A Multivariable Decision Rule for Weight-Rocking-Driven Onset Detection Method Bias on Bilateral Force Plates Across 32,952 Countermovement Jump Trials.

Bilateral vertical ground reaction force (vGRF) plates are the dominant sensor for field-deployed neuromuscular monitoring in athletes, with the countermovement jump (CMJ) being the dominant test. Detecting movement onset on the vGRF trace is a single decision that propagates through impulse-momentum integration to bias jump height. Existing onset method comparisons used fewer than 100 athletes, and bilateral weight rocking, a form of pre-jump postural sway during quiet standing, has not been characterized at scale as a method-specific source of disagreement. We analyzed 32,952 NCAA Division I CMJ trials from 579 athletes across 15 teams recorded on bilateral vGRF plates. Five hypotheses were pre-specified, and a per-trial decision rule was developed under leave-one-athlete-out cross-validation. Pooled, 45.89% of trials showed false-early firing on at least one of five non-first-derivative onset methods, relative to a rate-of-change reference, at a 30 ms threshold. On the rocking-amplified subset, a bidirectional body weight band method overestimated jump height by 0.943 cm versus the reference. The decision rule reached an area under the receiver operating characteristic curve of 0.71 with near-perfect calibration. It is a per-trial screening flag, not a deterministic classifier, and deployment should anchor on balanced-accuracy or high-precision operating points, holding the onset method constant across an athlete's monitoring timeline.