A simple thought experiment. You do “3×10 with the heavy band” today. Next week, your hand position drifts an inch closer to the anchor. The band’s starting tension is lower, the stretch through the rep is shorter, peak tension comes earlier. Same color band, same rep count on the page. A fundamentally different stimulus.
Now scale that across cables, selectorized stacks, suspension trainers, and any setup where the resistance varies through the range of motion. The traditional set log — sets × reps × weight — was a tool built for the bar. For everything else, it has been telling you a story that does not match what your muscle did.
What impulse actually is
Impulse is the integral of the force–time curve over a rep, set, or session. Plainly: how heavy was the load, multiplied by how long it was on you, summed up across the whole interval.
The unit is newton-seconds (N·s). A 200 N load held for 5 seconds is 1000 N·s. A 100 N load held for 10 seconds is also 1000 N·s. A 200 N load that ramps from 100 to 300 N over 5 seconds is, again, about 1000 N·s. Different muscular signatures; the same total mechanical work-time.
In tensr, every set produces a per-rep impulse number, a set total, a session total, and — once you log enough movements — a weekly impulse rolled up by movement and muscle group. They all derive from the same integral. The number is honest because nothing is being inferred from a label. It is what the sensor saw.
Why it matters
Three things break loose once you have impulse on a screen.
Variable resistance gets a unit. “3×10 with the heavy band” is finally something you can compare to “3×10 with the medium band” — or to last month’s cable session, or to next week’s selectorized stack. Banded work has been guess-and-feel for decades because reps and weight don’t capture the curve. Impulse does.
Cross-modality progression becomes real. A band session today, a cable session next week, a stack session the week after — different machines, different resistance profiles, the same N·s readout. You stop asking “did I work harder than last week” by feel.
Weekly volume is denominated in your unit, not your spreadsheet’s unit. “Add a set” or “swap a movement” change the weekly impulse rolled up by muscle group in measurable amounts. Your training plan is finally written in a language that survives equipment changes.
What to track together
Impulse lives at five scopes. Each one answers a different question.
| Scope | What it tells you |
|---|---|
| Rep impulse | The shape of one rep — concentric vs eccentric impulse, how much of the rep was hard. |
| Effective impulse | Impulse counted only where force was ≥ 80% of rep peak. The “stimulating” portion. |
| Set total impulse | The dose of one set, including the easy reps and the hard ones. |
| Session total impulse | Today’s total mechanical exposure, summable across movements. |
| Weekly impulse / muscle group | Volume in the unit your training plan should have always used. |
You don’t watch all five at once. You watch the level that matches the decision you’re trying to make. Set impulse for “did this set hit the dose”; weekly impulse per muscle group for “is my back getting enough work this week”; rep-level for “did the eccentric carry its weight.”
(TUT, the previous post’s metric, is one input to impulse — TUT counts seconds, impulse weights those seconds by force. Use TUT when load is constant. Use impulse when it isn’t, which is most of the time.)
What gear it needs
Honest at every supported sample rate. Impulse is an integral, and integrals are forgiving — under-sampling smooths out at the level of an integrated curve. An 8 Hz crane scale gives you set impulse you can trust to within a few percent. Sub-rep features (the shape of force build-up, eccentric vs concentric split) get noisier at low rates, but the headline number is solid everywhere.
This is the metric that lets a $30 BLE scale actually replace your set log.
What to do tomorrow
Pick one movement. Record one session. Note its set total impulse for each working set, and the session total. Write the numbers down. Next week, beat them.
That’s it. Don’t worry yet about how the impulse was distributed within the set — tempo, effective reps, and within-set decay are the next three posts. For now: the integral exists, it has a unit, and you just acquired a target.
The set log measures what you planned. Impulse measures what your muscles did. Train to the second one.