Evaluating a prototype device designed to alleviate night vision goggle induced neck strain among military personnel

Determining the effects of AR/VR HMD design parameters (mass and inertia) on cervical spine joint torques

This study aimed to determine gravitational and dynamic torques and muscle activity of the neck across a series of design parameters of head mounted displays (mass, center of mass, and counterweights) associated with virtual and augmented reality (VR/AR). Twenty young adult participants completed five movement types (Slow and Fast Flexion/Extension and Rotation, and Search) while wearing a custom-designed prototype headset that varied the three design parameters: display mass (0, 200, 500, and 750 g), distance of the display's center of mass in front of the eyes (approximately 1, 3, and 5 cm anteriorly), and counterweights of 0, 166, 332, and 500 g to balance the display mass of 500 g at 7 cm. Inverse dynamics of a link segment model of the head and headset provided estimates of the torques about the joint between the skull and the occiput-first cervical vertebrae (OC1) and joint between the C7 and T1 vertebrae (C7). Surface electromyography (EMG) measured bilateral muscle activity of the splenius and upper trapezius muscles. Adding 750 g of display mass nearly doubled root mean square joint torques across all movement types. Increasing the distance of the display mass in front of the eyes by 4 cm increased torques about OC1 for the Slow and Fast Rotation and Search movements by approximately 20%. Adding a counterweight decreased torques about OC1 during the rotation and search tasks but did not decrease the torques experienced in the lower cervical spine (C7). For the flexion/extension axis, the magnitude of the dynamic torque component was 20% or less of the total torque experienced whereas for the rotation axis the magnitude of the dynamic torque component was greater than 50% of the total torque. Surface EMG root mean square values significantly varied across movement types with the fast rotation having the largest values; however, they did not vary significantly across the headset configurations.

The effects of whole-body vibration and head supported mass on performance and muscular demand

For military rotary-wing aircrew, little is known about the interactive effects of vibration exposure and the addition of head supported mass (HSM) on target acquisition performance, head kinematics, and muscular demand. Sixteen healthy male participants wore an aviator helmet with replica night vision goggles and completed rapid aiming head movements to acquire visual targets in axial and off-axis movement trajectories while secured in a Bell-412 helicopter seat mounted to a human-rated shaker platform. HSM configuration (with or without a counterweight (CW)) and vertical whole-body vibration (WBV) conditions (vibration or no vibration exposure) were manipulated as independent variables. WBV exposure degraded target acquisition performance and lengthened time to peak velocity of head movements. For yaw peak velocity in the axial movement trajectory, peak velocity was 9.9%, 11.6%, and 8.4% higher in the noCW + WBV condition compared to the CW + WBV, CW + noWBV, and noCW + noWBV conditions, respectively.Practitioner summary: The majority of military helicopter aircrew use a counterweight to counteract the anteriorly displaced load of night vision googles. This study was undertaken to better understand how helicopter vibration and counterweight use interactively affect performance and health-related measures during rapid scanning head movements.

Influence of night vision goggles with white and green phosphor screens on selected parameters of the eye and fatigue

In modern aviation, in particular in the military context, increasingly many aviation tasks are performed at night. To improve the safety of night flights, night vision goggles (NVGs) are commonly used. This study aimed to examine whether changes in ophthalmic parameters during NVGs use vary depending on phosphor screen type (green or white coded as P43 and P45 respectively). Thirteen participants were studied during a 2-h visual task in a night vision laboratory. Before and after NVGs use, we examined visual acuity, pachymetry, critical flicker-frequency thresholds, stereoscopic and contrast vision. During the use of NVGs, visual acuity, intra-ocular pressure and eye refraction were measured. We found no difference in visual performance between NVGs with green and white phosphor screens; however, NVGs use in general may lead to subjective eye fatigue, neck pain and headaches associated with the time of wearing and the weight of the helmet with additional equipment attached. Practitioner summary Night vision goggles (NVGs), widely used to improve the safety of night flights, were examined according to the applied type of the phosphor screen. There was no difference in visual performance between a white and green phosphor screens; however, NVGs and helmet manufacturers should strive to design these devices to be as lightweight as possible. Abbreviations: ANOVA: analysis of variance; CCT: central corneal thickness; CFF: critical flicker frequency; CNS: central nervous system; CS: contrast sensitivity; FOV: field-of-view; I2: image intensifier; IOP: intra-ocular pressure; NVGs: night vision goggles; SV: stereoscopic vision; VA: visual acuity; VAS: visual analog scale.

Posture and Helmet Configuration Effects on Joint Reaction Loads in the Middle Cervical Spine

