~1.9
- TRIMP ≈ 60 × 0.78 × 1.9 ≈ 89 TRIMP points
The exponential weighting is the key innovation: it captures the fact that 60 minutes at 90% max HR is not merely twice as stressful as 60 minutes at 45% max HR — it is many times more stressful, matching the non-linear physiology of high-intensity exercise.
Monotone vs. Exponential TRIMP
Banister's original formula (monotone TRIMP) applied a simple linear weighting, which underweights high-intensity work. Modern implementations — including Fitiv's — use exponential TRIMP (also called Lucia TRIMP after researcher Alejandro Lucia), which applies an exponential curve to the zone weighting and better captures the physiological cost of Zone 4–5 training.
TSS: Training Stress Score (Power-Based Load)
TSS (Training Stress Score) was developed by Dr. Andrew Coggan and Hunter Allen specifically for cyclists with power meters. It calculates training load from power output relative to the athlete's FTP (Functional Threshold Power), providing a load score comparable across different athletes and workout types.
The TSS Formula
TSS = (Duration in seconds × Normalized Power × Intensity Factor) / (FTP × 3600) × 100
Where:
- Normalized Power (NP): A weighted average of power that accounts for the metabolic cost of variable efforts (roughly equivalent to the constant power that would produce the same physiological stress)
- Intensity Factor (IF) = Normalized Power / FTP
- 3600 converts FTP from watts/hour to watts/second for dimensional consistency
TSS benchmarks:
- TSS < 150: Low — easy to moderate session, recovery within 24 hours
- TSS 150–300: Moderate — can be sustained daily with adequate nutrition
- TSS 300–450: High — significant fatigue, 24–36 hours recovery required
- TSS > 450: Very high — multi-day recovery; appropriate only for periodically planned big training days
Example calculation: A cyclist with FTP of 280W completes a 90-minute ride with Normalized Power of 245W.
- IF = 245 / 280 = 0.875
- TSS = (5400 × 245 × 0.875) / (280 × 3600) × 100
- TSS = (1,157,625) / (1,008,000) × 100 ≈ 115 TSS
RPE-Based Training Load
For training sessions without heart rate or power data — strength training, yoga, swimming without a waterproof monitor — Rate of Perceived Exertion (RPE) provides a practical load estimate.
Session RPE (sRPE) method: Training Load = Session RPE (1–10 scale) × Duration (minutes)
A 45-minute strength session at RPE 7 = 315 arbitrary units.
The sRPE method, developed by Carl Foster at the University of Wisconsin, has been validated in numerous sports and produces load estimates that correlate well with TRIMP and power-based metrics when athletes consistently calibrate their RPE against objective data. Fitiv allows you to log RPE after any session, including strength training and non-monitored workouts, to include them in your total training load calculation.
RPE scale (Borg CR10):
- 1–2: Very easy (active recovery)
- 3–4: Easy to moderate (Zone 2)
- 5–6: Moderate to hard (Zone 3)
- 7–8: Hard (Zone 4, threshold)
- 9–10: Very hard to maximal (Zone 5, VO2 max)
Chronic Training Load (CTL): Your Fitness Number
CTL (Chronic Training Load), often called "fitness" in training platforms, is a rolling average of daily training load over the past 42 days (6 weeks). It represents your body's current capacity to absorb and perform training.
Mathematically: CTL is an exponentially weighted moving average with a 42-day time constant, giving greater weight to recent training than distant training.
What CTL tells you:
- Rising CTL = fitness is building
- Stable CTL = fitness is being maintained
- Declining CTL = detraining (or deliberate taper before a goal event)
CTL benchmarks for cyclists (TSS-based):
- 20–40: Recreational, moderate fitness
- 40–70: Trained enthusiast, strong club racer
- 70–100: Serious competitor, cat 3–4 racer
- 100–120: Category 1–2, elite amateur
- 120+: Professional level
For runners and multi-sport athletes, TRIMP-based CTL uses different scales but follows the same principles.
Acute Training Load (ATL): Your Fatigue Number
ATL (Acute Training Load), often called "fatigue," is a rolling average of daily training load over the past 7 days. It responds much faster to changes in training than CTL.
What ATL tells you:
- Rising ATL = you are accumulating fatigue from recent hard training
- ATL > CTL by a large margin = you are significantly more fatigued than your baseline fitness
- High ATL after a training block precedes adaptation — if recovery is adequate
ATL is not inherently bad. You cannot build fitness without periodically elevating ATL above CTL. The problem occurs when ATL remains elevated for too long without recovery phases, preventing the supercompensation that produces actual fitness gains.
Training Stress Balance (TSB): Your Form Number
TSB (Training Stress Balance) = CTL – ATL
TSB is the most important number for race planning and daily training decisions.
Interpreting TSB:
- TSB positive (fresh): You have rested more than you have trained recently. Good for races, not ideal for training adaptation.
