A set of 12. Rep 1 peaks at 400 N. Rep 12 peaks at 240 N. The first six reps came in above 350 N; the last three came in below 270. If you had stopped at rep 9, the set would have looked the same on a count log. It would have looked very different on a force–time curve.
The middle reps were the work. The last three were noise.
What set decay actually is
Set force decay is the gap between the first rep’s peak and the last rep’s peak in the same set, expressed as a percentage of the first peak. A set that goes 400 N → 240 N decayed 40%.
The single number is useful. The shape of the decline is more useful, and decomposes into a few related views:
- Decay slope — peak force vs rep index, fitted as a line. The slope tells you whether the decay was steady or accelerating.
- Tempo decay — eccentric (or full-rep) duration vs rep index. Slower reps usually precede weaker ones.
- Fatigue index — peak of the last 25% of reps divided by peak of the first 25%. A robust summary number when individual reps are noisy.
- Set consistency — standard deviation of rep peaks. High consistency means the set was uniform; high variance means something was changing.
- Form-shape variance — does each rep’s force–time curve look like the others? Form usually breaks before peak does.
Why it matters
Decay is not failure. It’s information.
Flat decay (5% or less) means the set was too easy. The load wasn’t enough to fatigue you across 10–12 reps, and your body adapted to almost none of it. If you’re chasing hypertrophy, that’s an empty set.
Smooth, gradual decay (15–30% across the set) is the productive shape. The load was challenging; you stayed above the stimulating threshold for most of the reps; the last few cost you something. This is what a working set looks like on the curve.
Cliff decay (peak force collapses on the last 2–3 reps, dropping below 60% of starting peak) means you stopped too late. The reps after the cliff were below the stimulating threshold and probably came at the cost of form, joint stress, or recovery you didn’t need to spend. They didn’t add stimulus — they added cost.
The shape names the load mistake. A flat session asks you to add weight or shorten rest. A cliff session asks you to cut a rep or two. A smooth session is the dose; do it again next week.
What to track together
The decay-and-fatigue cluster is one diagnostic view of a set. Individually any one number is noisy. Together they tell a story.
| Metric | What it tells you |
|---|---|
| Set force decay (%) | The headline: how much weaker were the last reps than the first? |
| Decay slope (N/rep) | Was the fall steady or did it accelerate? |
| Tempo decay (s/rep) | Slower reps usually precede weaker ones — early warning. |
| Fatigue index | A noise-resilient summary; better for week-over-week comparison. |
| Set consistency | Low variance + low decay = clean set. High variance = something changed mid-set. |
| Form-shape variance | Did every rep look like the others, or did the shape break? |
Read them in this order: start with decay percent, look at shape via decay slope and form-shape variance, sanity-check with fatigue index. Tempo decay is the early warning; consistency is the sanity check.
What gear it needs
Decay-by-peak and decay-by-impulse are honest at every sample rate. These are aggregates over reps; they smooth out under-sampling. An 8 Hz crane scale gives you set decay percent and decay slope reliably.
Tempo decay and form-shape variance want 80 Hz. Tempo decay needs sub-second resolution per rep to be meaningful; form-shape variance compares whole-curve shapes that get pixelated at low sample rates.
For lifters on a low-rate sensor: trust the percent and the slope, treat tempo decay as approximate, skip form-shape variance until you upgrade.
What to do tomorrow
Look at one of last week’s working sets. Ignore the rep count. Look at the curve.
- Was the decay flat? The load was too light. Add weight or volume next time.
- Smooth and gradual? Repeat the prescription. That’s a working set.
- Did peak force fall off a cliff in the last few reps? You stopped too late. Next time, cut one or two reps from the back of the set.
That’s the whole loop. The metric doesn’t tell you the set was bad — it tells you what shape mistake to fix. There’s a place for grinding sets, and tensr won’t lecture you about them; just don’t run grinding sets when you meant to run productive ones, and don’t call a flat set a hard one because it felt heavy on the first rep.
The set ended where the curve says it ended. The rep count was a coincidence.