Making the Most of Your Cycling Data: A Beginner's Guide to FTP, CTL, ATL, and TSB

You have a power meter. You are uploading rides to a training platform. The numbers are there. But if you're honest about it, you are not entirely sure what most of them mean or what to do when they change.

You are not alone. The four metrics that matter most in data-driven cycling training, FTP, CTL, ATL, and TSB, are not intuitive from their names. But once you understand what each one represents, the rest of training analysis starts to make sense quickly. This is the explainer most cyclists wish they had been given when they first started training with power.

FTP: Your Ceiling

FTP stands for Functional Threshold Power. It represents the highest power output you can sustain for approximately 60 minutes in a maximal effort.

Think of FTP as the ceiling of your aerobic engine. Every training zone is expressed as a percentage of your FTP. Zone 2 (56-75% of FTP) is easy endurance. Zone 4 (91-105% of FTP) is threshold work. Zone 5 and above (over 106% of FTP) is VO2 max territory.

Without an accurate FTP, your training zones are inaccurate, which means every session prescription based on those zones is imprecise.

How to measure it: The most accessible method is a ramp test, available in most training platforms. Power increases steadily until you cannot maintain it. Your FTP is estimated at approximately 75% of your best one-minute power from the test. More details in a separate article on FTP testing.

What it means when it changes: A rising FTP over a training season is the clearest single indicator that your training is working. A falling FTP despite consistent training is a sign that something is wrong: too much fatigue, not enough intensity, or an unreliable test methodology.

How often to test: Every 6 to 8 weeks during structured training is sufficient. Testing more frequently is not more informative and uses training time that would be better spent training.

CTL: Your Fitness

CTL stands for Chronic Training Load. It is a rolling 42-day average of your daily training stress, expressed in units called Training Stress Score (TSS).

Think of CTL as the fitness engine you have built. A higher CTL means you have a larger aerobic base, built through more sustained training over the preceding months. CTL rises when you train consistently and falls when you reduce training.

The decay rate of CTL is slow: it decreases at approximately 2% per day without training. One week off the bike costs roughly 14 points of CTL. Two weeks off costs more than that because the daily percentage applies to the remaining CTL. This is why off-season fitness preservation matters: it takes far less time to lose CTL than to rebuild it.

A practical scale for road cyclists: - CTL below 50: developing base, limited racing readiness - CTL 50-70: solid club cyclist level, capable of completing most sportives - CTL 70-90: competitive amateur level, race-ready for most categories - CTL 90-120: high-level amateur or amateur elite - CTL 120+: elite amateur or semi-professional

These numbers vary by the training platform and how TSS is calculated, but the scale gives a useful relative reference.

What it means when it changes: CTL rising steadily over 8 to 12 weeks reflects an effective training block. CTL plateauing despite consistent training suggests the training stimulus is not sufficient, either in intensity, volume, or specificity. CTL falling while you feel you are training hard often means your recovery is insufficient and you are not absorbing the training.

ATL: Your Fatigue

ATL stands for Acute Training Load. It is a rolling 7-day average of your training stress.

Think of ATL as the fatigue meter. It tracks how hard you have been working in the short term. ATL rises rapidly when you do hard or high-volume training and falls rapidly when you rest or reduce training.

The 7-day decay rate makes ATL very sensitive to recent training. A single day of hard training raises ATL noticeably. A single rest day lowers it noticeably. This responsiveness makes ATL useful for understanding your current fatigue state.

A practical interpretation: - ATL significantly above your CTL: you are working harder than your fitness level is accustomed to. This is sometimes necessary in training camps or build blocks but is not sustainable for long. - ATL near or below your CTL: you are in a manageable training state relative to your fitness level.

What it means when it changes: ATL rising steadily means you are accumulating training stress. In the right context (a planned build block), this is exactly right. ATL remaining stubbornly high without falling on rest days suggests you are not recovering adequately, which is a flag worth investigating.

TSB: Your Form

TSB stands for Training Stress Balance. The formula is: TSB = CTL minus ATL.

Think of TSB as your daily form indicator. It tells you whether you are rested or fatigued relative to your fitness level.

• Positive TSB (CTL greater than ATL): You are rested. Your fitness is higher than your recent fatigue. You are likely to feel good on the bike and produce good power numbers.
• TSB near zero: Moderate freshness. Well suited to quality training days.
• TSB negative: You are carrying more fatigue than your current fitness can fully absorb. Hard training will feel difficult. This is normal during training blocks.
• TSB deeply negative (below -30): Significant fatigue accumulation. If this persists for more than 2 to 3 weeks without a recovery week, overreaching risk is elevated.

What to do with it:

On days with positive or near-neutral TSB, hard sessions will produce better quality and more adaptation. If you have flexibility in your schedule and your TSB is well into positive territory, that is the day for a key quality session.

On days with deeply negative TSB, especially if HRV is also suppressed, reconsider hard sessions. A recovery day or Zone 2 ride will cost you less in the long run than a poorly executed quality session.

Putting It Together: The Daily Training Decision

Here is how the four metrics interact in practice.

Suppose it is Tuesday. Your FTP is 260W. Your CTL is 78. Your ATL is 95. Your TSB is therefore -17. Your plan calls for a 3x12 threshold session.

The TSB of -17 suggests moderate fatigue. Not excessive, but not fresh. The session is appropriate if you are in a build phase and the fatigue is expected. Check your HRV and how you feel. If both support it, proceed with the session at your normal FTP-based targets.

Now suppose the same scenario but your TSB is -38, you slept poorly, and your HRV is significantly below baseline. The data is pointing in the same direction from three sources. Replace the threshold session with a Zone 2 ride or take a rest day. Come back to that session in two days. The training block is not worth more than your body's ability to absorb it.

This is data-driven training at its most practical: not blindly following numbers, but using metrics to inform decisions that would otherwise be made purely on feel.

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