Early Postural EMG Dynamics During Optic Flow: Effects of Time, Velocity, Retinal Location and Laterality

Background/Objectives: Optic-flow velocity is known to influence postural sway, whereas corresponding electromyographic (EMG) effects may be difficult to detect when muscle activity is averaged over long stimulation periods. We therefore investigated the early temporal organisation of muscle activity following optic-flow onset and examined whether this response was modulated by stimulus velocity, retinal stimulation location, and laterality. Methods: Surface EMG recordings from the tibialis anterior (TA), soleus (SOL), trapezius descendens (TD), and paraspinal C4 muscles were reanalysed from a previously published dataset of 18 healthy adults exposed to radial optic flow at five velocities and three retinal stimulation configurations. RMS activity was quantified in two consecutive post-stimulus windows, 0-200 ms and 200-400 ms, and analysed using repeated-measures ANCOVA with baseline activity entered as a covariate. Results: All four muscles showed greater activation during the 200-400 ms window than during the 0-200 ms window, indicating a consistent temporal increase in EMG activity shortly after optic-flow onset. Additional effects were muscle-specific. In TA, the temporal increase was greater during large-field than peripheral stimulation. In SOL, temporal modulation varied with velocity and body side. TD showed a significant effect of velocity. C4 activity was greater on the right than on the left side. Conclusions: Early postural muscle activity shows a common temporal increase during the first 400 ms after optic-flow onset, together with significant stimulus modulation and laterality. These findings suggest that time-resolved bilateral EMG can reveal aspects of the early postural response that may be hidden when muscle activity is averaged over longer periods.