Postural control in response to simultaneous six-dimensional pseudorandom platform perturbations

Introduction: Postural sway evoked by pseudorandom platform perturbations (Peterka, 2002) has strong potential to predict fall risk (Maki et al., 1994). To model naturalistic environments that help quantify fall risk, novel two-dimensional (2D) platform motions were recently developed (Wagner et al., 2024). However, no prior study has characterized postural responses when 6D platform perturbations are provided simultaneously. Such a model is critical for understanding how the central nervous system (CNS) weighs and integrates concurrent vestibular and proprioceptive inputs. Classically, increasing 1D pitch amplitude leads to a reduction in frequency response functions (FRFs) due to the relative down-weighting of proprioceptive feedback (Peterka, 2002). We hypothesized that increasing perturbation amplitude would induce a global reduction in FRFs across six dimensions during simultaneous 6D platform perturbations.Methods: Twelve healthy young adults (ages 19–34) were subjected to 6D platform perturbations, each of which was constructed by summing six non-harmonically related frequencies (0.02–1.36 Hz). This sum of sines (SoS) approach enabled spectrally separated stimulation across six dimensions – anterior-posterior (AP), medial-lateral (ML), earth-vertical (EV) translations, right anterior-left posterior (RALP), left anterior-right posterior (LARP), and yaw rotations – using a Moog motion platform (Nieuw-Vennep, Netherlands). Testing was done with eyes closed and arms in a relaxed position at the side. Each participant completed one quiet stance trial and three perturbation trials (low, medium, and high peak-to-peak amplitudes) in random order. Kinematic responses were captured using a 9-camera Qualisys system (Gothenburg, Sweden) that tracked a marker cluster placed over the sacrum to approximate center of mass (CoM) location. 6D displacement of the sacral cluster was processed using a discrete-time Fourier transform (DFT), and FRFs were computed by normalizing spectral responses to the stimulus at each of the discrete perturbation frequencies.Results: Spectral analyses indicated that postural responses at all stimulus frequencies were significantly increased for medium and high peak-to-peak amplitudes compared to quiet stance (Benjamini-Hochberg adjusted p < 0.05). Additionally, postural responses were significantly greater in the perturbed plane compared to those in the unperturbed planes, which suggests that humans can generate spatially distinct postural responses in each dimension during simultaneous 6D platform perturbations. A two-way repeated measures MANOVA on the complex-valued FRFs revealed a significant effect of perturbation amplitude on AP, ML, EV, and yaw responses (Pillai’s Trace, AP: p < 0.001, ML: p = 0.001, EV: p = 0.002, yaw: p = 0.001). However, RALP and LARP responses did not show significant amplitude-dependent modulation (RALP: p = 0.474; LARP: p = 0.094) for the range of amplitudes tested. Specifically, increasing stimulus amplitude led to increased AP, ML, and EV FRFs but decreased yaw FRFs.Conclusions: The use of 6D SoS stimuli spectrally separated 6D postural responses that increased with stimulus amplitude. When normalized by the stimulus, we observed that increasing perturbation amplitudes elicited significantly increased FRFs in response to each of the three translation stimuli (AP, ML and EV) and reduced FRF in response to yaw rotation. These data suggest (1) that the inertial effects associated with increased translation stimuli yield greater normalized sway of the CoM at higher frequencies and (2) that the larger yaw rotation stimuli may yield more effective cues from the semicircular canal. Unlike early literature (Peterka, 2002), the effect of amplitude was not statistically significant for the two tilts (RALP and LARP). The lack of significant modulation in RALP and LARP tilts above 0.5 Hz could reflect simultaneous up-weighting of otolith contributions and down-weighting of canal contributions to postural control. The impact of amplitude on RALP and LARP tilts below 0.5 Hz might be associated with down-weighting of ankle proprioceptive contributions. This 6D platform perturbation paradigm may offer a high-resolution balance assessment for identifying the impact of sensory deficits on postural control in patients such as individuals with vestibular dysfunction or peripheral neuropathy. This work establishes the feasibility of expanding the paradigm to 6D platform + 6D visual perturbations, enabling even more naturalistic motions to assess fall risk.