When you set up a session, the contraction-intent setting tells the analysis layer what kind of contraction the patient is going to produce. That decision changes which metric tiles render on the result and which metrics roll into the patient's longitudinal dashboard. The setting is not cosmetic. The same trial computed as if it were an MVIC versus computed as if it were a ballistic RTD-only trial produces different surfaces because the metrics that mean something on one contraction type don't mean anything on another.
Three options:
- MVIC (maximal voluntary isometric contraction). Maximum effort, ramp to peak, sustained plateau, controlled release. Produces all six metrics.
- RTD / ballistic (sometimes called "pulse-like"). Maximum effort, explosive rise to peak, then immediate release. No sustained plateau. Produces peak and the RTD windows; steadiness tiles are correctly hidden.
- Submaximal hold. Target force well below maximum, held steadily for several seconds. No explosive rise to characterize. Produces steadiness metrics computed against the target; RTD windows are hidden.
Why the contraction type changes the metrics that appear
The metric definitions assume a specific signal shape. Force off these expectations doesn't produce a wrong number, it produces a meaningless one.
MVIC. The signal has an onset, a ramp, a plateau, and a release. Every metric is well-defined: peak is the maximum value, RTD windows are time-locked slopes off onset, plateau metrics are statistics over the sustained region.
RTD / ballistic. The signal has an onset and a sharp rise, but the rise is the goal, not a means to a plateau. There is no plateau to compute nRMSE, CV, or Yank across, because the patient released as soon as they hit peak. Showing those tiles for a ballistic trial would be reading variability metrics off a falling release tail, which is unrelated to the patient's motor control. The ballistic trial is also where the literature shows the largest rate-of-force-development values, with peak RFD running roughly 21% higher than the same patient's ramp-and-hold contractions on knee extension (Kozinc et al. 2022). When the assessment goal is RFD, ballistic is the right protocol; when both RFD and peak force are wanted, run them as separate sets.
Submaximal hold. The signal has a slow controlled rise (often a pretension followed by a deliberate climb to the target), a long sustained hold at submaximal force, and a release. Peak is uninteresting (it's whatever the patient happened to hit during the hold, possibly the target plus a small overshoot). The RTD windows are not the protocol's goal: the patient was instructed to ease into the target, not to fire explosively. Steadiness across the hold is what's being measured.
What the curves look like
The same patient can produce all three shapes with the same dynamometer, just different cues.
The ballistic trial has the early-window dynamics of an MVIC without the plateau. The submaximal hold has the plateau without the explosive rise. The platform's job is to compute only the metrics that belong on the curve in front of it.
Why this matters for longitudinal tracking
Sessions are grouped by joint and action, but they are also grouped by contraction intent. An MVIC peak from one visit and a ballistic peak from the next visit are not directly comparable: the ballistic peak will usually be lower (the patient released before reaching the MVIC ceiling). Trends on the patient dashboard separate by contraction intent for this reason. If you set up a follow-up visit with the wrong intent, the new session shows up on the wrong trend and the dashboard reads as if the patient regressed.
This is also why the contraction-intent selector matters even when you're confident the patient is going to produce something close to your expected shape. The setter is the contract between you and the analysis layer, not a label on the trial.
What the literature says
The choice between contraction types is not a stylistic preference; it changes the underlying neuromuscular phenomenon being probed. Even motor cortex activity reflects this directly: high-density recordings during a force-tracking task show that different force-profile families (static holds, slow ramps, fast oscillations, chirps) engage distinct neural state-space regions, with the same muscle activation potentially driven by very different neural dynamics depending on the subskill being recruited (Amematsro et al. 2025). When the protocol changes from "fast-and-hard" to "fast-only", the patient is performing a different motor task at the cortical level, not the same task with a different label. The metric set has to reflect that.
The early-window neural-vs-contractile dependence further reinforces the protocol stratification: the 0 to 50 ms window is neurally driven and the 100 ms window is contractile (Del Vecchio 2022), so the windows themselves are answering different questions even within an MVIC trial. Splitting the protocol into ballistic-only versus hold-only is one way to interrogate those questions more cleanly.
What to do next
- Pick the contraction type before the patient pushes, not after. The setting is part of the protocol, not a post-hoc label.
- For return-to-sport batteries, run a sequence: MVIC for peak and strength, ballistic for RFD, submaximal hold for steadiness. Three separate sets across the same joint. Each one contributes a different facet to the patient's profile.
- If a trial's curve does not match the intent you set (e.g. you asked for a ballistic and the patient held), exclude the trial and re-run with a clearer cue. Don't reframe an MVIC as a ballistic post-hoc; the trial is on the patient's record under the intent you set.
For the broader context on why RTD is read across two windows in the first place, see what RTD Early actually means.
References
- Kozinc Ž, Pleša J, Djurić D, Šarabon N. Comparison of rate of force development between explosive sustained contractions and ballistic pulse-like contractions during isometric ankle and knee extension tasks. Applied Sciences. 2022;12(20):10255. doi:10.3390/app122010255
- Amematsro EA, Trautmann EM, Marshall NJ, et al. Motor cortex flexibly deploys a high-dimensional repertoire of subskills. bioRxiv. 2025. doi:10.1101/2025.09.07.674717
- Del Vecchio A. Neuromechanics of the rate of force development. Exerc Sport Sci Rev. 2022. doi:10.1249/JES.0000000000000306