TRIMP, TSS, and Training Load Explained: The Metrics That Prevent Overtraining

TL;DR

Training load metrics like TRIMP and TSS quantify how much stress each workout puts on your body, while derived metrics like CTL, ATL, TSB, and ACWR track the balance between fitness and fatigue over time. Keeping your acute-to-chronic workload ratio between 0.8 and 1.3 is the single most actionable way to train hard without tipping into overtraining or injury.

What Is Training Load and Why Does It Matter?

Every training session imposes a cost on your body. Sprint intervals tax your anaerobic system and central nervous system differently than a two-hour easy run taxes your aerobic system and musculoskeletal structures. Training load is the attempt to quantify that cost — to assign a number to how much stress each session places on the athlete.

Without tracking training load, athletes are essentially guessing. They might feel fine after a hard week, only to break down in week three when accumulated fatigue catches up. Or they might play it too safe, never pushing into the productive discomfort zone where adaptation actually happens.

Training load metrics solve this by giving coaches and athletes a shared language for dosing stress. When used properly, they help answer the most important question in training design: "How much is enough, and how much is too much?"

TRIMP: Training Impulse

The Origins of TRIMP

TRIMP (Training Impulse) was developed by Dr. Eric Banister in the 1970s as one of the first attempts to quantify internal training load using heart rate data. The fundamental insight behind TRIMP is that both the duration and the intensity of exercise contribute to the overall physiological stress, but intensity contributes disproportionately.

A 60-minute session at 85% of maximum heart rate is not merely 1.7 times harder than the same duration at 50%. The metabolic, hormonal, and neuromuscular costs scale exponentially as intensity rises. TRIMP captures this through a weighting factor that increases exponentially with heart rate.

How TRIMP Is Calculated

The basic Banister TRIMP formula is:

TRIMP = Duration (minutes) x Heart Rate Reserve fraction x Weighting factor

Where the Heart Rate Reserve (HRR) fraction is calculated as:

(Average HR - Resting HR) / (Maximum HR - Resting HR)

The weighting factor uses an exponential function that differs slightly for males and females, reflecting differences in lactate response curves. For males, the multiplier is 0.64 x e^(1.92 x HRR fraction). For females, it is 0.86 x e^(1.67 x HRR fraction).

TRIMP Variants

Over the years, several TRIMP variants have been developed to address limitations of the original formula:

Banister TRIMP — The original formula described above. Uses average heart rate for the entire session.
Edwards TRIMP (also called zone-based TRIMP) — Divides the session into heart rate zones and assigns multipliers (Zone 1 = 1, Zone 2 = 2, Zone 3 = 3, Zone 4 = 4, Zone 5 = 5). Total TRIMP is the sum of minutes in each zone multiplied by the zone's factor.
Lucia TRIMP — Uses three intensity zones based on ventilatory thresholds (VT1 and VT2). Zone 1 = 1, Zone 2 = 2, Zone 3 = 3. Simpler but validated against blood lactate responses.
Individualized TRIMP (iTRIMP) — Uses the athlete's personal lactate-heart rate curve to compute a continuous weighting factor. Considered the most physiologically accurate but requires lab testing.

For most recreational and competitive athletes, Edwards TRIMP or Lucia TRIMP provide a practical balance between accuracy and simplicity. The key requirement is consistency: pick one method and stick with it across all sessions and training cycles.

Practical TRIMP Examples

To illustrate how TRIMP captures the difference between sessions:

Session	Duration	Avg HR Zone	Edwards TRIMP
Easy 60-min run	60 min	Zone 2	120
Tempo 45-min run	45 min	Zone 3/4	~158
30-min intervals	30 min	Zone 4/5	~135
90-min long run	90 min	Zone 2	180
Recovery 30-min jog	30 min	Zone 1	30

Notice that a 30-minute interval session produces a higher TRIMP than a 60-minute easy run despite being half the duration. This reflects the disproportionate physiological cost of high-intensity work.

TSS: Training Stress Score

TSS and Functional Threshold Power

Training Stress Score (TSS) was developed by Dr. Andrew Coggan as part of the power-based training framework for cycling. While TRIMP uses heart rate, TSS uses power output relative to the athlete's Functional Threshold Power (FTP) — the highest average power sustainable for approximately one hour.

The TSS formula is:

TSS = (Duration in seconds x Normalized Power x Intensity Factor) / (FTP x 3600) x 100

Where:

Normalized Power (NP) accounts for the variable nature of power output during a ride, giving more weight to harder efforts.
Intensity Factor (IF) is the ratio of Normalized Power to FTP.

