Sensorimotor integration for backward walking balance control

Introduction: Backward walking is challenging as motor output and sensory input are reversed in order and direction. Compared to forward walking, backward walking tends to be more cautious with slower, shorter and wider steps, more time spent in double support phase, and larger trunk and centre of mass (COM) movements (Soangra & Rajagopal, 2021).The altered control demands paired with unfamiliar sensory feedback during backward walking present unique challenges to balance with potential improvements by augmenting sensory input.Visual information is used in a feedforward manner to modulate limb movement, direction, and velocity of movement during walking (Patla, 1997). Without vision, gait speed, step length, and step width decrease, while trunk movement increases (Iosa et al., 2012). Haptic feedback from cutaneous and proprioceptive inputs orients an individual to the walking surface and the source of the haptic input (Jeka & Lackner, 1994). Adding haptic input can positively impact forward walking balance by reducing the size of trunk and COM excursions, and movement variability (Oates et al., 2017).As backward walking is used to evaluate and train walking balance control (Laufer, 2003), examining the influence of vision and haptic input in step and whole-body stability measures during backward walking with varying sensorimotor integration demands is important for understanding implications for people with balance impairments. The primary aim of this study was to examine the effects of added haptic input and availability of vision on step and balance control parameters during backward walking in young, healthy adults. Gender also has an influence on backward walking with older women walking slower and with shorter steps compared to older men (Hao et al., 2024).The secondary aim was to examine the influence of gender on backward walking with and without vision and added haptic input.Methods: Fifty-five participants (37 women, 28.1 (9.9) years) performed five trials each of backward walking for 10 m at their preferred speed, with eyes open and eyes closed, and with and without added haptic input. Haptic input was provided via haptic anchors (small weights, ~125 grams) attached to strings held by participants and dragged along the ground. Kinematic data from a 3-D motion capture system (Vicon Nexus) were used to generate a 12-segment full-body model (de Leva, 1996) using custom-written scripts in MATLAB (R2019b, MathWorks, Natick, MA, USA). Participant gender was assigned based on perceived gender identity.The step parameters analyzed included stride velocity (nSV) normalized to the square root of (lo *g), where g = 9.81 m/s² and lo= leg length; relative time spent in double support (%DS); step length (nSL) normalized by leg length; and step width (SW). Balance control parameters included the horizontal instantaneous dynamic margins of stability (MOS) (Hof et al., 2005): The anterior-posterior MOS (AP_MOS) was calculated as the distance between the extrapolated centre of mass (xCOM) and the toe of trailing limb. The mediolateral MOS (ML_MOS) was calculated as the distance between the xCOM and the closest lateral edge of the BOS.A repeated measures ANOVA compared the effect of vision (eyes open vs. eyes closed), haptic input (with or without the haptic anchors), and participant gender (woman vs. man) (α≤.01). Significant interaction effects were analyzed further using paired t-tests (vision or haptic input, α≤.0025). Participant height and mass were compared between women and men using independent t-tests. ANCOVAs using mass and/or height as a covariate were used to follow up on main effects of gender.Results: Main effects of vision revealed reduced nSV (p<.001), greater SW (p<.001), larger %DS values (p<.001), and a reduced AP_MOS (p<.001) when walking backwards with eyes closed compared to eyes open. Main effects of haptic input included reduced ML_MOS with the haptic anchors (p<.001) compared to without. Follow-up analyses of a vision x haptic input interaction for nSL (p<.001) showed shorter steps with eyes closed compared to eyes open (p<.001) except for when participants were using the haptic anchors when there were no differences in nSL (p=.073).Mass (p=.007) and height (p<.001) were significantly greater for men compared to women. There was a main effect of gender for AP_MOS (p=.003) and ML_MOS (p<.001) with men having larger values than women. Follow-up ANCOVAs with mass and height as covariates eliminated the main effect of gender for both AP_MOS (p=.417) and ML_MOS (p=.023) and showed that mass was a significant covariate for ML_MOS (p=.01).Conclusions: Visual input was used to control step parameters and stability during backward walking as evidenced by slower and wider steps, more time in double support, and placement of the xCOM closer to the trailing edge of the base of support. Adding haptic input reduced lateral stability but limited the impact of no vision on step length. Gender differences in walking stability were influenced by gender-based differences in body size with particular influence of mass on lateral stability. Overall, results point to vision as important sensory input for backward walking with the impacts of eyes closed being partially mitigated by the addition of haptic input. Examining influence of sex and gender should be continued for people with balance impairments.