Two athletes pull on a load and hit the same peak force — say, 800 N. The strength log shows the same number for both. They are not the same athlete.
One got to 800 N in 180 ms. The other took 700 ms.
In any sport that ends in under a second — sprinting, jumping, throwing, climbing, contact — the second athlete’s late-arriving peak force never participates. The rep ended before they could use it.
Strength is a number on a stack. Force you can use right now is something different.
What rate of force development actually is
Rate of force development (RFD) is the slope of the force–time curve in the first ~200 ms of a max effort. Plainly: how quickly your nervous system can get a muscle from rest to its high-tension state.
Three closely related metrics:
- Concentric peak slope (RFD) — the maximum dF/dt during the first ~0–200 ms of the concentric phase, in N/s.
- Time to peak (TTP) — milliseconds from rep onset to rep peak.
- Eccentric loading rate — how fast force builds during the eccentric (catching/yielding) phase. Often higher than concentric RFD; trained eccentric stiffness is a different animal.
Why it matters
The velocity-based-training crowd has been chasing this exact concept for a decade — through bar-mounted accelerometers, linear position transducers, AI-powered phone cameras. All proxies for “how fast did the movement happen, and what does that tell us about the athlete’s force-production machinery.”
A force sensor measures it directly. There’s no estimation, no derived velocity. The slope is the slope.
Three places it matters:
Sport-specific contact time. A vertical jump foot-on-ground time is 200–400 ms. A boxing punch is sub-100 ms. A climbing pull-up off a small hold is similarly fast. Peak force you can produce in 600 ms is irrelevant; what arrives in 200 ms is what shows up in the sport.
Detraining detection. RFD drops faster than peak force when training stops. A lifter coming back from a layoff often has 90% of their max strength back in two weeks but only 60% of their RFD. Conditioning watches the slope, not the headline.
Programming for explosiveness. If you train slow heavy work exclusively, RFD declines even as max strength rises. You become slower-strong. Whether that’s desirable is sport-dependent — but it should be deliberate, not a side effect.
What to track together
The explosive triplet, viewed as one block.
| Metric | What it tells you |
|---|---|
| Concentric RFD (N/s) | The headline. How fast force comes online. |
| Time to peak (ms) | The duration view. Same idea, different unit. |
| Eccentric loading rate (N/s) | How fast you absorb. The “stiffness” component. |
The reading worth attending to first is concentric RFD. TTP is the same number expressed in time rather than rate; pick whichever your athletes’ coaches think in. Eccentric loading rate is a separate quality — a strong eccentric isn’t the same skill as a fast concentric, and athletes can be high in one and low in the other.
What gear it needs
80 Hz only. RFD is a slope metric, and slopes fall apart at low sample rates. At 8 Hz you have one sample every 125 ms; a 200-ms RFD window is barely two samples. The number you’d compute is more noise than signal.
The sensor honesty matrix marks RFD as ❌ on the 8 Hz scales — meaning we don’t ship the metric there. A Tindeq Progressor (~80 Hz) or a C100 with the high-rate firmware retrofit is the floor.
This is the cleanest example in the whole metric catalog of “ship it where it works, don’t ship it where it lies.” A WH-C06 owner gets honest TUT, honest impulse, honest set TUT — but the RFD readout is hidden until they have hardware that can support it. That’s the contract.
What to do tomorrow
A single max-effort isometric pull, recorded.
Set up the sensor for a movement you can grip with full force from a stable position — a deadlift bar at mid-shin height, a cable at chest height, a hangboard edge. On the count of three, pull as hard as you can for 5 seconds. tensr captures the curve; the slope from rest to peak is your concentric RFD baseline.
Re-test weekly. The number should be stable session-to-session within ±10%; bigger drops flag detraining, illness, or sleep debt before subjective wellness catches up to it.
Strength is what you can do given as much time as you want. RFD is what you can do right now.