What is TT Lab?
TT Lab automatically calculates science-backed running metrics from your activity data. Metrics that used to require expensive devices like Garmin or Apple Watch are now available for free in TT Runner.
Tap the info button (i) next to any metric in the app to see a quick explanation. This guide goes deeper — explaining what each metric means and how to apply it to your training.
Training Load Metrics
These metrics tell you how much stress today's workout put on your body. Essential for avoiding overtraining and planning recovery.
📊 Training Stress Score (TSS)
A single number that represents how hard a workout was on your body. Higher means more demanding.
The same 10K can have very different TSS values depending on intensity. TSS combines intensity × duration into one score.
🟡 50–150 — Moderate training (tempo run, long run)
🔴 Over 150 — High-intensity training (race, hard intervals)
❤️ Cardiac Effort
How much stress the workout put on your cardiovascular system, calculated from intensity and duration.
While TSS measures overall body load, Cardiac Effort focuses specifically on cardiovascular fatigue.
🔵 Moderate — Good stimulus
🟡 Somewhat high — Use caution
🔴 High to very high — Full recovery essential
⏱️ Recovery Estimate
Estimated hours your body needs to fully recover before the next workout.
After a race or hard interval session, wondering "should I rest tomorrow?" — check this number.
🟡 24–48h — Moderate intensity, one rest day ideal
🔴 Over 48h — High intensity, full recovery needed
Fitness Level Metrics
These answer the question "how fit am I right now?" Track them over time and watch your progress curve.
🏃 VDOT
Your running fitness expressed as a single number, based on Jack Daniels' formula. Higher = faster runner.
With just a 5K time, you can estimate your marathon potential. TT Runner's training pace recommendations are all based on your VDOT.
🏃 40–50 — Intermediate
🏃 50–60 — Advanced
🏃 60+ — Elite
🫁 VO2max (Maximum Oxygen Uptake)
The maximum amount of oxygen your body can use per kilogram per minute. The gold standard of fitness measurement.
This is the same metric shown on Garmin and Apple Watch. When external device data is available, TT Lab prioritizes it for accuracy.
🟡 40–50 — Average
🔴 Under 40 — Needs improvement
Aerobic & Anaerobic Effect
These metrics tell you how much this run actually contributed to improving your fitness.
💚 Aerobic Effect
How much this workout helped improve your cardiovascular endurance. Scored out of 5.
Tends to be high during easy runs and long runs. 3–4 is the sweet spot for building endurance.
3–4 — Effective for fitness improvement (easy run, long run)
5 — Maximum stimulus (race-level intensity)
🔥 Anaerobic Effect
How much the workout contributed to speed and power development. Goes up during high-intensity efforts.
Near zero on easy runs, highest during 400m intervals or hill sprints.
3–4 — Effective for speed improvement (intervals, tempo)
5 — Maximum stimulus (all-out sprint)
Heart Rate Metrics
These require a heart rate sensor (Apple Watch, Garmin, etc.) and are key for qualitative assessment of your runs.
📈 Heart Rate Drift
How much your heart rate increased while running at the same pace. A measure of fatigue.
If your 30-minute easy run shows 7% drift, either you're fatigued or your easy pace is too fast.
🟡 3–5% — Average
🔴 Over 5% — Accumulated fatigue signal
💓 Heart Rate Recovery
How quickly your heart rate drops in the first minute after stopping. Faster drop = healthier heart.
If this number is trending upward over weeks, your aerobic fitness is definitely improving.
🟡 20–30 bpm — Average
🔴 Under 20 — Poor recovery
Running Efficiency Metrics
These measure "faster and farther with the same energy" — providing evidence for form correction.
⚡ Efficiency Factor (EF)
How fast you can run at a given heart rate. Higher means more efficient running.
Track your EF on the same route each week. A steady increase means your aerobic fitness is improving.
🎯 Running Efficiency
A composite score of cadence, ground contact time, and vertical oscillation. Scored out of 100.
More accurate when Garmin Running Dynamics data is available.
🟡 60–80 — Average
🔴 Under 60 — Needs improvement
↕️ Vertical Ratio
How much you bounce up and down relative to forward distance. Lower = less wasted energy.
If you've been told "you bounce too much when you run," watch this metric. Aim for under 8%.
🔵 6–8% — Good
🟢 8–10% — Average
🟠 Over 10% — Needs improvement
💡 Which metrics should I focus on first?
If you're new to this, start with VDOT and TSS. VDOT tells you your level, TSS tells you how hard today's workout was.
If you have a heart rate sensor, add Heart Rate Drift. It's the most practical way to check if your easy runs are actually easy.
You don't need to track all 12 metrics at once. Learn one or two at a time and build your own routine.
FAQ
Are TT Lab metrics free?
Yes. TT Lab metrics are calculated automatically when you record a run. You can watch a short ad to unlock AI analysis with a detailed coaching report.
Do I need heart rate data for all metrics?
Pace-based metrics like VDOT and TSS work without heart rate data. Heart Rate Drift, Aerobic Effect, and HR Recovery require a heart rate sensor like Apple Watch or Garmin.
Are TT Lab metrics the same as Garmin metrics?
Some metrics share similar names but may use different calculation methods. TT Lab uses VDOT-based running science formulas and prioritizes data from connected devices like Garmin for more accurate results.
See your running data in TT Lab
Just run — 12 metrics are calculated automatically.