On neck load among helicopter pilots : effects of head worn equipment, whole body vibration and neck position

Abstract

Introduction: Helicopter pilots complain of pain originating from the neck region. The causes are still basically unknown, but the ergonomic situation when flying a helicopter, with unfavorable load caused by static neck and body positions, whole-body vibration and heavy head-worn equipment, has been suggested as a risk factor.

Aim: The aim of the work reported in this thesis was to quantify the effects of external loads on helicopter pilots necks, and to evaluate different methods for measuring neck load.

Methods: Thirty-nine Swedish military helicopter pilots participated in the five studies. The effects of different neck and body positions, head-worn equipment and vertical whole-body vibration were evaluated concerning neck muscle activity, induced mechanical load and seat-to-head transmissibility. Surface electromyograms (EMG) were recorded from upper and lower dorsal neck muscles, the sternocleidomastoid, and the upper trapezius. The induced load on the lower cervical spine was calculated using a sagittal, static, biomechanical model, and vibration transmissibility was calculated as the ratio of recordings from helmet-mounted accelerometers and vertical vibration acceleration measured at the seat. The neck and body positions evaluated were: neutral, neck flexion 20° (for muscle activity, induced load and transmissibility), neck rotation 30° (EMG), and trunk inclination 20° (EMG). The head-worn equipment evaluated was: helmet alone, helmet and night vision goggles (NVG), and helmet, NVG and counterweight (all evaluated using EMG, induced load and transmissibility). Vibration was evaluated at different frequencies (2.5-30 Hz) and magnitudes (0.5, 1, and 2 m/s 2) using EMG and transmissibility.

For the reliability testing of a neck fatigue protocol, the pilots performed isometric contractions in neck flexion and extension for 45 s, sustaining a force representing 75 % of maximum strength in a seated position. Subjective fatigue was rated using the Borg CR-10 scale. The test was repeated twice the first day and then two additional times with one-week intervals. Variables analyzed were the slope of the median frequency change, the normalized slope, and the ratings after 15, 30 and 45 s; and also the initial median frequency (IMDF). The intra-class correlation (ICC) and the measurement error (S w), intra- and inter-day were calculated.

Results: Dorsal neck muscle activity increased by 3-4 % of maximum voluntary electrical activation (MVE) as a cause of neck rotation, 2-3 % of neck flexion, and 1.5-2.5 % of trunk inclination. The use of NVG increased muscle activity in upper neck by 0.5-1.5 % and in lower neck by about 0.5 %. Results with added counterweight were about the same as with NVG. Muscle activity increased by about 0.5-1 % MVE as a function of vibration at frequencies around 4-5 Hz, with the higher levels when the neck was flexed. Muscle activity was also affected by vibration magnitude, where lower-neck-muscle and trapezius activity increased at the highest vibration level at frequencies around 4-5 Hz. The induced load was also affected by both neck flexion and NVG. The load at 20° flexion increased by about 8 % of maximum voluntary contraction (MVC) compared to neutral and by about 3 % MVC when adding NVG compared to using helmet only. The load decreased somewhat when a counterweight was added. The transmissibility peak in a vertical direction was highest when the head was in neutral position and the fore-and-aft transmissibility peak was highest when the head was flexed. There were no effects of head-worn equipment concerning vertical transmissibility, but the fore-and-aft transmissibility peak level was lower with NVG. Different magnitudes of vibration gave only minor effects on transmissibility. The best reliability for the slope was found for the 45 s intra-day analysis, taking all measurements into account (ICC 0.65-0.83). Reliability after 30 s was poorer but still acceptable (ICC 0.52-0.71). For the subjective ratings, the highest reliability was found after 30 s inter-day (ICC 0.86-0.88). IMDF showed generally high reliability for the intra-day analyses (ICC 0.63-0.80).

Conclusion: All three proposed risk factors caused measurable changes in muscle activity, induced load and seat-to-head transmissibility. Of the three, neck and body position caused the highest response. EMG and seat-to-head transmissibility responded somewhat different as function of vibration indicating that effects of vibration should be measured using more then one outcome measure. The protocol for measuring neck muscle fatigue can be considered reliable for use in further research. Since performing a contraction of 75 % of maximum was quite strenuous, it is recommended that the protocol period be shortened to 30 s.