Sagittal and Frontal Inter-Plane Balance Control Variability is Minimised at Preferred Walking Speed in Healthy Young Adults.

Reduced preferred walking speed (PWS) is a known indicator of increased fall risk, particularly sideways falls in individuals with reduced physical capacity, often attributed to decreased mediolateral stability in the frontal plane. The underlying mechanisms and the impact of walking speed on inter-plane balance control coordination remain unclear. The current study aimed to develop a novel analytical framework for quantifying the sagittal and frontal inter-plane coordination of COM-COP control and its variabilities during level walking via continuous relative phase (CRP) angles and to study the walking speed effects. Fifteen young adults walked at PWS, slower and faster, while the COM-COP inclination angles (IA) and their rates of change (RCIA) were measured to calculate inter-plane CRP patterns. The deviation phases (DP) of CRP curves quantified the variability of inter-plane coordination. The inter-plane balance control coordination and variability were significantly influenced by walking speed, with minimum DP values occurring during double limb support at PWS. Walking at one's PWS represents the best compromise for minimising control variability between the sagittal and frontal planes. This novel approach highlights the role of inter-plane balance control variability in determining PWS in young adults and emphasises the importance of monitoring these variables for fall risk assessment.