FITIV Pulse turns each workout into a training load number, so the feeling that "that was a hard workout" becomes something you can track, add up, and compare across days, weeks, and months. Workouts aren't equal: a 30-minute easy run and a 30-minute threshold run share the same duration, but the threshold run costs your body far more, takes longer to recover from, and gives a different training stimulus.
FITIV offers several methods to suit different sports and data. You pick one at the top of the Training Load screen (tap the Training Summary card on the Today tab): TRIMP, Effort, TSS, or All Day, which counts your whole day's active calorie burn rather than just your workouts. The help center's guide to the different training load methods covers the pros and cons of each.
What is Training Load?
Training load is a quantitative measure of the physiological stress imposed by a training session or a period of training. It combines the intensity, duration, and frequency of exercise into a single number that can be compared across time.
Tracking it does three jobs. It helps keep any single week from getting excessive compared with your habitual load. It shows the fitness base you're building, which lets you keep increasing training over time. And it shows how your current fitness compares with your current fatigue.
A load number only makes sense next to your own training history: a week that would devastate an untrained athlete is routine for an elite marathoner.
TRIMP: Training Impulse
TRIMP (Training Impulse) is a heart rate-based training load metric developed by Eric Banister in the 1970s and subsequently refined. It accounts for both the intensity and duration of a session by weighting time spent at higher heart rates more heavily than time at lower intensities.
The original Banister TRIMP formula multiplies training duration by an intensity factor derived from how far your exercise heart rate sits between your resting and maximum heart rate, weighted exponentially so harder effort counts disproportionately. So an hour at Zone 4 threshold pace scores far more than an hour in Zone 2, often more than double, reflecting the disproportionate stress of higher-intensity work.
FITIV calculates exponential TRIMP from your heart rate throughout the workout, making it available for any activity where heart rate is recorded: running, cycling, rowing, swimming with Apple Watch, cross-country skiing, and more. Because it needs only heart rate data, TRIMP works with every heart rate source FITIV supports.
The limitation of TRIMP is that heart rate is an imperfect proxy for internal load. In hot conditions, heart rate is artificially elevated relative to effort. In the early stages of a long ride, heart rate may not reflect the metabolic demand of the effort. For sports where better load metrics are available (particularly cycling), more precise alternatives exist.
TSS: Training Stress Score
TSS (Training Stress Score) is a cycling-specific training load metric developed by Dr. Andrew Coggan that uses power output rather than heart rate to quantify training stress. It is the standard load metric in competitive cycling and is superior to heart rate-based metrics for power-metered cycling because power is an objective, immediate measure of external workload.
TSS is based on Normalized Power (an intensity-weighted average of power that accounts for the cost of variable-intensity riding), your Functional Threshold Power (FTP, the maximum power sustainable for approximately one hour), and workout duration.
A one-hour ride at exactly FTP produces a TSS of 100 by definition. A two-hour moderate endurance ride at 65% of FTP produces approximately TSS 85. A 60-minute threshold session at 92% of FTP also produces approximately TSS 85: similar total load, very different physiological character.
FITIV calculates TSS from power for cycling and running workouts with power data. Without power data, FITIV calculates a heart rate-based TSS instead.
RPE-Based Load
RPE (Rate of Perceived Exertion) load multiplies your own effort rating after a workout by the workout's duration. The Session RPE method (CR10 scale × duration in minutes) has been validated against more sophisticated methods in multiple sports and gives reliable load estimates where objective data is unavailable, particularly team sports, strength training, and activities not well-suited to heart rate or power monitoring.
After a workout, FITIV asks for your Perceived Effort on a 1–10 scale and multiplies it by the workout's duration to give Effort Minutes. If you don't rate a session, FITIV estimates the effort from your heart rate. That gives you a load estimate for any workout type, and it records how hard the session felt to you, which varies between athletes, alongside the heart rate data.
Chronic vs. Acute Training Load
The most important application of training load data is comparing your recent load (acute) to your habitual load (chronic):
Acute Training Load (ATL): A rolling weighted average of daily training load over approximately 7 days. Reflects recent fatigue.
Chronic Training Load (CTL): A rolling weighted average of daily training load over approximately 42 days (FITIV uses 28 days with the Effort method). Reflects accumulated fitness.
