Quantifying locomotor instability in healthy adults: Insights from treadmill-induced slips and trips using a dynamic criterion.

BACKGROUND

Understanding how the musculoskeletal and nervous systems maintain balance during walking remains challenging. Instability is difficult to quantify in real-life settings and no fully developed wearable system is currently available to measure it outside the laboratory. Treadmills provide a safe, compact, standardized solution to generate surrogate slips and trips, enabling precise and repeatable measurements. Accurate assessment is critical for identifying at-risk of falls populations and evaluating rehabilitation.

OBJECTIVE

To this aim, we tested a 3D dynamic criterion - the dynamic instability vector (DIV), which norm (DIVnorm) quantifies the distance between the body's center of mass and the minimal moment axis of external forces. We aim to capture spatiotemporal patterns of instability in response to external perturbations, including magnitude and timing of extrema.

METHOD: Thirty-eight healthy adults walked on a dual-belt instrumented treadmill (1.2 m·s-1) and experienced 8 slips and 8 trips at heel strike (target speeds: 2.04 and 0.36 m·s-1), each at two intensity levels (accelerations: 3 and 10 m·s-²), for a total of 608 randomized perturbations.

RESULTS

Perturbations caused a rapid increase in instability, with distinct response patterns across conditions. Surrogate slips elicited larger and faster DIV extrema than trips, including a characteristic two-peak response (DIVnorm), whereas trips showed a single or absent peak structure. These effects were modulated by perturbation intensity (p < 0.001), with significant interactions between perturbation type and intensity (p < 0.05, p < 0.001).

DISCUSSION: These findings highlight the ability of the DIV to discriminate distinct instability signatures across perturbation types and levels. Future work will focus on wearable implementation for real-world gait monitoring.