INTRODUCTION: Between 43 and 97% of helicopter pilots in the Canadian Armed Forces report neck pain. Potential contributing factors include the weight of their helmet, night vision goggles (NVG), and counterweight (CW) combined with deviated neck postures. Therefore, the purpose of this investigation was to quantify changes in neck loads associated with posture, helmet, NVG, and CW.METHODS: Eight male subjects volunteered. They undertook one of five deviated neck postures (flexion, extension, lateral bending, axial rotation) times four configurations (no helmet, helmet only, helmet and NVG, and helmet, NVG, and CW). 3D kinematics and EMG from 10 muscles (5 bilaterally) drove a 3D inverse dynamics, EMG-driven model of the cervical spine which calculated joint compression and shear at C5-C6.RESULTS: The compression in the neutral posture was 116.5 (5.7) N, which increased to 143.7 (11.4) N due to a 12.7 N helmet. NVGs, weighing 7.9 N, also generated this disproportionate increase, where the compression was 164.2 (3.7) N. In flexion or extension, the compression increased with increasing head-supported mass, with a maximum of 315.8 (67.5) N with the CW in flexion. Anteroposterior shear was highest in the lateral bending [34.0 (6.2) N] condition, but was generally low (< 30 N). Mediolateral shear was less than 5 N for all conditions.DISCUSSION: Repositioning the center of gravity of the helmet with either NVGs or CW resulted in posture-specific changes to loading. Posture demonstrated a greater potential to reposition the head segment's center of gravity compared to the helmet design. Therefore, helmet designs which consider repositioning the center of gravity may reduce loads in one posture, but likely exacerbate loading in other postures.Barrett JM, McKinnon CD, Dickerson CR, Laing AC, Callaghan JP. Posture and helmet configuration effects on joint reaction loads in the middle cervical spine. Aerosp Med Hum Perform. 2022; 93(5):458-466.

Predicting Cervical Spine Compression and Shear in Helicopter Helmeted Conditions Using Artificial Neural Networks

OCCUPATIONAL APPLICATIONSMilitary helicopter pilots around the globe are at high risk of neck pain related to their use of helmet-mounted night vision goggles. Unfortunately, it is difficult to design alternative helmet configurations that reduce the biomechanical exposures on the cervical spine during flight because the time and resource costs associated with assessing these exposures in vivo are prohibitive. Instead, we developed artificial neural networks (ANNs) to predict cervical spine compression and shear given head-trunk kinematics and joint moments in the lower neck, data readily available from digital human models. The ANNs detected differences in cervical spine compression and anteroposterior shear between helmet configuration conditions during flight-relevant head movement, consistent with results from a detailed model based on in vivo electromyographic data. These ANNs may be useful in helping to prevent neck pain related to military helicopter flight by facilitating virtual biomechanical assessment of helmet configurations upstream in the design process.

A Novel Biopsychosocial Approach to Neck Pain in Military Helicopter Aircrew

INTRODUCTION: Flight-related neck pain (FRNP) is a frequently reported musculoskeletal complaint among military helicopter aircrew. However, despite its prevalence and suspected causes, little is known of the underpinning pain mechanisms or the impact of neck pain on aircrews in-flight task performance. The biopsychosocial (BPS) approach to health, combined with the contemporary conceptualization of musculoskeletal pain, in which injury and pain are not necessarily synonymous, provides a relatively new holistic framework within which to consider the problem of FRNP in military helicopter aircrew. Combining these concepts, a new conceptual model is proposed to illustrate how biopsychosocial factors may influence pain perception, potentially affecting aircrews capacity to process information and, therefore, threatening in-flight task performance. Recommendations are made for considering the underlying pain mechanisms of FRNP to aid prognoses and guide the development of holistic evidence-based countermeasures for FRNP in military helicopter aircrew. Development of instruments able to measure psychosocial factors, such as self-efficacy and functional ability, validated in the military helicopter aircrew population, would assist this task.Vail RE, Harridge SDR, Hodkinson PD, Green NDC, Pavlou M. A novel biopsychosocial approach to neck pain in military helicopter aircrew. Aerosp Med Hum Perform. 2021; 92(5):333341.

A novel passive neck orthosis for patients with degenerative muscle diseases: Development & evaluation

The current study evaluated the effect of a passive neck orthosis, developed for patients suffering from progressive muscular diseases, on neck muscle activity in 10 adult healthy participants. The participants performed discrete head movements involving pure neck flexion (-10 to 30°), pure neck rotation (up to 30° left and right) and combined neck flexion-rotation (-10 to 30°) in steps of 10° by moving a cursor on a screen to reach predefined targets and staying on target for 10 s. Surface electromyography (EMG) was recorded from upper trapezius and sternocleidomastoid muscles and amplitudes were averaged over the static phases in trials with and without the orthosis. Moreover, the variability in head position and time required to perform the tasks were compared between conditions. Wearing the orthosis caused significant reductions (p = 0.027) in upper trapezius activity (a change of 0.2-1.5% EMGmax) while working against gravity. The activity level of the sternocleidomastoid muscle increased (p ≤ 0.025) by 0.3-1.0% EMGmax during pure and combined rotations without any pain reported. The orthosis showed potential to reduce the activity level of the upper trapezius muscle, the main load bearing muscle of the neck. Further study will be carried out to evaluate the effect in different patient groups.

Validation of a three-dimensional visual target acquisition system for evaluating the performance effects of head supported mass

Night vision goggles (NVGs) enable aircrew to complete missions in the cover of night, but dramatically increase and alter the distribution of mass borne by the head. Our novel approach to visual target acquisition, based on Fitts' Law, was used to assess differences across three different performance metrics between low (L) and high (H) head supported mass (HSM) conditions. Fifteen healthy male participants completed time-optimal and reciprocal visual target acquisitions between target pairs arranged in four different movement trajectories. A significant interaction effect was found and subsequent post hoc analysis revealed that participants required more time to acquire the 20 mm target in the H-HSM condition. In the H-HSM condition participants had a higher error index during target acquisition and required more time to move off the target. Our approach demonstrates great promise in distinguishing performance decrements associated with the use of helmeted systems that include NVGs.