Training Load Tracking: TRIMP, TSS & RPE
Not all workouts create equal training stress. A 30-minute easy run and a 30-minute threshold run involve the same duration and similar heart rates on the surface — but their physiological cost, recovery demand, and adaptive stimulus differ by an order of magnitude. Training load quantification translates the qualitative sense of "that was a hard workout" into a number that can be tracked, summed, and compared across days, weeks, and months. Fitiv Pulse supports multiple training load calculation methods to accommodate different sports and data availability.
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.
The purpose of tracking training load is threefold:
- Acute load management: Ensuring any single week's training is not excessive relative to habitual load
- Chronic load development: Building the accumulated fitness base that allows progressive training increases
- Fitness-fatigue balance: Understanding the relationship between current fitness level and current fatigue state
Training load is neither good nor bad in isolation — it must be interpreted relative to your chronic training history. A load 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 the ratio of exercise heart rate to maximum heart rate. A 60-minute run at Zone 2 might produce a TRIMP of 50; the same duration at Zone 4 threshold pace might produce a TRIMP of 120 — more than double, reflecting the disproportionate stress of higher-intensity work.
Fitiv calculates TRIMP from beat-to-beat heart rate data, making it available for any activity where heart rate is recorded — running, cycling, rowing, swimming with a waterproof HR monitor, cross-country skiing, and more. TRIMP is the default training load metric for most activities in Fitiv because it requires only heart rate data, which all supported devices provide.
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 calculated from Normalized Power (an intensity-weighted average of power that accounts for the metabolic cost of variable-intensity riding), Functional Threshold Power (FTP, the maximum power sustainable for approximately one hour), and workout duration:
TSS = (Duration in seconds × NP × IF) ÷ (FTP × 3600) × 100
Where IF (Intensity Factor) = NP ÷ FTP.
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 45-minute threshold interval session at 95% of FTP produces approximately TSS 85 — similar total load, very different physiological character.
Fitiv calculates TSS for cycling workouts when a power meter is connected. Without power data, Fitiv uses a heart rate-based TSS approximation that produces comparable values for most athletes.
RPE-Based Load
RPE (Rate of Perceived Exertion) load uses the athlete's subjective effort rating after a workout, multiplied by workout duration, to estimate session load. The Session RPE method (CR10 scale × duration in minutes) has been validated against more sophisticated methods in multiple sports and produces reliable load estimates for activities where objective data is unavailable — particularly team sports, strength training, and activities not well-suited to heart rate or power monitoring.
Fitiv prompts for an RPE rating after every workout. This input serves two purposes: it provides a load estimate for workouts lacking objective data, and it calibrates the AI coaching model to your individual perception of effort, which varies between athletes.
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. 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 from sports medicine — initially in cricket, subsequently replicated across multiple sports — consistently identifies an ACWR above 1.5 as associated with significantly elevated injury risk. 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.
Fitiv tracks ATL, CTL, and ACWR across all logged workouts and displays them in the training load dashboard. When the ratio approaches or exceeds 1.5, the AI coaching system flags an elevated load warning regardless of subjective readiness.
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 and uses it to inform training recommendations in the weeks leading up to goal events that users specify in their profile.
Why Training Load Monitoring Matters for Athletes
Without quantified training load, athletes rely on subjective sense of effort to manage training volume and intensity — which is demonstrably unreliable. The most common training errors — overtraining, insufficient progressive overload, and inadequate taper before competition — all result from inaccurate training load assessment.
Load monitoring does not replace training intelligence, but it provides the quantitative foundation that good training decisions require. Knowing your CTL allows you to set an upper limit on week-to-week load increases (10-15% maximum is the commonly recommended guideline). Knowing your ATL:CTL ratio tells you whether the planned hard week is reasonable or reckless.
Frequently Asked Questions
Q: Which training load method is most accurate — TRIMP, TSS, or RPE? A: 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. Fitiv uses the most appropriate method for each activity type and converts all outputs to a common load scale for weekly and monthly summaries.
Q: What is a sustainable weekly training load increase? A: The conventional guideline is no more than a 10% increase in weekly training load from one week to the next. The sports science literature supports this as a conservative upper bound for injury risk management. Fitiv calculates week-over-week load change and flags increases that approach or exceed this threshold.
Q: Can I see my chronic training load from before I started using Fitiv? A: Fitiv can import historical workout data from Apple Health, Garmin Connect, and Strava. If your historical workouts are available with heart rate data, Fitiv will calculate retrospective training load back through your history, providing a complete CTL picture from the start.
Q: How does training load for strength training get calculated without heart rate zones? A: Strength training load in Fitiv is calculated from volume load (total sets × reps × weight), RPE-based session load, and heart rate where available. Volume load captures progressive overload in a way heart rate cannot — a heavier lift at the same perceived effort represents genuinely greater muscular stress, which TRIMP would miss. The strength and endurance load figures are displayed separately and combined into a total daily load estimate.