What do TSS, CTL, and ATL actually mean?

If you train with a power meter, you’ve seen TSS, CTL, and ATL. Used well, these metrics help you pace training stress, build fitness, and arrive fresh for goals. Used blindly, they can send you into a hole. Here’s how to read them like a coach.

The building blocks: FTP, NP, IF, and TSS

Before CTL and ATL make sense, the foundation matters.

• FTP (functional threshold power): The best power you can sustain for about an hour. All stress metrics scale from this. If FTP is wrong, everything downstream is wrong.
• NP (normalized power): A rolling calculation that estimates the metabolic cost of variable efforts better than average power.
• IF (intensity factor): How hard the ride was relative to FTP. IF = NP / FTP.
• TSS (training stress score): A unit of work combining duration and intensity. Roughly, 100 TSS ≈ a one-hour ride done at FTP.

For power-based rides:

TSS = time_hours × IF² × 100
IF = NP / FTP

Quick examples:

• 90 min sweet spot at IF 0.88 → 1.5 × 0.88² × 100 ≈ 116 TSS.
• 1 h endurance at IF 0.65 → 1.0 × 0.65² × 100 ≈ 42 TSS.
• 2.5 h hilly group ride at IF 0.72 → 2.5 × 0.72² × 100 ≈ 130 TSS.

There are heart-rate and run equivalents (hrTSS, rTSS), but for cyclists with power meters, TSS from power is the most precise.

CTL, ATL, and TSB: fitness, fatigue, freshness

Think of your training like a bank account. CTL is your long-term balance, ATL is your recent spending, and TSB is how flush you feel today.

• CTL (chronic training load): An exponentially weighted average of your daily TSS over ~42 days. It trends with fitness. Units: TSS/day.
• ATL (acute training load): An exponentially weighted average over ~7 days. It reflects fatigue from recent work. Units: TSS/day.
• TSB (training stress balance): TSB = CTL − ATL. Positive = fresher; negative = more fatigued.

Rules of thumb: aim to grow CTL by about 3–7 TSS/day per week; keep an eye on very negative TSB (e.g., < −25) for more than a few days; target a small positive TSB on an A‑race day.

How to use these metrics in your week and season

Plan weekly load with purpose

• Start with goals and constraints: How many hours can you ride? Which days can you go long? Map potential TSS by day.
• Distribute stress across zones: Mix endurance (Zone 2), threshold (sweet spot/tempo), and high-intensity (VO2max) so total weekly TSS climbs steadily without stacking too much intensity back-to-back.
• Watch ATL to manage fatigue: If ATL spikes and TSB dives below −20 for several days, insert recovery rides (30–60 min easy) or a rest day.
• Grow CTL gradually: Increase weekly TSS so CTL rises ~3–7 TSS/day per week. Hold that growth for 2–3 weeks, then schedule a down week (−30–40% TSS).

Use TSB to time freshness

• Weekday hard sessions: It’s normal to do quality work at TSB between −5 and −20 if sleep and fueling are good.
• Taper into events: Reduce TSS for 7–14 days so ATL drops faster than CTL. Many riders feel best on race day with TSB around +5 to +15 for one‑day events.
• Stage races or training camps: Start near neutral TSB (−5 to +5). You’ll go negative during the block; plan recovery afterward.

Checkpoint: is your FTP set correctly?

• Retest or auto‑update every 4–6 weeks: Use a 35–60 min best effort, a well-executed 20 min test (×0.95), or a reliable ramp protocol.
• Sanity checks: If endurance rides at Zone 2 feel too hard or TSS seems unusually high/low for perceived effort, your FTP may be off.
• Seasonal drift: Heat, altitude, and fatigue can shift FTP. Small adjustments keep TSS honest.

Practical examples

• Building toward a gran fondo (12 weeks): Start CTL 45. Progress weekly TSS so CTL rises ~5 TSS/day per week for two weeks, then a down week. Repeat twice. Final 10–12 days taper: drop weekly TSS 30–50%, keep 2 short intensity touch-ups. Target race-day TSB +10.
• Back from illness: Reset expectations. Start with 40–60 TSS rides, cap ATL so TSB does not go below −10 for the first two weeks. Let CTL stabilize before reintroducing VO2 work.
• Busy week at work: Replace a long ride with 2 × 45 min sweet spot sessions. Similar TSS, less time, manageable ATL.

Common pitfalls and fixes

• Chasing CTL for its own sake: Fitness is specific. Combine CTL growth with event-specific intensity and terrain.
• Ignoring recovery: High CTL with chronically negative TSB and poor sleep is a red flag. Schedule a down week.
• Overtrusting NP/TSS in spiky rides: Short, surge-heavy sessions can inflate NP. Use feel and heart rate alongside power.
• Comparing CTL across riders: CTL reflects your training history and FTP setting. Use it to track your trends, not to compete.
• Neglecting fueling: Underfueling inflates perceived effort for the same TSS. Aim for 60–90 g carbs per hour on hard/long rides and prioritize recovery.

Bottom line: set a realistic FTP, plan steady TSS progression, and use CTL, ATL, and TSB to balance workload and freshness. They won’t replace feel, but they will make your decisions sharper.