Is the room moving? Muscle responses following visual perturbations

Postural adjustments are essential for balance control and to reduce risk of falling. One emerging method to train reactive postural control consists in exposing individuals to safe and controlled destabilizing perturbations that intend to simulate changing conditions that can lead to falls. Studies using virtual reality suggest that visual perturbations engage mechanisms of motor adaptation, increase electrocortical activity and modulate balance performance. What is not yet clear is the impact of trunk and limb muscles activation on the postural adjustments responsible to maintain balance control. This paper aims to map the response of trunk and limb muscles to visual perturbations, and compare them to those of physical perturbations. Additionally, our study includes vertical perturbations (i.e. balance disturbances in the vertical plane) known to be a major cause of falling. Therefore, this paper also compares muscles responses to both horizontal and vertical perturbations. Fourteen healthy participants (ten males; age: 27±4; BMI: 23.8±2.6 kg/m²) stood on a moveable platform within a virtual reality system projecting visual scenes over a 360° dome-shaped screen such that the participant appeared to be standing in the middle of a room. Concomitantly, the electrical activity of tibialis anterior, gastrocnemius, rectus femoris, hamstring, rectus abdominis, paraspinal, external oblique and deltoid muscles was captured. Amid a larger protocol, this paper reports on randomly presented 1) visual perturbations; i.e. the virtual room moves during 0.35 seconds a distance corresponding to 14 cm in four directions (forward - FP, backward - BP, upward - UP, downward - DP), each repeated three times; and 2) physical perturbations (12cm displacement in one second) for the four directions and two sensory conditions: static camera (SC; virtual room remains static) and dynamic camera (DC; corresponding transitions in the visual scenery). We calculated three muscle activation parameters: onset latency, duration of activation, and magnitude. Separate 2-factor repeated-measures ANOVA were applied for each outcome measure across factors of perturbation direction (FP, BP, UP and DP) and condition (VIS, SC, DC). Forward visual perturbations led to longer onset latencies when compared to upward and downward visual perturbations (e.g. in the gastrocnemius: respectively, 443±56.6 ms vs. 326±39.6 ms and 334±51.1 ms, P<0.05). Duration of activation was longer following downward visual perturbations than after backward visual perturbations in the rectus femoris (respectively, 630±120 ms vs 335±81.2 ms, P<0.05). All lower limbs and the paraspinal muscles presented with a longer onset latency in response to visual perturbations in comparison to both types of physical perturbations (SC and DC) (P<0.05). The magnitude of activation following visual perturbations was smaller than both types of physical perturbations in all muscles (P<0.05). Duration of activation was also longer in the gastrocnemius following visual perturbations when compared to both SC and DC conditions of physical perturbations (P<0.05). Overall, magnitude of responses was often larger following horizontal perturbations in comparison to vertical perturbations. Our results suggest that visual perturbations alone activate limb and trunk muscles. Although perturbation direction seems to regulate the timing of response following visual perturbations in some limbs muscles, no differences were observed in the magnitude of activation within visual perturbations. Physical perturbations significantly increased EMG responses compared with visual perturbations. Overall, horizontal perturbations often led to faster and more intense responses than vertical perturbations. Our findings that different types of perturbations lead to more or less intense muscle responses and to different activation timing may have translational benefits for the optimization of rehabilitation strategies implementing destabilizing perturbations and oriented to persons at risk of falling.