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Joint reaction forces across all postures and helmet conditions. A single asterisk denotes significant differences at P < 0.05, double asterisks P < 0.01, and triple asterisks P < 0.001. A) Compression. The dashed horizontal line indicates 10% of the estimated compressive tolerance (390 N) of the C5-C6 functional spinal unit (3.9 kN). B) Anteroposterior shear acting on C5. These can be thought of as the required anteriorly directed force sustained by the joint to prevent C5 from accelerating posteriorly. The bold dashed line (50 N) indicates 10% of the estimated shear tolerance (500 N) of the C5-C6 functional spinal unit. C) Mediolateral shear.
A) Experimental set-up for MVE trials. The robotic arm was attached to the posterior aspect of the helmet and subjects performed exertions in flexion, extension, rotation, and lateral bending from this position. B) (Top row) The helmet conditions examined in this study. From right to left, No Helmet, helmet only (hOnly), helmet and night vision goggles (hNVG), and the helmet, night vision goggles, and counterweight (hNVG + CW). (Bottom row) The OpenSim cervical spine model demonstrating the five posture conditions used in the study. There is no representative geometry for the helmet, but it is represented mathematically.
Center of mass (CoM) positions for the helmet only condition (hOnly) compared to the night vision goggles (hNVG) without and with (hNVG + CW) conditions.
Changes in the flexion-extension moment with posture and helmet condition. Significance is annotated as indicated in Fig. 1, with a single asterisk denoting P < 0.05, double asterisks P < 0.01, and triple asterisks P < 0.001.