- TSB 0 to -10 (optimal training state): Moderate fatigue on a rising or stable fitness base. This is where most effective training occurs.
- TSB -10 to -25 (fatigued): Heavy training block. Performance is temporarily suppressed but fitness is building if intensity is maintained.
- TSB below -30 (overreaching risk): Significant cumulative fatigue. Increased injury risk, suppressed HRV, declining performance. Requires recovery intervention.
For race peaking: Athletes aim to arrive at goal races with TSB between +5 and +20 — positive enough to be fresh, not so positive that fitness has significantly detrained. A 7–14 day taper typically moves TSB from -15 to -20 up to this range.
Using Training Load to Prevent Overtraining
The most valuable application of training load metrics is early detection of excessive stress accumulation — weeks before subjective feelings of overtraining emerge.
Warning signs in the metrics:
- ATL significantly elevated above CTL for more than 10–14 consecutive days
- TSB declining below -30 without a planned rest day
- CTL declining despite continued training (adaptation failure)
- HRV trend suppressed in parallel with elevated ATL
Practical load management principles:
- The 10% rule: Increase weekly training load by no more than 10% per week on average. Larger jumps overwhelm recovery capacity.
- Every 3rd or 4th week: Build load for 2–3 weeks, then reduce volume by 30–40% for one recovery week.
- Big weeks must be followed by recovery: A training camp or overload week (ATL spikes significantly) must be followed by reduced training before the fitness gain can be expressed.
How Fitiv Calculates and Displays Training Load
Fitiv Pulse automatically calculates training load for every recorded workout, choosing the most appropriate metric based on available data:
- GPS run or ride with heart rate: Exponential TRIMP calculation
- Cycling with power meter: TSS calculation
- Strength, HIIT, or unmonitored sessions: RPE-based load (user-entered or estimated from session type and duration)
- All modalities: Combined into a single unified load score for cross-sport athletes
Fitiv's training load dashboard shows:
- Daily load: Each workout's contribution to total daily stress
- 7-day ATL (fatigue): Your current acute load relative to your baseline
- 42-day CTL (fitness): Your chronic load trend — the fitness number
- TSB (form): The CTL-ATL difference, color-coded green (fresh), yellow (moderate fatigue), red (high fatigue)
- Load progression chart: 90-day view of how fitness, fatigue, and form have evolved
Fitiv's readiness score integrates TSB with HRV and sleep score to generate a single daily recommendation. An athlete with a TSB of -25 and suppressed HRV receives a different readiness signal than an athlete with a TSB of -25 but elevated HRV — because the former has physiological fatigue while the latter may simply have high fitness-building stress without systemic recovery debt.
Frequently Asked Questions
Q: What is a good weekly TRIMP target? A: For recreational athletes training 5–8 hours per week, a weekly TRIMP of 300–500 is typical. Competitive amateur athletes training 8–12 hours might accumulate 500–800 TRIMP per week. The most important factor is not the absolute number but the week-to-week change — keep weekly increases below 10–15% on average.
Q: How is TRIMP different from TSS? A: TRIMP uses heart rate data and is applicable to any HR-monitored sport. TSS uses power data and is specific to cycling (or running with a running power meter). Both estimate the physiological stress of a session, but TSS is generally more precise for cycling because power responds instantly to effort while HR lags. For multi-sport athletes, TRIMP is more universally applicable; Fitiv uses whichever metric is most appropriate for each session.
Q: Can I use training load if I only do strength training? A: Yes, using the RPE-based session load method (RPE × duration). Strength training does not lend itself to HR or power-based load calculation, but RPE load has been validated in resistance training contexts and provides a useful estimate. Include all sessions — including strength — in your load calculation to get a complete picture of systemic fatigue.
Q: How much should I reduce load during a recovery week? A: Reduce total training volume by 30–40% while maintaining some intensity. A recovery week should include 1–2 quality sessions (short threshold or Zone 4 intervals) within a reduced total volume. Eliminating intensity entirely during recovery weeks can leave athletes feeling flat and slow to rebound. The goal is to drop ATL and ATL-to-CTL ratio, not to detrain.
Q: How long before a race should I start tapering? A: For events lasting 1–4 hours (marathon, century ride, Olympic triathlon): taper for 10–14 days, reducing volume by 40–50% while maintaining some intensity. For longer events (Ironman, 100-mile ultramarathon): taper begins 2–3 weeks out. The goal is to bring TSB from a negative value to approximately +5 to +20 on race day.
Q: Does training load work for team sport athletes? A: Yes. Session RPE-based training load is widely used in football, basketball, rugby, and other team sports where HR monitoring is inconsistent. Some teams use GPS-derived metrics (distance, high-speed running) in addition to RPE load. The CTL/ATL/TSB model applies to any sport where session load can be quantified consistently.