Acute:Chronic Workload Ratio (ACWR): ATL ÷ CTL. This ratio quantifies how much harder you are training recently compared to your habitual training level.
Research in sports medicine, starting with cricket and rugby, has linked an ACWR above about 1.5 with elevated injury risk, though the strength of that link is still debated. Below 0.8 indicates undertraining relative to habitual load. The "sweet spot" for productive training with manageable injury risk is approximately 0.8-1.3. The popular "10% rule" for weekly increases hasn't held up as an injury-prevention tool either: in a trial of novice runners, a gradual 10%-a-week build-up didn't reduce injuries (Buist et al., 2008).
FITIV tracks your short-term load (ATL), long-term load (CTL), and the ratio between them, which FITIV calls your Training Load Ratio (TLR). The Training Summary card on the Today tab shows your current ratio. Tap it for the Training Load screen, where you can filter by activity type and see your short-term (ATL) and long-term (CTL) load, Today's Training Status (Undertraining, Optimal, Peaking, Overreaching, or At Risk, or Baseline Needed until you've recorded enough workouts to set a baseline), and a Daily Training Status chart showing where your load sits against the Optimal Training Range.
Coming back from a quiet week with three hard sessions in four days is a typical case. If that pushes your recent load well above your usual load, your Training Readiness score drops to reflect it, however ready you feel.
Fitness vs. Fatigue: The Performance Model
Chronic training load is a reasonable proxy for fitness: an athlete carrying high CTL has recently been training consistently and has accumulated aerobic adaptation. Acute training load is a proxy for fatigue: high ATL relative to CTL means recent training has been unusually heavy.
The theoretical form (performance readiness) at any point is approximately: Form = CTL − ATL.
When ATL is much higher than CTL (negative form), the athlete is fatigued: fitness may be there but performance is suppressed. When ATL drops below CTL, as it does during a taper before a race, form is positive: fitness is retained while fatigue clears, and peak performance is possible.
FITIV tracks this relationship through your short-term and long-term load and your Training Status, so you can see whether you're building fitness, holding steady, or piling on fatigue in the weeks before a goal event.
Why Training Load Monitoring Matters for Athletes
Without numbers for training load, you rely on feel alone to manage volume and intensity, which makes a gradual build-up of fatigue easy to miss. The most common training errors (overtraining, insufficient progressive overload, and inadequate taper before competition) all result from inaccurate training load assessment.
Load numbers give your own judgment something solid to work from. Knowing your CTL helps you keep week-to-week load increases gradual (10% is a commonly cited rule of thumb). Knowing your ATL:CTL ratio tells you whether the planned hard week is reasonable or reckless.
Frequently Asked Questions
Which training load method is most accurate: TRIMP, TSS, or RPE?
Each is most accurate in its intended context. TSS is the most accurate for power-metered cycling because it is based on objective workload rather than a physiological proxy. TRIMP is best for other endurance activities where heart rate is the primary available signal. RPE is most valuable for strength training and team sports. In FITIV, you choose which method drives your training load view, and every workout counts in that method's units, so runs, rides, and gym sessions add up to one picture.
What is a sustainable weekly training load increase?
A common guideline is no more than about a 10% increase in weekly training load from one week to the next. Research hasn't pinned down one safe number; in one study of novice runners, following it didn't reduce injuries compared with a faster build-up. The bigger risk is a sudden spike well above what you're used to. FITIV's Training Status shows when your recent load has climbed well above your long-term load, moving from Optimal to Peaking, Overreaching, or At Risk.
Can I see my chronic training load from before I started using FITIV?
Partly. FITIV can import past workouts from Apple Health, including sessions recorded by other apps and devices. Imported workouts with heart rate data count toward your training load. If you connect Garmin Connect in FITIV, new Garmin workouts sync in from the day you connect, but older Garmin history doesn't come in through Garmin Connect. FITIV doesn't import directly from Strava.
How does training load for strength training get calculated without heart rate zones?
Two ways. With heart rate from your Apple Watch or a chest strap, strength sessions count through TRIMP just like any other workout. And if you rate your Perceived Effort after the session, the Effort Minutes method (Perceived Effort × duration) captures how hard the session felt, which heart rate alone can miss during heavy lifting. Your sets, reps, and weight are tracked separately in your workout history, where FITIV highlights PRs and progressive overload.
