$19.95 a month, every month. That’s what TrainingPeaks charges to see three numbers on a chart. There are free ways to get those same numbers, and one of them is better than the paid version for most athletes.
This guide covers every option. It shows the math, compares the tools side by side, and explains the one confusion that trips up nearly every Garmin user.
How to Track CTL, ATL, and TSB Without TrainingPeaks Premium
Three solid paths exist. Pick based on what you care about:
- Free with no setup: Intervals.icu. Full chart, identical math, connects to Garmin and Strava automatically.
- Free with AI explanation: AthleteOS. Full Performance Management Chart plus plain-English interpretation of what your numbers mean today.
- Build it yourself: A Google Sheet. Takes about an hour to set up. You own the numbers forever.
TrainingPeaks Basic, which is free, shows you none of this. Zero. The Performance Management Chart is locked behind the $19.95/month paywall. Many athletes don’t realize this until they’ve already connected their Garmin and spent 20 minutes looking for the chart.
What the PMC Chart Actually Shows (and Why Athletes Pay for It)
The Performance Management Chart (PMC) plots three numbers over time: your fitness score (CTL), your fatigue score (ATL), and your form score (TSB). Together they describe where you are in the build-peak-taper cycle.
CTL is a 42-day exponentially weighted moving average of your daily Training Stress Score (TSS). Think of it as a slow-moving tide that rises with consistent training and drops if you take a week off. Its half-life is about 29 days, meaning a workout you did four weeks ago still counts for roughly half its original value.
ATL runs on a 7-day window. It responds fast: one hard week sends it up quickly, one easy week brings it back down. Half-life is about 4.85 days. ATL is your recent workload — it resets in less than a week of easy training.
TSB is simple: TSB = CTL minus ATL. Positive TSB means you’re fresher than your baseline. Negative TSB means you’re carrying more fatigue than usual. For a target race, Joe Friel recommends landing at +15 to +25 on race morning.
The Math You Can Build in a Spreadsheet
The formulas come from Banister’s 1975 impulse-response model, later refined by Morton, Fitz-Clarke, and Banister in 1990. Coggan adapted them to use TSS instead of heart-rate-based load.
CTL today = CTL yesterday × exp(-1/42) + TSS today × (1 - exp(-1/42))
ATL today = ATL yesterday × exp(-1/7) + TSS today × (1 - exp(-1/7))
TSB today = CTL today - ATL today
The multiplier exp(-1/42) equals about 0.976 — each day, your fitness score keeps 97.6% of its prior value. ATL’s multiplier, exp(-1/7), equals about 0.867. It forgets faster, which is why CTL’s half-life is 29 days and ATL’s is just 4.85 days.
| Metric | Time constant | Daily retention | Half-life |
|---|---|---|---|
| CTL (fitness) | 42 days | 97.6% | ~29 days |
| ATL (fatigue) | 7 days | 86.7% | ~4.85 days |
Put those two formulas in columns C and D of a Google Sheet, seed CTL and ATL at zero, paste in a column of daily TSS values, and you have a functioning PMC. No subscription required.
One important caveat: these time constants are population averages. Hellard et al. found in 2006 that the 95% confidence interval for the fitness time constant spans 25 to 61 days across individual athletes. Your 42 might actually be 30, or 55. The default number is a starting point, not a personal truth.
Spreadsheet Caveats Worth Knowing
A DIY sheet has sharp edges. Rest days must be entered as TSS = 0, not left blank — a blank cell breaks the formula chain. Multi-sport athletes need to sum bike TSS and run rTSS manually each day. And stale FTP values make every TSS reading noisy. Intervals.icu handles all of this automatically. AthleteOS adds the interpretation on top.
The Full Comparison: Six Options Side by Side
| Tool | Monthly Cost | PMC Chart | AI Interpretation | Mobile App | Key Limit |
|---|---|---|---|---|---|
| TrainingPeaks Premium | $19.95 | Yes | No | Yes | Most expensive; chart only |
| TrainingPeaks Basic | $0 | No | No | Yes | Zero CTL/ATL/TSB visibility |
| Intervals.icu | $0 (or $4/mo supporter) | Yes | No | Web only | Steep learning curve |
| Strava Premium | $11.99 | Yes | Limited | Yes | Costs more than Intervals.icu |
| Garmin Connect | $0 | No (EPOC-based) | Training Status label | Yes | EPOC numbers not comparable to TSS |
| AthleteOS | $0 | Yes | Yes | Yes | Fewer third-party integrations than Intervals.icu |
Intervals.icu: The Free PMC Most Athletes Don’t Know About
Intervals.icu replicates the Performance Management Chart exactly. It uses the same exponential decay constants as TrainingPeaks. The metric names differ because TSS, CTL, and ATL are registered trademarks of Peaksware (TrainingPeaks’ parent company). So Intervals.icu calls them Load, Fitness, Fatigue, and Form. Same math, different labels.