TSS Benchmarks

A useful rule of thumb for interpreting TSS values:

TSS Value	Recovery Impact
Under 150	Low; recovered by next day
150-300	Medium; some residual fatigue next day
300-450	High; residual fatigue likely for 2 days
Over 450	Very high; several days of recovery needed

A 60-minute session at exactly FTP yields a TSS of 100, providing a convenient reference point. An easy endurance ride might yield TSS of 40-60 per hour, while a criterium race might hit 120+ per hour.

TSS Limitations

TSS works exceptionally well for cycling, where power meters provide direct measurement of external work. Adapting it to running and other sports requires proxies like heart rate or pace, which introduce additional assumptions. Running TSS (rTSS) uses pace relative to threshold pace, while heart rate-based TSS (hrTSS) is available for any activity where heart rate is measured.

The practical takeaway: use power-based TSS for cycling if you have a power meter, and use TRIMP or hrTSS for running, swimming, and other activities. The specific metric matters less than consistent tracking over time.

The Performance Management Chart: CTL, ATL, and TSB

Understanding the Three Curves

The most powerful application of training load data is the Performance Management Chart (PMC), which tracks three derived metrics over time:

Chronic Training Load (CTL) — also called "fitness." This is the exponentially weighted moving average of your daily training load (TRIMP or TSS) over the last 42 days. CTL represents the cumulative training you have absorbed and adapted to. A higher CTL means a more trained athlete.

Acute Training Load (ATL) — also called "fatigue." This is the exponentially weighted moving average of your daily training load over the last 7 days. ATL represents your recent training stress and correlates with how tired you are right now.

Training Stress Balance (TSB) — also called "form." Calculated simply as CTL minus ATL. TSB represents the balance between your accumulated fitness and your current fatigue.

TSB = CTL - ATL

Interpreting TSB

TSB is positive: You are rested. CTL (fitness) exceeds ATL (fatigue). This is the taper state — where athletes typically perform their best in competition.
TSB is near zero: You are in a balanced state. Training load roughly matches your body's capacity to absorb it.
TSB is negative: You are fatigued. Current training stress exceeds your adapted capacity. This is normal during build phases but should be managed.

Practical TSB Guidelines

TSB Range	State	Implication
+15 to +25	Peak form	Ideal for competition or time trials
+5 to +15	Fresh	Good for quality sessions and testing
-10 to +5	Balanced	Normal productive training range
-10 to -30	Fatigued	Building fitness; monitor recovery closely
Below -30	Overreaching	High risk; recovery block likely needed

The specific numbers vary by individual, but the patterns are consistent. Productive training operates in the mildly negative TSB range. Peaking for events requires letting TSB rise by reducing ATL while CTL remains elevated.

The Acute-to-Chronic Workload Ratio (ACWR)

What ACWR Tells You

The Acute-to-Chronic Workload Ratio is perhaps the most practically useful training load derivative for injury prevention. Originally developed by Tim Gabbett for team sport athletes, it has since been validated across many sports.

The calculation is straightforward:

ACWR = Acute Load (this week) / Chronic Load (4-week rolling average)

The Sweet Spot

Research consistently identifies an ACWR "sweet spot" between 0.8 and 1.3:

Below 0.8: Undertraining relative to your recent history. Detraining risk, and paradoxically, slightly elevated injury risk when you do return to full training.
0.8 to 1.3: The optimal zone. Training load is appropriately matched to your body's current capacity.
Above 1.3: Danger zone. You have spiked your training load significantly beyond what you have been doing. Injury risk climbs sharply.
Above 1.5: High-risk zone. Injury probability increases by 2-4 times compared to the sweet spot.

The 10% Rule Revisited

The old coaching heuristic of "never increase weekly volume by more than 10%" is a rough approximation of the ACWR concept. An ACWR of 1.1 represents a 10% increase over your recent average. The advantage of ACWR is that it is based on actual load data rather than volume alone, and it accounts for your specific recent training history.

For athletes returning from illness or a planned break, the ACWR framework provides clear guidance: ramp training load gradually so that the ratio stays below 1.3, even if the absolute loads seem modest. The body's vulnerability to injury is relative to recent preparation, not absolute fitness.

How Coach Uses Training Load Metrics

Automated Load Monitoring

When your Garmin device syncs with Coach, every session's heart rate data is processed to calculate training load metrics automatically. You do not need to manually log workouts or compute TRIMP values. The system maintains your running CTL, ATL, and TSB curves and monitors your ACWR in real time.

