CTL / ATL / TSB Explained: Unified Training Load Tracking Across Platforms

What Are CTL, ATL, and TSB?

These three metrics come from the PMC (Performance Management Chart) model in exercise physiology:

• CTL (Chronic Training Load): 42-day exponentially weighted moving average of TSS. Represents your baseline fitness. Higher CTL = greater capacity to handle training volume.
• ATL (Acute Training Load): 7-day exponentially weighted moving average of TSS. Represents your current fatigue. A sharp ATL spike means you've been training hard recently.
• TSB (Training Stress Balance): TSB = CTL - ATL. Positive means fresh and race-ready; negative means you're in a fatigue-building phase.

Core formula: TSB = Yesterday's CTL − Yesterday's ATL

Why Do Different Platforms Show Different Numbers?

The #1 question: "My CTL is 65 on Intervals.icu but 52 on Garmin — which is right?" The answer isn't "data error" — it's different inputs:

• TSS algorithms differ: TrainingPeaks and Intervals.icu use power-based rTSS; Garmin uses heart-rate-based hrTSS and its proprietary EPOC model. The same ride can yield TSS values 10-20% apart.
• CTL time constants differ: Default is 42 days, but some platforms let you customize. If you set 60 days in Intervals.icu and Garmin uses a fixed 42, your CTL will diverge.
• Incomplete data: Garmin only has your runs, Strava only has your rides — each platform sees a partial picture, so training load is undercounted.

Which is real? — All are valid models with different inputs. The key is picking one platform and tracking the trend, not obsessing over absolute values.

Consistent Tracking with GarSync

Option 1: Built-in TSB charts. Tap Health/Balance on the Today page to view Fitness (CTL), Fatigue (ATL), and Form (TSB) trends directly in GarSync. Data comes from all your locally aggregated activities — runs, rides, swims included — unaffected by any single platform's incomplete data.

Option 2: Sync to pro tools. For deeper analysis, GarSync can funnel activities from Garmin, Strava, Zwift, COROS, and more into Intervals.icu. All activities feed into a single PMC model — the trend curve reflects your real training load.

FAQ

Q: Which platform's CTL should I trust?

A: GarSync has built-in CTL/ATL/TSB charts — tap Health/Balance on the Today page to view Fitness (CTL), Fatigue (ATL), and Form (TSB) trends, with time ranges from 42 days to 2 years. Unlike web-based tools like Intervals.icu that require a browser, GarSync gives you the data directly in the app — more convenient for quick daily checks.

Q: How negative should TSB get before I rest?

A: It varies by individual, but general guide: TSB -10 to -30 is a normal training load range; below -30 warrants attention to recovery; +5 and above is ideal for race day or taper. Focus on trends, not absolute numbers.

Q: Which is more accurate — HR-based TSS or power-based TSS?

A: Power-based TSS (rTSS) is more precise because power is an objective physical measurement. Heart rate is influenced by sleep, caffeine, stress, and emotions. If you have a power meter, use rTSS as your standard. If you only have HR data, hrTSS still provides meaningful reference.