Reading drift warnings

Two warning banners can appear on a session after the metrics compute. Pre-loaded tare warns that the strap was already loaded before the contraction began (which biases peak and shifts onset). Post-release rebound warns that the trace did not return to baseline after the release (which usually means the strap migrated under load). This walks what each banner is detecting, what it means clinically, and whether to act on it.

Updated May 15, 2026

The platform inspects every recorded trial for a small set of baseline-quality issues that aren't obvious from looking at the metric tiles. When it finds one, it surfaces an inline warning banner on the session page. The banners describe a property of the recording, not a failure of the platform; they exist so a clinician can decide whether the trial is still usable given the issue, override the onset if needed, or exclude and re-run.

Two warnings come up often enough to be worth understanding in detail.

Pre-loaded tare warning

What it says. "Possible tare drift detected. Force increased by [N] over [ms] before the intentional onset."

What the platform is detecting. A measurable linear drift in the baseline between the start of the recording and the auto-detected onset. The detector fits a regression line to the pre-onset window and reports the slope plus R² of the fit. When the slope is steep enough that the implied force change exceeds a threshold over the rest window's duration, the banner fires.

What it usually means clinically. Three common physical causes:

  1. The strap was tightened during the rest window. The clinician adjusted the strap after the device was tared but before the patient pushed. The dynamometer reads the increasing load as a "drift" because the tare baseline is now stale.
  2. The strap is slowly creeping under static load. Some webbing strap materials elongate slightly under continuous tension over a few seconds. If the rest window is long enough, the elongation shows up as a slow rise in the baseline.
  3. The patient is pre-tensioning involuntarily. This is harder to spot at the chart level but produces the same pattern: a slow rise in force during the rest window because the patient is gradually loading the strap without realizing it.

How to read the banner.

  • If the drift is small (a few Newtons over a one to two second rest window), the platform's baseline correction probably absorbed it cleanly. The metrics are usable. The banner is informational.
  • If the drift is large (tens of Newtons over the same window), the auto-detected onset shifted. The early-window RTD is computed against a baseline that doesn't reflect the patient's actual resting state. Consider overriding the onset or excluding the trial.
  • If the banner appears on every trial in a set, the cause is procedural rather than incidental. Retare between trials, check whether the strap is being adjusted mid-set, or switch to a strap material with less creep.

For why some resting pretension is actually desirable on tension dynamometers (it makes countermovement visible), see taring the dynamometer and what a good baseline looks like. Pretension and tare drift are different things: pretension is a static offset the baseline correction handles, drift is a changing baseline over the rest window that biases onset detection.

Post-release rebound warning

What it says. "Post-release force did not return to baseline."

What the platform is detecting. After the patient releases at the end of the trial, the force trace should return to (approximately) the same level it started at. The detector compares the post-release tail of the signal to the pre-onset baseline and fires the warning when the difference exceeds a threshold.

What it usually means clinically. Two common physical causes:

  1. The strap migrated under load. During the maximum-effort contraction, the strap shifted distally or proximally on the limb, and now the resting load on the strap is different from where it started. The dynamometer is reading a different baseline post-release because the rig geometry has changed.
  2. The strap stretched under load. Less common with modern straps, but webbing or buckle hardware can take a small permanent set after a maximum effort. The next trial in the set starts from a different baseline.

Why it matters even though peak and plateau are computed against the pre-onset baseline. The trial's own metrics are usually fine because the platform's baseline correction subtracts the pre-onset offset. The problem is the next trial in the set, which inherits the new baseline. If you don't retare between trials, the next trial's pre-onset window starts at the wrong level, and the platform's correction can't tell that the level shifted between trials versus during a trial.

How to read the banner.

  • If it appears on a single trial within a set, retare before the next trial and continue. The trial in question is usually still usable; the metrics on it were computed against the right (pre-onset) baseline.
  • If it appears on multiple trials in a set, the strap is slipping under load. Tighten the strap, switch to a different strap or attachment point, or accept that this session is going to need a re-tare between every trial.
  • If the trace post-release sits substantially below baseline (rather than above), the gauge may be in a position where the strap is now pulling against it in compression. Check the rig geometry; this is unusual but happens occasionally with creative strap setups.

When neither banner fires but the baseline still looks wrong

The platform's warning thresholds are tuned to surface clear issues without crying wolf. There's a quiet middle ground where the baseline is slightly off but not enough to trip a banner. Two things to watch for visually on the chart:

  • The pre-onset baseline is noisy (fluttering rather than flat). Usually a patient who hasn't settled or a loose strap. The detector still finds an onset, the metrics still compute, but the baseline-noise envelope is wider than usual and onset placement is less precise than usual.
  • The post-release trace doesn't quite settle back to pre-onset baseline but the difference is small. Subthreshold strap migration. Retake the trial if you can, accept it if you can't, but be aware that the trial's "baseline" is partly a feature of the rig at this point.

For the broader discussion of when these patterns warrant exclude, see when to exclude a trial.

What to do next

  • A banner is information, not an alarm. Read it, decide, move on. Most banners on a single trial within an otherwise-clean session don't require any action beyond awareness.
  • Repeated banners across a set point at procedure, not at the patient. Retare habits, strap material, or rig setup are the things to change.
  • Pre-loaded tare warnings and post-release rebound warnings are independent. A trial can fire one, the other, both, or neither. The pre-onset and post-release windows are different parts of the signal and the platform inspects them separately.

References

  • Tillin NA, Pain MTG, Folland JP. Identification of contraction onset during explosive contractions. Response to Thompson et al. J Electromyogr Kinesiol. 2013;23(4):991-994. doi:10.1016/j.jelekin.2013.04.015
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