Metabolic Demands and Kinematics During Level Walking in Darkness With No Vision or With Visual Aid

Abstract

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.

Increased Metabolic Demand During Outside Walking in Darkness With No Vision or With Visual Aid

“…The notion that acute, temporary impairment of vision reduces the mechanical efficiency during walking was subsequently confirmed in treadmill experiments, during which blindfolding resulted in approximately 20% increase in energy expenditure during level walking, both at slow 1 and moderate pace. 2 …”

“… 10–12 During level walking on a treadmill, i.e., at a preset walking speed, the elevated metabolic demand induced by blindfolding the walker was to a large extent attributable to the concomitant reduction in step length. 1 , 2 In addition, increased vertical foot clearance and, to a lesser degree, increased step width likely contributed to the reduced mechanical efficiency in the unsighted walker. 1 Previously, step variability has also been reported to increase during overground walking in persons with closed eyes.…”

“…Moreover, during treadmill-level walking in complete darkness, neither the elevated metabolic demand nor the reduced step length of unsighted walking was completely counteracted by using a visual aid, i.e., night vision goggles (NVGs). 2 Since the NVGs provide good conditions for visual acuity but a visual field that is limited to 40°, this suggests that even under highly predictable and obstacle-free conditions, not only the foveal vision but also the peripheral visual field plays a key role in adjusting the walkers’ movement pattern so as to minimize energy expenditure. 2 …”

“… 2 Since the NVGs provide good conditions for visual acuity but a visual field that is limited to 40°, this suggests that even under highly predictable and obstacle-free conditions, not only the foveal vision but also the peripheral visual field plays a key role in adjusting the walkers’ movement pattern so as to minimize energy expenditure. 2 …”

“…We reasoned that the aforementioned 20% increase in metabolic demand observed in unsighted subjects walking at a preset pace on the treadmill 1 , 2 may not accurately reflect the effects of vision on metabolic demand in more realistic conditions. Hence, a walker with restricted vision might adjust to a less energy-demanding movement pattern if the speed is not consistently enforced.…”