- Go to intervals.icu and sign in with your Strava or Garmin account. No credit card required.
- Authorize the data sync. Intervals.icu pulls up to three years of historical activities automatically.
- Open the Fitness tab. Your CTL, ATL, and Form lines appear within minutes.
No configuration required to see the chart. The deeper settings — custom time constants, dashboard panels, power curves — can come later.
It connects to Garmin, Strava, Wahoo, Polar, Suunto, Coros, Zwift, Amazfit, WHOOP, Oura, and about a dozen more platforms.
One technical difference worth knowing: TrainingPeaks calculates your form score using yesterday’s CTL and ATL values. Intervals.icu uses today’s values. Your numbers will be a few points apart between platforms. Neither is wrong. They just use different reference points in the calculation.
For a detailed head-to-head, see AthleteOS vs Intervals.icu.
Strava Fitness and Freshness: Is It the Same Thing?
Yes, mathematically. Science4Performance reverse-engineered Strava’s constants and confirmed that Strava uses exp(-1/42) for fitness and exp(-1/7) for fatigue. Identical to TrainingPeaks.
Strava calls the three metrics Fitness, Fatigue, and Form. It calibrates them to your FTP when power data is available, or uses heart rate-based Relative Effort when it isn’t.
The catch: Strava requires a paid subscription for Fitness and Freshness. As of 2025, that’s $11.99/month or $79.99/year. That’s more expensive than Intervals.icu for the same PMC functionality.
Garmin’s Acute and Chronic Load: Not the Same Thing (Here’s Why)
This is the most common point of confusion. Garmin shows you an Acute Load and a Chronic Load. They look like ATL and CTL. They are not.
Garmin uses EPOC (Excess Post-Exercise Oxygen Consumption) as its load unit. TrainingPeaks and Intervals.icu use TSS (Training Stress Score). According to forum comparisons on Intervals.icu, the same ride that shows an Exercise Load of 118 on Garmin might show a TSS of 78 on Intervals.icu — a 51% difference. A Garmin Chronic Load of 250 tells you nothing about your CTL.
Garmin’s Chronic Load also uses a 28-day window, not 42 days. The time constant is shorter. The numbers are not comparable across systems, even for the same athlete.
Garmin does give you one directionally useful signal: the Load Ratio (Acute divided by Chronic). A ratio of 0.8 to 1.3 is the productive zone — Garmin marks it green. At 1.5 or above, injury and illness risk climbs significantly — marked red. This is Garmin’s proxy for TSB, built on Firstbeat’s Acute:Chronic Workload Ratio framework.
Vermeire et al. showed in 2021 that different training load methods applied to the same athlete over eight weeks produced peak performance predictions up to 19.6 days apart.
A Real Example: One Age-Group Triathlete’s Switch
After four seasons paying for TrainingPeaks Premium, Marcus (a 38-year-old age-group triathlete from the Pacific Northwest, name changed) canceled when his annual renewal jumped to $134.99 in spring 2025. His use case was simple: checking the PMC before race week to confirm his form score was trending positive. He had never used the Annual Training Plan builder. His coach sent workouts by email.
One Saturday morning, Marcus created a free Intervals.icu account and connected it to Strava, which already aggregated his Garmin activities. Within two minutes the full fitness/fatigue/form chart populated with three years of history. His Fitness sat at 74, his Fatigue at 82, his Form at -8. Nearly identical to what TrainingPeaks had shown the day before.
Six months later Marcus had one complaint he couldn’t ignore: no plain-language explanation of what a Form of -8 actually means for his training decisions that week. Without a coach, he was back to staring at a number and guessing.
That’s the gap AthleteOS fills.
Where AthleteOS Fits
AthleteOS includes the full Performance Management Chart in the free tier. Fitness score, fatigue score, form score, full date range. Same math as TrainingPeaks.
The difference is the AI layer. Instead of a number sitting on a chart, the AI coach generates a plain-English explanation of what your specific form score means today and when your form window is projected to peak before your target race.
Imbach et al. noted in 2022 that machine-learning approaches extend the Banister fitness-fatigue framework by interpreting individual patterns rather than assuming a population average applies. The 42-day constant may not be your constant. A system that learns from your history can account for that.
For a deeper look at the science behind CTL, ATL, and TSB, see CTL, ATL, and TSB Explained. For context on how training load tracking fits into a broader readiness picture alongside HRV, see HRV Readiness Trend.
Which Option Is Right for You
If you want the PMC chart and nothing else, Intervals.icu is the answer. It’s free, it’s accurate, and it works with almost every platform you already use.
If you want the PMC chart and want to understand what the numbers mean without a coach, AthleteOS is the right call. Both are free.
If you need structured workout sync to a Garmin or Wahoo device, a coaching integration, or the Annual Training Plan builder, TrainingPeaks Premium earns its price. Those features don’t exist elsewhere at the same quality.
Paying $19.95/month for a number you could get free is a real choice — legitimate if you need the ecosystem around it. But you don’t have to.