Background:​ When working in darkness, helicopter pilots often use night vision goggles (NVG) and a counterweight (CW) attached to the helmet. In addition, missions also entail whole-body vibration and ergonomically unfavorable neck and body postures. The present aim was to assess the mechanical load and compare it with previously published data comprising recordings of muscle activity induced by different head-worn equipment used by helicopter pilots.

Methods:​ Activity in left upper and lower dorsal neck muscles was recorded in 14 Swedish helicopter pilots, using electromyography (EMG) when wearing (i) helmet, (ii) helmet and NVG, and (iii) helmet, NVG and CW in a laboratory setting. The activity was recorded in a random order with the head in neutral position and at 20° neck flexion. The induced load for the cervical spine in these conditions was calculated using a sagittal, static, biomechanical model.

Results:​ The induced flexing load moment and muscle activity increased with the NVG and decreased from that level with the CW. However, the magnitude of the induced load was higher than the muscle activity indicated, especially when related to maximum strength.

Conclusion:​ At low loads, as apparent in the present study, substantial increases in induced load moment are entailed, with only a comparatively small increase in neck-muscle activation levels. This must be considered when using EMG in the assessment of work load and in ergonomic interventions.

Relevance to industry:​ As EMG is a common method of assessing excessive muscle activity as a sign of workload exposure, it is essential to note the non-linear relation between the neck-muscle activity and the induced flexing load moment caused by head-worn equipment.