Predictive neuromuscular simulations of slip-perturbed sit-to-walk reveal the combined effects of quadriceps weakness and sensorimotor delay.

Abstract

Sit-to-walk (STW) is an unstable transition in which age-related quadriceps weakness and slowed sensorimotor processing may combine to elevate slip-related fall risk. This study quantified how vastus weakness and neural transmission delay, alone and together, affect stability and recovery mechanics during slip-perturbed STW. We used predictive neuromuscular simulations of a sagittal-plane musculoskeletal model to impose a standardized slip-like disturbance (20 N anterior force to both heels for 0.1 s at t = 1.0 s) across a 5 × 5 factorial grid: vastus strength scaled to 100/95/90/85/80% and neural delay increased by 0/+10/+20/+30/+40%. Outcomes over three phases (Stand-up and Preparation, Initial Perturbation Response, and post-perturbation stepping phase) included center-of-mass displacement/velocity summary metrics displayed as heatmaps, joint kinematics, model-predicted activations, joint loads, and bilateral asymmetry indices. Nineteen of 25 conditions completed the task; six failed, all under combined deficits. Isolated impairments (weakness up to -20% or delay up to +40%) were tolerated. Failures clustered at (-10% strength, +20% delay), (-10% strength, +30% delay), and all +40% delay combinations with any tested weakness, defining a clustered failure region within the tested grid rather than a singular threshold. Delay increased Phase-2 center-of-mass excursions, while weakness shifted coordination toward hip-dominant strategies and increased asymmetry; combined deficits markedly amplified hip joint-load peaks. Within this controller and perturbation setup, single-axis deficits were largely compensable, but their combination produced a concentrated failure region and high-cost, hip-dominant post-perturbation stepping strategies. These results should be interpreted as sagittal-plane, model-based hypotheses that warrant future experimental validation.