Helmeted head and neck dynamics under whole-body vibration.

Abstract

The military aviator's helmet functions as both protective gear and as an equipment mounting platform for devices that enhance aviator performance. These devices include night vision goggles, head-up displays, sighting systems, and other gear such as multiple visors and chemical protective masks. This additional equipment alters the helmet mass and center of mass, which in turn, increases the load on the muscles of the neck. This study investigated sagittally symmetric changes in head supported mass (M) and center of mass (CM) for effects on head and neck posture, acceleration, and neck myoelectric responses under whole-body vibration. Twelve volunteer male aviators were exposed to short duration axially swept sinusoidal vibration while wearing twelve different helmet configurations. These configurations consisted of three masses of 2, 3, and 4 Kg, and four center of masses of $-$2, 0, 2, and 4 cm measured relative to the head center of mass in the Anterior/Posterior (AP) direction. Acceleration measurements were made obtaining pitch, AP, and axial motion at T1 and the AO complex. Postural measurements were made using high speed video recordings of the subject's profile. Myoelectric responses were measured using bipolar surface electrodes placed over the right and left infrahyoid, sternocleidomastoid, splenius capitus, and trapezius muscles. Results showed that postural static moments remained constant with no significant changes in head, neck, or trunk angle as a function of vibration frequency, M, or CM parameters. Acceleration measurements showed significant increases in head pitch acceleration at resonance, but no changes in resonant frequency, for changes in M and CM. Consistent resonant frequencies suggest the head and neck system is not a passive system but is actively controlled by the muscles of the neck. Myoelectric responses in the neck showed significant increases in peak RMS activity for posterior muscles and were highly correlated with head pitch acceleration (r $>$ 0.80). Force calculations about the combined head and helmet center of mass were correlated (r $>$ 0.85) with calibrated posterior neck muscle responses. Acceleration, myoelectric, and force calculation results suggest that significant differences are seen in the responses when added helmet mass exceeds 82 $\pm$ 23 N$\cdot$cm relative to the AO complex.