Training load puts a number on how much stress a workout puts on your body, based on how hard you went and for how long. Tracked over weeks, it shows the fitness you've built and the fatigue you're carrying, so you can decide when to push, when to back off, and when you're close to your peak. There are three common ways to score a session (TRIMP, TSS, and RPE-based load), and two running averages, chronic and acute load, that turn those scores into fitness and fatigue.
What Is Training Load?
Training load is the measured stress of exercise. Stress here just means the stimulus your body adapts to. Too little load and nothing changes. Too much for your recovery to handle and you slide into overreaching and overtraining.
The basic equation:
Training Load = Intensity × Duration
By this math, a 30-minute Zone 4 run and a 2-hour Zone 2 ride might produce similar loads, even though they feel completely different. You need both numbers. Volume alone misses how hard you went, and intensity alone misses 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. Time at higher heart rates counts for more, because harder exercise produces disproportionately more physiological stress.
That weighting is what sets TRIMP apart. Sixty minutes at 90% of max HR is many times more stressful than 60 minutes at 45%, far more than double, and the weighting reflects that non-linear physiology.
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, and its default: you'll see it as TRIMP in the method picker at the top of the Training Summary detail.
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 how much training your body can currently absorb and perform.
Technically, CTL is an exponentially weighted moving average with a 42-day time constant, so recent training counts more than training from weeks ago. A rising CTL means fitness is building, a flat one means you're maintaining it, and a falling one means you're detraining or deliberately tapering 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.
Rising ATL means you're accumulating fatigue from recent hard training, and ATL well above CTL means you're far more fatigued than your baseline fitness. That isn't a problem in itself. High ATL after a training block comes before adaptation, as long as you recover, and you can't build fitness without periodically pushing ATL above CTL. Trouble starts when ATL stays high too long without recovery phases, which blocks the supercompensation that produces real 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. FITIV doesn't show TSB; it compares the same two loads as a ratio instead, its Training Load Ratio.
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
Load metrics are most useful as an early warning: they can show stress piling up weeks before you start to feel overtrained.
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
In FITIV, the Training Summary detail does this check for you with your Training Load Ratio (TLR): short-term load divided by long-term load. Today's Training Status labels it Undertraining, Optimal, Peaking, Overreaching, or At Risk, and the Daily Training Status chart marks the Optimal Training Range and Peak Training Range, so a stretch of Overreaching or At Risk days stands out.
Managing load comes down to a few habits. 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. Build for 2–3 weeks, then cut volume by 30–40% for one recovery week, so every 3rd or 4th week is easier. And follow any big week with recovery. Say you spend a long weekend at a cycling camp and your ATL spikes. The fitness from that camp only shows up after you've trained lighter for a while.
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, and experienced athletes tolerate higher loads but still need gradual progression to keep improving.
So read your training load against your own history. Your long-term load is the baseline, and whether today's number is high or low depends on what your body is used to.
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 made up mostly of low aerobic work is usually sustainable; it's how endurance athletes build volume. The same load heavy on high-intensity work is much harder to recover from. And rising load with your focus drifting toward intensity is a common path to fatigue, so shift some sessions back to easy aerobic work. To check whether your body is keeping up with both, look at your Recovery score.
How FITIV Calculates and Displays Training Load
FITIV Pulse calculates training load for every recorded workout. Tap the Training Summary card on the Today tab and choose which method drives it in the picker at the top:
- 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: 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. For help choosing, see the help center's guide to training load methods.
The Training Summary shows:
- Short Term (ATL): an exponentially weighted 7-day average of your recent training
- Long Term (CTL): an exponentially weighted average over the past 42 days (28 days with Effort)
- 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
- Daily Training Status: a chart of your short-term load and each day's status over time, against your optimal and peak training ranges
FITIV's Training Readiness score combines your Recovery score (overnight HRV, resting heart rate, and sleep) with your recent Strain, your Battery, and your Training Load Ratio. 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 first is showing physiological fatigue and the second 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.
