When Larry and I built the first Plyomat prototypes, we expected small differences compared with existing systems. What we didn’t expect was a consistent inflation of 0.05–0.08 seconds in flight time; the Just Jump controller was reading jumps roughly 4–7 inches higher than ours…which was both confusing and discouraging to say the least!
Oscilloscope tests confirmed it; the discrepancy wasn’t random.
At first we blamed hardware latency, but that margin was too large to be explained by circuitry alone. The deeper we looked, the more we realized this wasn’t a flaw, it was a legacy.
Many side-by-side validations, using modern switch mats, force plates, and even an oscilloscope to detect signal timing have confirmed that the Just Jump system consistently overestimates flight time by 0.05–0.08 seconds. This results in jump heights being inflated by 4–7 inches compared to modern, validated systems.
It’s important to note that the Just Jump calculates jump height the same way as other switch mat technology (shown below); however, there is specific inflation added to it’s firmware. Plyomat was able to discern this inflation, approximately 11.19% difference in flight time values and add it as a feature within the app to toggle it on or off if the user wishes to do so.
Important note:
How switch mats calculate jump height - Flight time is detected the moment both feet leave the mat and return, with an actuation force of about 15-20lbs to trigger the plates inside the switch mat to touch and produce a signal. Like all switch mat technology, the physics equation below translates that flight time into vertical jump height.
The Just Jump Mat, introduced in the early 1990s by Paul Mackovjak, arrived when the Vertec “jump-and-reach” system was still the accepted gold standard. Because the Vertec naturally yields higher readings (it includes reach extension, body lean, and visual estimation error), early coaches expected bigger numbers.
My working theory is that Mackovjak adjusted the Just Jump’s internal equation so that its flight-time output matched the Vertec benchmarks coaches already recognized. In other words, the inflation was intentional, a bridge for credibility in an era before validation papers or force-plate comparisons existed.
That design choice worked; the Just Jump became the industry staple for thirty years. Unfortunately, that same inflation later became associated with all switch-mat technology, even though no other system reports jump heights that high.
Force plates calculate jump height by integrating vertical ground-reaction force to determine take-off velocity, then applying
h = (v²) / (2g)
This method isolates the true airborne phase. Switch mats, including the Just Jump and Plyomat, estimate jump height from flight time using
h = (g × t²) / 8
A switch mat is a binary circuit, not an analog one.
It operates on a simple open/closed state:
The controller’s microprocessor records the exact timestamps of each transition (from closed → open and open → closed).
It doesn’t “sample continuously” in the analog sense like a force plate does with load cells, which record voltage changes at high sampling frequencies (often 1000+ Hz).
Plyomat’s switch circuit operates with sub-25 millisecond latency and near-zero debounce, capturing exact open and close events without delay.
While force plates sample continuously at high frequency, Plyomat records discrete transition times with equal consistency and precision for flight-time calculations.
|
System |
Signal Type |
Sampling |
Output |
|
Force Plate |
Analog (continuous voltage from load cells) |
Continuous at 500–2000 Hz |
Force-time curve (impulse integration possible) |
|
Switch Mat (Plyomat) |
Digital (binary on/off state) |
Event-driven |
Flight time between transitions (no force data) |
Three key studies frame the discussion.
True height (m) = (0.8747 × JJS height) – 0.0666
In short, the Just Jump is reliable but consistently high. Their data validated what we had already seen firsthand.
Investigated the criterion validity and test–retest reliability of the Plyomat compared to a force plate in 48 Division III football players. Results showed a very strong correlation (r = 0.95 [0.91–0.97], p < 0.001) with the force plate and excellent test–retest reliability (ICC = 0.85 [0.69–0.92]; SEM = 2.36 cm). Jump heights from the Plyomat were only about 1 cm higher on average, a negligible difference (d = 0.16).
Together, these findings confirm that while earlier systems inflated values, the Plyomat provides force-plate-level validity and reliability, offering coaches a practical and affordable field alternative for vertical-jump testing.
We knew coaches had decades of historical Just Jump data; erasing that would erase context. To preserve continuity, we created a selectable calibration inside the Plyomat controller called Just Jump Mode.
When toggled on, the system multiplies true flight-time values by 1.1119 before converting to vertical-jump height. That coefficient came from hundreds of simultaneous trials between both systems. The result reproduces the inflated Just Jump output almost exactly.
In other words, Plyomat can operate in two realities:
|
Parameter |
Plyomat Mode |
Just Jump Mode |
Difference |
|
Average Flight Time |
0.505 s |
0.562 s |
+11.19% |
|
Reported Jump Height |
24.8 in |
29.2 in |
+4.4 in |
|
RSI/GCT Tracking |
Full |
Full |
None |
|
Purpose |
Precision |
Continuity |
— |
“Just Jump Mode preserves historical data while Plyomat Research Mode aligns with force-plate standards.”
Every jump-testing system converts flight time to height using h = (g × t²) / 8. Because the Just Jump’s firmware lengthens t, we reverse-engineered the scale factor that would recreate its results.
t (Just Jump) = 1.1119 × t (Plyomat)
That single multiplier expands flight time by 11.19 percent, yielding the same jump height inflation of 4–7 inches observed historically. It’s built into the firmware and perfectly reversible, allowing users to toggle accuracy or continuity with one setting.
For coaches managing data across systems, I’ve also created a conversion spreadsheet that instantly translates numbers to and from Just Jump and Plyomat values. You can access it here:
👉 Just Jump ↔ Plyomat Conversion Sheet
As Coach Mike Boyle often reminds us, data is only useful if you can compare it to something. His teams have collected Just Jump numbers for decades, and those values form the language of his program. Changing devices without a translation would break that continuity.
That principle shaped our decision: modern accuracy should never invalidate historical progress. The Plyomat lets coaches preserve their legacy metrics while moving forward with better precision.
|
Situation |
Best Setting |
Rationale |
|
Starting a new testing program |
Plyomat Mode |
Matches force-plate data; preferred for validation and RSI analysis |
|
Maintaining a multi-year Just Jump database |
Just Jump Mode |
Preserves continuity with past records |
|
Comparing different facilities or technologies |
Use conversion sheet |
Normalizes all data to a single standard |
Telling an athlete they have a “30-inch vertical” means something. Deflating that to 25 inches for the sake of technical purity can hurt morale, especially if their recruitment depends on it. So consider than when making the change and it’s worth the battle.
Even if inflated, the numbers are consistently inflated. That makes them trustworthy for relative progress tracking.
In summary, The Just Jump made jump testing affordable and accessible in the field, and as a practitioner myself, I am forever grateful for that! Plyomat makes it accurate and historically compatible. Precision matters, but so does continuity.
Just Jump Mode exists so coaches never have to choose between the two.