Virtual reality does not fool the brain only: spinal excitability changes during virtually simulated falling

Virtual reality (VR) is known to induce substantial activation of brain's motor regions. It remains unclear to what extent virtual reality can trigger the sensorimotor system, and more particularly, whether it can affect lower nervous levels. In this study, we aimed to assess whether VR simulation of challenging and stressful postural situations (Richie's plank experience) could interfere with spinal excitability of postural muscles in 15 healthy young participants. The H-reflex of the triceps surae muscles was elicited with electrical nerve stimulation while participants were standing and wearing a VR headset. Participants went through several conditions, during which stimulations were evoked: standing still (noVR), standing in VR on the ground (groundVR), standing on the edge of a building (plankVR), and falling from the building (fallingVR). Myoelectrical activity of the triceps surae muscles was measured throughout the experiment. Leg and head movements were also measured by means of accelerometers to account for body oscillations. First, no differences in head rotations and myoelectrical activity were to be noted between conditions. Second, triceps H-reflex (H_MAX/M_MAX) was not affected from noVR to groundVR and plankVR. The most significant finding was a drastic decrease in H-reflex during falling (-47 ± 26.9% between noVR and fallingVR, P = 0.015). It is suggested that experiencing a postural threat in VR efficiently modulates spinal excitability, despite remaining in a quiet standing posture. This study suggests that simulated falling mimics the neural adjustments observed during actual postural challenge tasks.NEW & NOTEWORTHY The present study showed a modulation of spinal excitability induced by virtual reality (VR). In the standing position, soleus H-reflex was downmodulated during a simulated falling, in the absence of apparent changes in body oscillations. Since the same behavior is usually observed during real falling, it was suggested that the visual cues provided by VR were sufficiently strong to lead the neuromuscular system to mimic the actual modulation.