Ankle Torque Variance Is a Better Indicator of Balance Control Performance Than Plantar Perceptual Sensitivity Threshold

We explored whether ankle torque variability or plantar perceptual threshold explains human balance control more effectively. We hypothesized that ankle torque variance is a better indicator of center of pressure (COP) velocity variance than plantar perceptual sensitivity. Two conditions were tested: loaded (23 kg vest added) and unloaded, as loading should diminish plantar sensitivity and increase COP velocity variability. We created a linear feedback model to assess the noise change in the sensorimotor loop induced by loading. Plantar sensitivity was quantified using a psychophysical approach while participants stood barefoot. A linear motor applied a force impulse on the participant's heel. A 'yes-no' method of limits was selected to identify plantar sole sensory thresholds in both conditions. We observed reduced plantar sensitivity in loaded compared to unloaded conditions. In the loaded condition, participants exhibited greater COP velocity variance, with significant positive Pearson's correlations confirming a substantial association between ankle torque and COP velocity variances for both loaded (variance accounted for (VAR): r 2 = 44.56%, p = 0.018) and unloaded conditions (VAF: r 2 = 58.83 %, p = 0.004). No significant correlation existed between COP velocity variance and plantar sensitivity threshold for both loaded (VAF: r 2 = 0.002 %, p = 0.99) and unloaded conditions (VAF: r 2 = 21.81%, p = 0.35). The model confirmed a ~88% rise in sensorimotor loop noise in the loaded condition. Ankle torque variance assesses the precision of non-perceptual and perceptual detection mechanisms in evaluating whole-body motions and the accuracy in converting sensory cues into ankle torque.