Supplementary Material for: Age-Related Associations Between Reactive-Balance Stepping Responses, Self-Induced Stepping and Gray Matter Volume

Background. The distinction between reactive and proactive balance control mechanisms in terms of age-related structural neural correlates is still scarce. From a biomechanical perspective, reactive stepping is a rapid response to sudden loss of stability, whereas proactive self-induced stepping requires anticipatory postural adjustments and longer duration. This study aims to explore how cortical and sub-cortical gray matter volume correlates with variables of reactive and proactive stepping responses among older and young adults; and whether these stepping responses can be distinguished from one another in terms of their structural neural correlates (i.e., cortical and sub-cortical gray matter volume).
Methods. Twenty-six older adults and nine young adults underwent structural MRI brain scanning. Self-induced stepping variables were derived from ground reaction force data, while kinematic parameters of reactive stepping, including step thresholds, were obtained using a three-dimensional motion capture system. Age-related differences in ground reaction force measures, stepping kinematics, and gray-matter volume in ten regions of interest were examined, followed by partial correlation analyses.
Results. Age-related impairments in reactive and proactive stepping performance were accompanied by significantly smaller gray-matter volume among older adults across all regions of interest (p≤0.003), except for the brainstem (p=0.026; post-correction significance level: p<0.005). Partial correlation analyses including both older and young adults revealed significant associations whereby longer balance recovery durations and lower stepping thresholds were associated with lower gray-matter volume in prefrontal and cortical regions, and in the putamen and amygdala (r=-0.41 to -0.60, p≤0.037; and r=0.38 to 0.43, p≤0.048, respectively). In self-induced stepping performance, longer preparation and step durations were significantly associated with lower pre-central, cerebellar and amygdala gray-matter volume (r=-0.51 – -0.56, p≤0.006).
Conclusion. Age-related differences in reactive stepping deficits were primarily associated with lower gray-matter volume in prefrontal, cortical, putamen, and amygdala regions, whereas self-induced stepping impairments were associated with precentral, cerebellar, and amygdala gray matter volumes. These findings suggest distinct neural substrates underlying reactive versus self-initiated balance control. Further investigation needs to explore whether intervention programs in older adults may change gray matter volume in these regions.