TSS, IF, and Variability Index: Reading Workout Quality Beyond Total Stress
5 min read
Learn how to read TSS, IF, and Variability Index together so you can judge workout quality beyond total stress and make one clear training adjustment.
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TSS shows total load, IF shows intensity, and VI shows how steady the ride was. Read all three before you judge workout quality.
A single ride can look successful on paper and still miss the point of the workout. TSS, IF, and Variability Index help you see whether the session delivered the right load, the right intensity, and the right shape.
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Training Stress Score, or TSS, is a single score for workout load. TrainingPeaks describes it as a way to quantify the stress of a workout or part of a workout.
In plain terms, TSS blends how long you rode with how hard you rode. It helps compare load across rides that do not share the same length or shape.
Intensity Factor, or IF, tells you how hard the session was relative to your Functional Threshold Power. That makes your FTP setting important, so use a sound FTP test before you lean too hard on IF.
Variability Index, or VI, compares Normalized Power with average power. A VI near one means steady power, while a higher VI means more surges and coasting.
For the broader metric stack, pair these numbers with power and ride data basics. They each answer a different question, so none should stand alone.
TSS: load from intensity and duration.
IF: intensity relative to your FTP.
VI: Normalized Power divided by average power.
Near-one VI means a steadier ride.
Use the trio to judge load, intensity, and steadiness before you change the plan.
TSS is the session’s total signal, IF is the signal’s average strength, and VI is how noisy the signal was.
High TSS with low IF and low VI usually reads as a long, steady aerobic session. The ride asked for time on task, not repeated spikes.
Moderate TSS with high IF and high VI tells a different story. That pattern fits short hard work, where sharp efforts raise intensity without a huge total load.
Low TSS with moderate IF and high VI can still matter. The session may be short, but the power shape shows punchy work rather than smooth endurance.
The key is not to rank workouts by TSS alone. For longer planning, put daily load inside CTL, ATL, and TSB context, then read IF and VI for session quality.
High TSS, low IF, low VI: long aerobic work.
Moderate TSS, high IF, high VI: hard interval work.
Low TSS, high VI: short punchy work.
Same TSS can hide different ride shapes.
TSS tells you how much you asked, IF tells you how hard you asked, and VI tells you whether the ask was steady or chaotic.
TSS = a single score combining intensity and duration to estimate physiological stress (useful for load tracking).
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When IF matched the target but TSS ran high, the workout likely gave the planned intensity plus extra load. Keep the next key intensity target, but trim easy volume around it.
When IF was high but TSS stayed low, the session was hard without much total time. Next time, hold the target intensity and use fewer repeats or shorter work blocks.
When VI was high but the goal was steady endurance, the effort shape missed the brief. Swap one hard day for controlled sub-threshold riding, or use steady sweet-spot work to rebuild pacing discipline.
Your next move should be clear, not dramatic. Keep the signal you wanted, then adjust the load or shape that drifted.
High TSS, target IF: cut easy volume next.
High IF, low TSS: reduce repeats next time.
High VI, endurance goal: ride steadier next session.
Low IF for weeks: add one focused intensity day.
The clean move is to keep the target stimulus and tune the stress around it.
Preserve the planned intensity, then adjust volume or structure based on whether the workout was noisy or simply long.
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Start with IF and ask whether the ride hit the intended intensity. If it did, do not overcorrect just because the session felt uneven.
Next, look at VI to see whether power was steady or jagged. This is where indoor and outdoor rides may differ, so compare like with like when you review training data across settings.
Then compare actual TSS with the planned load. If load ran high, protect the next block by cutting easy time before you cut the key stimulus.
Finish with how you felt, because numbers are not the whole ride. If readiness is low, use your next-session readiness cues before adding more work.
Check IF against the session target.
Check VI for steadiness or spikes.
Check TSS against planned load.
Check RPE before adding work.
TSS, IF, and VI do not tell you whether you slept well, ate enough, or started the ride fresh. They are ride metrics, not a full recovery screen.
They also depend on sound inputs. Bad power data, an outdated FTP, or mismatched device settings can skew Normalized Power, IF, and TSS.
Use them with heart rate, RPE, and longer load trends. For recovery timing, pair the file review with training and recovery balance, not just one ride score.
The numbers should narrow the next decision. They should not replace your plan, your notes, or the wider training system around the workout.
Do not ignore sleep, soreness, or RPE.
Check power data before changing the plan.
Update FTP when evidence supports it.
Use weekly load trends for bigger choices.
Day 0: Confirm NP, IF, TSS, and VI, then check sensor consistency. Log RPE and sleep. If IF matched target but TSS overshot, reduce the next easy ride rather than cutting the next key stimulus.
Days 1–3: Keep one session at the same targeted intensity. Reduce total ride time across the next block if VI was high or TSS overshot, and use easy recovery if fatigue feels unusual.
Days 4–7: Bring planned volume back gradually. If high VI or high RPE keeps showing up, swap one quality session for a steady sub-threshold ride.
End of week: Compare actual and planned TSS, IF, VI, and your recovery notes. If the metrics realign, resume the plan; if not, keep volume lower while preserving one targeted intensity session.
TSS shows total load, IF shows how hard the ride was against your FTP, and VI shows whether power was steady or spiky. Read them together, then make one calm adjustment: keep the intended stimulus, and change the volume or structure that drifted.
Not always. Same TSS can hide different intensity and power shapes, so compare IF and VI before you call the workouts equivalent.
No. Low VI is useful when the goal is steady endurance or pacing control, but interval work will often create a higher VI by design.
First check whether your FTP and power data are current. IF depends on those inputs, so bad settings can make a normal ride look off.
No. They describe the ride file, while sleep, RPE, and readiness help explain how your system handled that load.