Peak Anteroposterior Heel Slip Acceleration Predicts Lateral Trunk Flexion During Unexpected Slip Perturbations

Slip-induced falls are a major contributor to hip fractures and injury, particularly during sideways falls where lateral trunk flexion drives impact to the hip. While past studies have focused on sagittal-plane slip mechanics, the mechanisms of inducing frontal-plane trunk excursion remain poorly understood. This study investigated which slip foot kinematic variables predict lateral trunk flexion during unexpected slips. Twenty-six healthy young adults experienced an unexpected slip while walking in a laboratory setting. Peak anteroposterior (AP) heel slip distance, velocity, and acceleration, as well as mediolateral (ML) slip distance, velocity, and acceleration, were measured using three-dimensional motion capture. A stepwise multiple linear regression identified peak AP heel slip acceleration as the sole significant predictor of lateral trunk flexion (p = .026, R ² = 0.19). One-way repeated measures ANOVA and one-tailed paired t-tests further revealed that peak heel acceleration occurred significantly earlier than both peak heel velocity (p = .012) and the onset of lateral trunk flexion (p < .001), supporting its role as an early destabilizing mechanism. In contrast, neither velocity nor slip distance predicted lateral trunk flexion magnitude. These findings suggest that peak AP heel acceleration is associated with inducing lateral trunk flexion movements that likely precede lateral falls. Reducing heel acceleration through slip-resistant flooring or footwear, or enhancing compensatory strategies such as reactive arm movements, may reduce the likelihood of losing sideways balance. This work highlights the need for balance training interventions that emphasize rapid responses to early-phase slip dynamics.