INTRODUCTION: Uniformed services commonly perform foot-borne operations at night, while using visual aid in terms of night vision goggles (NVG). During slow-level walking, complete lack of visual input alters kinematics and markedly increases the metabolic demand, whereas the effect on kinematics and energy expenditure of restricting the peripheral visual field by wearing NVG is still unknown. The purpose was to evaluate whether metabolic demands and kinematics during level walking are affected by complete darkness with and without visual aid.

MATERIALS AND METHODS: Eleven healthy men walked on a treadmill (inclination: +2.3°, velocity: 4 km/h) with full vision in a lighted laboratory (Light), and in complete darkness wearing either a blindfold (Dark), or restricting the visual field to about 40° by wearing monocular (Mono) or binocular (Bino) NVG. Oxygen uptake ($\dot{\text{V}}$O2) was measured to evaluate metabolic demands. Inertial measurement units were used to estimate kinematics, and the outcome was validated by using a motion capture system. Ratings of perceived exertion, discomfort, and mental stress were evaluated after each condition using a Borg ratio scale. Physiologic and kinematic variables were evaluated using repeated measures analysis of variance (ANOVA), whereas ratings were evaluated using non-parametric Friedman ANOVA.

RESULTS: $\dot{\text{V}}$ O2 was 20% higher in the Dark (1.2 ± 0.2 L/min) than the Light (1.0 ± 0.2 L/min) condition. Nominally, $\dot{\text{V}}$O2 in the Mono (1.1 ± 0.2 L/min) and Bino (1.1 ± 0.2 L/min) conditions fell in between those in the Light and Dark conditions but was not statistically different from either the Light or the Dark condition. Step length was shorter in the Dark (-9%, 1.22 ± 0.16 m) and Mono (-6%, 1.27 ± 0.09 m) conditions than in the Light condition (1.35 ± 0.11 m), whereas the Bino (1.28 ± 0.08 m) condition was not statistically different from either the Light or the Dark condition. The three conditions with no or limited vision were perceived more physically demanding, more uncomfortable, and more mentally stressful than the Light condition, and the Dark condition was perceived more mentally stressful than both NVG conditions.

CONCLUSIONS: The study confirms that complete lack of visual cues markedly reduces the mechanical efficiency during level walking, even under obstacle-free and highly predictable conditions. That $\dot{\text{V}}$O2 and step length values for the NVG conditions fell in between those of the Light and Dark conditions suggest that both foveal and peripheral vision may play important roles in optimizing the mechanical efficiency during level walking.