Understanding TSS, CTL, and ATL — A Runner's Guide

You’ve spent time with Sequorr’s analytics dashboard, you’ve seen three numbers that sit at the center of your training picture: TSS, CTL, and ATL. These aren’t arbitrary — they’re the foundation of how structured training actually works, and understanding them puts you in control of your fitness trajectory instead of guessing.

This guide breaks down what each metric means, how Sequorr’s algorithm works, and most importantly how you can use them to build readiness, manage fatigue, and peak at race day.

What Is Training Stress Score (TSS)?

TSS is a single number that captures how hard a workout was relative to what you’re capable of. It accounts for both the duration and the intensity of your session — a short, explosive tempo run and a long easy effort can produce similar TSS values for very different reasons.

Think of it as a cost model. Every session you do has a cost. The question is whether you’re spending wisely relative to what you’re trying to build.

TSS Formula

TSS = (Duration x Intensity Factor² x 100) / Threshold Reference

Where Intensity Factor is the ratio of your session's normalized output to your threshold output (Functional Threshold Pace or Power). A workout done entirely at threshold produces ~100 TSS per hour.

Some practical anchors to help you calibrate:

Easy 60-min run — ~40-50 TSS
Threshold 60-min run — ~100 TSS
Long run (2.5 hrs, moderate) — ~150-180 TSS
Marathon race effort — ~250-350 TSS

Why this matters

TSS strips away the noise. You stop thinking "was that run hard enough?" and start asking "did today's cost fit my plan?" It lets you compare sessions that feel completely different — like an interval workout versus a long run — on the same scale.

CTL: Chronic Training Load (Your Fitness)

CTL is an exponentially-weighted rolling average of your daily TSS over roughly the last 42 days. It represents your accumulated training — what your body has absorbed and adapted to over weeks of consistent work.

CTL Calculation

CTL_today = CTL_yesterday + (TSS_today - CTL_yesterday) / 42

This is an exponential moving average with a 42-day time constant. More recent training carries more weight, but fitness takes time to build — which is exactly what this calculation models.

The key property of CTL is that it rises slowly and falls slowly. It takes consistent loading over weeks to move the needle. But the flip side is reassuring: a few missed days don’t erase what you’ve built.

ATL: Acute Training Load (Your Fatigue)

ATL works identically to CTL but with a 7-day time constant. It represents your short-term training load — essentially, how fatigued you are from recent sessions.

ATL Calculation

ATL_today = ATL_yesterday + (TSS_today - ATL_yesterday) / 7

ATL reacts quickly. A big training day spikes it; a rest day brings it down fast.

ATL is your early warning system. When it’s significantly higher than your CTL, your body is accumulating stress faster than it can absorb.

TSB: Training Stress Balance (Your Form)

This is where it all comes together. TSB is simply the difference between your fitness and your fatigue:

TSB (Form)

TSB = CTL - ATL

Positive TSB = you're rested relative to your fitness. Negative TSB = you're carrying fatigue. The sweet spot for race day is typically +10 to +25.

Race day target

Most athletes perform their best when TSB is between +10 and +25. Too low and you're still carrying fatigue. Too high and you've rested too long — your CTL has eroded. The taper is about threading this needle.

The Bottom Line

TSS, CTL, ATL, and TSB give you something powerful: a shared language for understanding training cycles that removes guesswork and replaces it with intentional signals. Sequorr puts the data in your hands. What you build with it is up to you.