Effects of an actuated ankle exoskeleton on walking stability in healthy adults: a controlled laboratory study

Background Ankle exoskeletons are widely used to reduce the metabolic cost of walking, yet their effects on walking stability during unperturbed gait remain insufficiently understood. Walking stability can be characterized using complementary measures that capture stride-to-stride variability, global temporal organization, and local dynamic stability. Understanding how ankle exoskeleton assistance influences these different aspects of gait stability is essential for the safe design and control of wearable robotic devices. Methods Eighteen healthy adults walked on a treadmill at a constant speed (1.1 m/s) with and without an actuated bilateral ankle exoskeleton in a randomized crossover design. Spatiotemporal variability was quantified using coefficients of variation (CoV) of stride length, step width, and stance ratio. Global gait stability was assessed using detrended fluctuation analysis of stride time. Local dynamic stability was evaluated using maximum Lyapunov exponent calculated for the trunk, hip, upper leg, lower leg, and foot. Paired-sample one-sided t-tests were used to compare conditions. Results Walking with the ankle exoskeleton resulted in increased stride-to-stride spatiotemporal variability, reflected by higher CoV values (stride length, p < 0.001; step width, p = 0.037; stance ratio: p = 0.002), while mean stride length and step width remained unchanged. Mean stance ratio was reduced in the exoskeleton condition (p < 0.001). Global gait stability did not differ between conditions, indicating preserved long-range temporal gait organization. Local dynamic stability increased at the lower leg (p < 0.001) and foot (p = 0.009) when walking with the exoskeleton, consistent with enhanced stability at segments close to the assisted ankle joint. In addition, local dynamic stability at the hip was significantly increased (p = 0.038). Conclusions Ankle exoskeleton assistance alters gait control across multiple levels during steady walking. While stride-to-stride variability increased, global gait stability remained unchanged. Local dynamic stability was increased at distal segments and the hip, suggesting both direct distal and indirect proximal effects of ankle-level assistance. These findings indicate that ankle exoskeleton assistance can affect local gait stability across body segments and provide insight for the design and control of ankle exoskeletons with respect to their stability-related effects during walking.