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.
How TRIMP Works
TRIMP combines how long you exercised with how hard your heart was working, relative to your resting and maximum heart rate. Greater weight is given to time spent at higher intensities, reflecting that exercise at higher heart rates produces disproportionately greater physiological stress.
The 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.
Linear vs. Exponential TRIMP
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, with separate constants for men and women, and better captures the physiological cost of Zone 4–5 training. It's the version FITIV uses.
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.
How TSS Works
TSS combines workout duration with how hard you rode relative to your FTP, using Normalized Power (a weighted average of power that accounts for the cost of variable efforts). One hour at exactly FTP scores 100, and every other workout is scaled against that reference.
TSS benchmarks:
- TSS < 150: Low — easy to moderate session, recovery within 24 hours
- TSS 150–300: Moderate — some fatigue may linger into the next day
- TSS 300–450: High — fatigue may last two days or more
- TSS > 450: Very high — multi-day recovery; appropriate only for periodically planned big training days
Example: A 90-minute ride at a solid tempo effort, a little below FTP, typically scores a little over 100 TSS: more than a one-hour ride at FTP because it lasted longer, even though it was easier.
RPE-Based Training Load
For training sessions without heart rate or power data — strength training, yoga, swimming without heart rate data — 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 lets you log your Perceived Effort after any session, including strength training and workouts without heart rate. When you choose Effort as your training load method, those ratings drive your load, so every session counts.
RPE scale (Borg CR10), with rough zone equivalents:
- 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)
Rough CTL ballparks for cyclists (TSS-based), as often quoted by coaches. Individual riders vary widely:
- 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 most useful for race planning: it shows whether you're carrying fatigue or arriving fresh.
Interpreting TSB: TrainingPeaks, which popularized this model with its Performance Management Chart, currently groups Form into these ranges:
- +30 or higher (recharging): Rested for a long time. Fitness is likely starting to slip, as in an off-season.
- +5 to +29 (fresh): Good for races. Not where most training adaptation happens.
- −10 to +4 (maintaining): Not much fatigue, not much new stimulus.
- −11 to −29 (optimal training): The typical range for a solid training block. You'll feel some fatigue while fitness builds.
- −30 or lower (overloading): Heavy cumulative fatigue. Increased injury and illness risk; plan recovery soon.
For race peaking: Athletes aim to arrive at goal races with TSB of roughly +5 to +29 — positive enough to be fresh, not so positive that fitness has started to slip. A taper of one to two weeks typically moves TSB from the negative training range up into 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:
- Build gradually: A common rule of thumb is to increase weekly load by no more than about 10%. Research hasn't confirmed one exact safe number, but sudden large jumps are consistently linked to higher injury risk.
- 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.
Why Training Load Feels Different for Different Athletes
The same load number doesn't mean the same thing for everyone:
- Endurance athletes can usually carry a higher total load, because much of it comes from long, low-intensity aerobic work.
- Strength-focused athletes build load in shorter, harder sessions, so their numbers tend to be lower even when they're working hard.
- Beginners feel a lot of stress from modest volume while their bodies adapt.
- Experienced athletes tolerate higher loads but still need gradual progression to keep improving.
That's why training load is always read against your own history. Your long-term load is the baseline; whether today's number is high or low depends on what your body is used to, not on anyone else's.
Training Load Measures How Much, Not What Kind
Sometimes your load stays steady but workouts feel harder, or fatigue builds even though total training time hasn't changed. That happens because training load captures the amount of stress, not its type. An hour of easy aerobic work and a shorter session of hard intervals can produce similar load but take very different tolls on recovery.
Your training focus fills that gap: how your recent training time splits between low aerobic work (Zones 2–3), high aerobic work (Zone 4), and anaerobic efforts (Zone 5). To see your mix, tap the Heart Rate Zones card on the Today tab and choose 30d or 90d; the totals show your time in each zone. As a general guide, most of your training time should be easy: endurance athletes typically do around 80% of their training at low intensity (see the 80/20 model). See heart rate zones for how the zones are set.
Read the two together:
- High load, mostly low aerobic: usually sustainable. This is how endurance athletes build volume.
- High load, heavy on high-intensity work: much harder to recover from, even at the same number.
- Rising load with focus drifting toward intensity: a common path to fatigue. Shift some sessions back to easy aerobic work.
Load tells you how much stress you're carrying, your time in each zone tells you what kind, and your Recovery score tells you whether your body is keeping up.
How FITIV Calculates and Displays Training Load
FITIV Pulse calculates training load for every recorded workout, and you choose which method drives your training load view:
- Effort: your 1–10 effort rating × duration, giving Effort Minutes. Works for any workout, including strength and HIIT. If you don't rate a session, FITIV estimates the effort from your heart rate
- TRIMP: heart rate relative to your maximum, with more weight on higher intensities. Ideal for cardio
- TSS: from power when you have it (cycling or running), or from heart rate when you don't
- All Day Calorie Burn: your daily active energy, including activity outside workouts
All workouts count in the same currency, so runs, rides, and gym sessions add up to one picture for cross-sport athletes.
FITIV's training load view shows:
- Short Term Load (ATL): an exponentially weighted 7-day average of your recent training
- Long Term Load (CTL): an exponentially weighted average over the past several weeks
- Training Status: whether your recent load is Undertraining, Optimal, Peaking, Overreaching, or At Risk relative to your long-term load, or Baseline Needed while FITIV gathers enough data
- Load history: how your short- and long-term load have evolved over time
FITIV's Training Readiness score combines your Recovery score (overnight HRV, resting heart rate, and sleep) with your recent Strain and the ratio of your last week's training load to your last six weeks. An athlete with a big spike in recent load and suppressed HRV receives a lower readiness signal than an athlete with the same load spike but HRV in their normal range — because the former is showing physiological fatigue while the latter may simply be absorbing fitness-building stress.
Frequently Asked Questions
What is a good weekly TRIMP target?
There's no universal target. Weekly TRIMP depends on how much and how hard you train, and different apps scale it differently, so compare against your own history rather than someone else's number. The most important factor is the week-to-week change: build gradually and avoid sudden spikes.
How is TRIMP different from TSS?
TRIMP uses heart rate data and is applicable to any HR-monitored sport. Standard TSS uses power data, so it's mainly for cycling (or running with a running power meter); FITIV also calculates a heart rate-based TSS when you don't have power. 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. In FITIV, you choose which method drives your training load view.
Can I use training load if I only do strength training?
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.
How much should I reduce load during a recovery week?
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.
How long before a race should I start tapering?
Research suggests a taper of about 8–14 days works well for most endurance events, cutting volume by roughly 40–60% while keeping some intensity and your usual training frequency. Marathoners and long-course triathletes often taper for 2–3 weeks. The goal is to bring TSB from a negative value to roughly +5 to +29 on race day.
Does training load work for team sport athletes?
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.
