# Optic Flow Simulating Self-Motion Does Not Modulate the Hoffmann Reflex in the Soleus During Upright Standing in Healthy Young Adults

## TL;DR

Optic flow increases postural sway but does not change the soleus reflex in healthy young adults during standing.

## Contribution

Shows that optic flow alone does not alter spinal reflexes, suggesting supraspinal mechanisms control posture in virtual environments.

## Key findings

- Optic flow increases postural sway but does not modulate the soleus H-reflex amplitude.
- Supported standing confirms that optic flow alone does not alter segmental reflexes.
- Postural control under optic flow likely relies on supraspinal mechanisms, not spinal reflex modulation.

## Abstract

What are the main findings?
Optic-flow stimulation increases postural sway but does not induce modulation of the soleus Hoffmann reflex, regardless of optic-flow complexity.Supported-standing trials confirm that optic flow alone (without postural engagement) fails to alter segmental reflex.

Optic-flow stimulation increases postural sway but does not induce modulation of the soleus Hoffmann reflex, regardless of optic-flow complexity.

Supported-standing trials confirm that optic flow alone (without postural engagement) fails to alter segmental reflex.

What are the implications of the main findings?
Optic flow alone is insufficient to alter Ia-monosynaptic reflex gain in non-threatening contexts.Postural control in response to virtual optic flow could mainly depend on supraspinal mechanisms under non-threatening conditions.

Optic flow alone is insufficient to alter Ia-monosynaptic reflex gain in non-threatening contexts.

Postural control in response to virtual optic flow could mainly depend on supraspinal mechanisms under non-threatening conditions.

Background/Objectives: Visual motion is a powerful contributor to postural control, yet its influence on modulation of the Ia afferent pathway remains to be confirmed. This study investigated whether optic-flow simulating self-motion modulates the soleus Hoffmann (H) reflex recorded in the soleus during upright stance in immersive virtual reality. Methods: Fourteen healthy adults completed two experimental sessions, each comprising four visual conditions of increasing optic-flow complexity. In one session, participants stood freely on a force platform (free standing) whereas in the other, postural sways were restricted (supported standing). Surface EMG, posterior tibial nerve stimulation, and force-platform recordings were collected. Results: During free standing, optic flow substantially increased postural sway [F(3,13) = 15.7, p < 0.001, η2 = 0.55], with higher sway in all optic-flow conditions (~13 mm/s) compared with static viewing (~10 mm/s). In contrast, soleus H-reflex amplitude was not modulated by optic flow [F(3,13) = 0.2, p = 0.57], remaining stable across conditions (~44% Mmax). Background EMG and CoP position preceding stimulation were similar across conditions. In supported standing, used to isolate the effect of optic flow independently to postural control, H-reflex amplitude again showed no condition effect [F(3,13) = 0.2, p = 0.86]. Conclusions: These findings indicate that postural perturbation induced by optic flow was not accompanied by a modulation of the Ia afferent-motoneuron transmission of the soleus under the used experimental conditions. The results suggest that postural control under virtual optic flow is mediated predominantly by supraspinal sensory-integration mechanisms, rather than by modulation of the Ia-monosynaptic reflex pathway.