This matters because consistency is the biggest challenge in training load monitoring. A metric that you only calculate when you remember to is far less useful than one that updates automatically after every session.

Intelligent Training Adjustments

The real value of training load tracking is not the numbers themselves but the decisions they inform. When your TSB drops below -25 and your HRV trend shows suppression, the AI coaching system can flag this confluence and suggest a modified session before you dig yourself into an overtraining hole.

Similarly, if your ACWR approaches 1.3 because you had a recovery week and are returning to full training, the system can recommend a bridging session — something between your recovery week intensity and your full training intensity — to smooth the transition.

Periodization Support

Training load metrics are essential for structured periodization. A well-designed training plan manipulates CTL, ATL, and TSB deliberately across mesocycles:

Base phase: Gradually increasing CTL through progressive volume. TSB stays mildly negative (-5 to -15).
Build phase: Introducing intensity while maintaining or increasing CTL. TSB may dip further (-15 to -25).
Peak/taper phase: Reducing ATL while maintaining CTL. TSB rises to +10 to +25 for race day.
Recovery phase: Reduced load across the board. CTL may decline slightly, but ATL drops rapidly, allowing supercompensation.

Understanding these dynamics helps you see the purpose behind every easy week and every hard block. Check out our guide on how this works in practice to see the full coaching workflow.

Putting It All Together: A Practical Weekly Monitoring Routine

Here is a straightforward weekly routine for athletes who want to use training load metrics effectively:

Daily (2 minutes)

Glance at your overnight HRV and sleep data
Note your subjective energy and motivation (mental check-in)
After each session, confirm the TRIMP/TSS was recorded and seems reasonable

Weekly (10 minutes)

Review your weekly TRIMP/TSS total compared to the previous three weeks
Check your ACWR — is it between 0.8 and 1.3?
Review your TSB trend — is it tracking where you expect for this phase of training?
Assess whether planned sessions for next week need adjustment based on recovery data

Monthly (20 minutes)

Review your CTL trend — is it progressing as planned for your goal event or fitness target?
Look for patterns: which session types generate the most load? Which have the best recovery profiles?
Evaluate if your current training structure matches the periodization intent

Pre-Competition (1 week out)

Confirm TSB is trending positive and will reach your target range by race day
Verify CTL has not dropped more than 5-10% during taper
Review sleep and recovery data for the taper week to confirm good rest quality

Common Mistakes in Training Load Management

Mistake 1: Chasing CTL

A rising CTL feels rewarding — it means your fitness is growing. But CTL should rise at a sustainable rate. Increasing CTL by more than 5-7 points per week (in TSS-based systems) is a recipe for breakdown. Patience in building the chronic load base is what separates sustainable improvement from the boom-bust cycle many athletes experience.

Mistake 2: Ignoring Load Spikes

A single big race or epic training day can spike your ATL dramatically. This is fine if it is planned and followed by appropriate recovery. It becomes problematic if you attempt to maintain that spiked load level in the following days. Always plan the recovery that follows the effort.

Mistake 3: Treating All Load as Equal

A TRIMP of 200 from a long Zone 2 run creates very different physiological stress than a TRIMP of 200 from high-intensity intervals. Most training load systems do not fully capture this distinction. Supplement your quantitative tracking with qualitative awareness of what type of stress each session imposed: muscular, metabolic, neurological, or psychological.

Mistake 4: Not Accounting for Life Stress

Training load metrics only capture exercise stress. Work deadlines, travel, family obligations, and poor nutrition all contribute to your total allostatic load. If life stress is high, your capacity to absorb training stress is reduced. The smartest athletes lower training load during high-stress life periods, even when their body feels ready for more.

Getting Started with Training Load Tracking

If you are new to training load monitoring, do not try to implement everything at once. Start with these steps:

Ensure consistent heart rate recording for every session. A chest strap is more accurate than optical wrist sensors for high-intensity work, but any consistent data is better than none.
Pick one metric — Edwards TRIMP is a good starting point — and track your weekly totals for four weeks to establish a baseline.
Calculate your ACWR weekly by dividing this week's total by your 4-week average.
Keep a simple log of how you feel versus what the numbers say. This calibration period helps you learn what the metrics mean for your body.

Or skip the manual work entirely. Platforms like Coach handle the computation automatically when connected to your Garmin device, and the AI coach translates the numbers into plain-language recommendations. Explore the pricing plans to find the option that fits your training goals.

The goal of training load tracking is not to turn every run into a math problem. It is to give you — or your coach — the objective data needed to train hard when you can, rest when you should, and arrive at your goal event in the best possible form.