Rotary wing pilot neck pain is increasing in prevalence. This may be due to the combined effects of additional head supported mass (e.g., night vision goggles (NVG), head mounted displays) and wholebody vibration (WBV). Not only is the one per revolution frequency of most helicopters within the range of frequencies where the human spine is most sensitive to injury/damage, but the neck is also constantly stabilizing the head to counter the aircraft vibrations. Previous flight tests indicated that a minimal detuning of the track-and-balance of the main rotor of the NRC Bell 412 (increased vibration by ~0.006 g) resulted in a substantial increase in vibration at the pilots head (a ~0.01 g increase). Although such increases in vibration were classified as safe (i.e., unlikely to result in any health issues) according to the ISO 2631-1 standard, physiological measures of the pilot indicated elevated stress (i.e., increased heart rate and heart rate variability ratio) with an increase in helicopter vibration and increased neck muscle fatigue over time. However, the magnitude of these physiological effects and the maximum safe exposure level of vibration remain unclear.
The current study examined the physiological responses of 12 pilots during and after 15-minute exposures to three vibration levels (-25%, normal, and +25% amplitude; Levels 1-3) on a human rated shaker platform which are representative of the NRC Bell 412 and CF Griffon helicopters in forward flight. In addition, this study compared the human physiological response to an Original Equipment Manufacturer cushion (OEM Cushion 1) and a vibration mitigating cushion (Mitigating Cushion 2) at each vibration level using a counter-balanced design across participants. As a measure of neck strain, neck electromyography (EMG) amplitude, force, and median frequency were each significantly influenced by vibration level and cushion type. The EMG amplitude was higher during Level 3 compared to Level 1 for both cushions, and during Level 2 compared to Level 1 for the OEM cushion. During Levels 2 and 3, the EMG amplitudes were lower, and median frequency higher, with the vibration mitigating cushion compared to the OEM cushion. Because muscle fatigue coincides with higher EMG amplitude and lower median frequency, significant differences in EMG amplitude and median frequency across vibration levels and cushions indicate that neck strain was higher with the OEM cushion and/or Level 3 vibration. Accelerations recorded at the seat pad and helmet confirmed that WBV increased significantly with increases in vibration level and low-frequency acceleration of the head was amplified relative to seat vibration. Notably, head acceleration was significantly lower with the vibration mitigation cushion compared to the OEM Cushion 1 during Level 3 vibration, and seat pad acceleration was significantly lower with the vibration mitigating cushion during both Levels 2 and 3 vibration. Despite significant effects of vibration level and cushion type on WBV, head and seat pad accelerations for all six Level/Cushion combinations fell within the same “fairly uncomfortable” range (0.5–1.0 m∙s⁻ ²) of the ISO-2631-1-1997 Standard comfort scale. Overall, heart rate was significantly higher during each of the vibration trials as compared to the initial resting rate. One of the measures of HRV (sample entropy) decreased from Epoch 1 (0-5 minutes) to Epoch 2 (5-10 minutes) during each 15-minute vibration trial and across the 6 vibration trials, indicative of increased sympathetic drive, or stress, with viration exposures. Psychomotor Vigilance Task (PVT) reaction times showed a reduced ability to maintain vigilance post-experiment (after all vibration trials). Discomfort ratings increased significantly over the duration of the six experimental trials, with significant differences between neck, shoulder, back, buttock, and thigh discomfort.
In conclusion, the changes in absolute EMG amplitude and median frequency measures indicate that increased vibration may be more fatiguing while the vibration mitigating cushion was effective in reducing such effects. While reducing WBV through aircraft tuning and/or vibration mitigating cushions would be expected to significantly reduce neck strain and related disorders, further work is required to evaluate the effects of prolonged vibration exposure, head supported masses, and alternative mitigation solutions.