Load Cell Hysteresis Is Making Your IMTP Scores Read Low: How to Diagnose and Re-Zero It

A strength staff runs the standard three-trial IMTP protocol on a college linebacker mid-preseason. Trial 1 comes in at 2,610 N. Trial 2, after the usual 60-second rest, comes in at 2,655 N - the athlete looks and sounds like he's pulling harder, chalk flying, bar creaking under the strap. Trial 3, same rest, same visible effort, same grunt at lockout: 2,538 N. On paper that reads like fatigue or a technique breakdown late in the session, and it gets written up that way. The athlete didn't get weaker between trial 2 and trial 3. The load cell's zero point did.

Every strain-gauge load cell used in IMTP and isometric squat rigs carries a small but real property called hysteresis: at the same actual applied force, the sensor doesn't output quite the same value depending on whether it's approaching that force from below or settling down to it from above. Load the cell to near-maximum and release it, and the internal strain element doesn't spring back to true zero instantly - it creeps back over several seconds to tens of seconds. If a tester zeroes the system during that creep window, the athlete's next trial gets measured against a stale, elevated baseline, and every subsequent force sample - including the number that ends up in the report - reads lower than what the athlete actually produced.

Hysteresis is not sensor malfunction - it's a documented property of every strain-gauge load cell, specified on the manufacturer's data sheet as a percentage of rated output. For the S-beam and shear-beam cells built into most IMTP rigs, that figure typically sits around 0.02-0.05% of full scale. On a 5,000 N cell, that sounds trivial: a loop width of maybe 1-2.5 N. The problem isn't the loop width itself - it's what happens to the internal signal in the seconds right after a near-maximal pull, before the strain element finishes settling.

Picture the loading history of a single IMTP session. The cell sits at true zero before trial one. The athlete pulls past 2,600 N, holds, then relaxes. Output doesn't fall straight back to zero - it decays along a curve, often still reading several newtons positive ten or fifteen seconds later, especially if a strap or chain hasn't been fully slackened between trials. If the software, or the tester manually, captures a ‘zero’ snapshot during that decay - exactly what happens when a fixed rest period cues the zero regardless of what the baseline is doing at that instant - it locks in an inflated reference point. By the time the athlete initiates the next pull a few seconds later, the true baseline has kept decaying downward, so every force sample from that trial gets compared against a zero that was already too high. The trial's peak, and any rate-of-force-development window built off that same baseline, comes out systematically low - not randomly noisy, but biased in one direction, trial after trial, for as long as the rig keeps carrying residual tension between attempts.

Two bodies of published work bear directly on this, even though neither sets out to measure hysteresis by that name. Comfort et al. (2019, Strength and Conditioning Journal) published the field's standardization paper for the IMTP, and one of its explicit procedural recommendations is to zero the load cell immediately before every single trial rather than once at the start of a testing session. The authors' rationale lines up exactly with the mechanism above: preload from the bar, the athlete's own body weight settling onto the handle, and residual tension left over from the prior effort can all shift what the system treats as zero, and re-taring only at session start lets that shift accumulate silently across every trial that follows. The limitation is that this is a synthesis and standardization document, not a controlled experiment - it tells a tester what to do and why it plausibly matters, but it doesn't isolate how many newtons a given rig's hysteresis alone is worth, because that number depends on the specific cell, its age, and how it's mounted.

Dos'Santos, Jones, Comfort and Thomas (2017, Journal of Strength and Conditioning Research) come at the adjacent problem from the analysis side: they compared several onset-threshold methods for identifying where a pull actually starts in the force-time trace, including thresholds set as a multiple of the resting baseline's own noise. Peak force proved relatively insensitive to threshold method, shifting only marginally, but early-phase variables - force at short fixed time points and rate of force development from onset - shifted considerably more, since both are calculated relative to wherever the software decides the baseline sits. That is the same vulnerability hysteresis-driven zero creep exploits: any variable measured relative to a resting baseline inherits whatever error is sitting in that baseline, and the earlier in the pull a variable is calculated, the larger the relative damage a few newtons of baseline error does to it. The limitation here is that the manipulation was analysis-side threshold choice in a single testing session, not a physical hysteresis stress-test of the transducer itself - it demonstrates the downstream consequence of a wrong baseline without measuring how large a real hysteresis-driven baseline error typically gets on a given rig, which is why that has to be characterized locally, per instrument, using the protocol below.

Run this once per load cell - it takes about fifteen minutes and only needs repeating after a recalibration, a hardware swap, or if drift complaints resurface months later.

Equipment. A calibration mass or certified reference weight (20-50 kg of plates works fine), the rig's raw unfiltered force-time output logged at 1,000 Hz or higher rather than just the summary peak-force number, and a stopwatch or the DAQ's own timestamp.

Procedure.

1. With no load on the cell, record 10 seconds of quiet baseline and note the raw zero value.
2. Apply the calibration mass at a loading rate similar to how an athlete actually pulls - not a slow, gentle set-down - hold it in place for 5 seconds, and record the raw output (this is the ascending reading).
3. Remove the mass in one motion and immediately begin logging the raw output continuously, without re-zeroing, for a full 60 seconds.
4. Compare the value logged in the instant after removal (the descending reading, at the same nominal load point as step 2 if you catch it on the way down) against the ascending reading from step 2 - the gap between them is the hysteresis loop width, in newtons and as a percentage of the cell's rated capacity.
5. From the same 60-second decay trace, find how many seconds it takes the raw output to settle back within a small band of the true zero recorded in step 1 - call this the recovery time.

Normal ranges. Under 0.1% of rated capacity (5 N or less on a 5,000 N cell), with recovery under 15-20 seconds, is healthy and rarely causes a meaningful under-read once trials are spaced past that recovery window. In the 0.1-0.3% range, or recovery stretching past 30-45 seconds, the standard rest period most IMTP protocols use isn't enough time to settle - fix it procedurally: lengthen the gap before zeroing, and zero from an averaged 2-3 second window instead of an instantaneous sample. Above roughly 0.3%, or a zero that never recovers even after 60+ seconds, points to diaphragm fatigue or physical damage that needs recalibration or replacement, not a scheduling fix.

One mistake shows up often enough to flag on its own: leaving the strap, chain, or clevis attachment between the bar and the load cell under partial tension between trials because it's ‘close enough’ to re-rack quickly. A cell under sustained partial load cannot complete its hysteresis recovery at all - it just keeps creeping at whatever rate that residual tension allows. Fully slacken the attachment between every trial, not just between sets.

Take the linebacker from the introduction. His rig's cell showed a loop width of about 0.18% of its 5,000 N rating (roughly 9 N) and a recovery time of 40 seconds when characterized with the protocol above - past normal, but not damaged. The team's testing SOP called for a fixed 60-second rest between trials and zeroed the system automatically at the 55-second mark, five seconds before the next pull cue. That five-second gap is where the error crept in.

Trial 3's baseline was still elevated by 11 N when the system zeroed, but by the time force onset actually occurred five seconds later, the cell had relaxed further to +7 N. The 4 N gap between those two numbers is what got subtracted incorrectly, understating trial 3 by roughly that amount - small on its own, but it compounds: trial 3 carried more residual drift than trial 2, which carried more than trial 1, because rest periods weren't long enough to let each trial's loading history fully clear before the next began. Extending the rest to 90 seconds and zeroing from a 3-second averaged window closed the gap to under 1 N across all three trials in a re-test the following week.