The aim of the studies presented in this dissertation was to study the degenerative changes in the cervical spine due to high Gz exposure;​ some suggested contributing factors on the neck muscle strain under Gz;​ and effects of certain countermeasures in reduction of strain under Gz.

The subjects were volunteer cadets undergoing training in the Air Force Academy (AFA) of Finland (Studies II, III, and V) and subsequently followedup during their later career (Study I), and AFA instructor pilots (Study IV). Experimental measures included cervical and lumbar magnetic resonance imaging (MRI) (Study I) and electromyography (EMG) for muscular electrical activity (Studies II-V) and the strength of the cervical flexor, extensor and rotator muscles. In addition to strength measurements and test flights, loads on the neck/shoulder muscles were simulated with a cervical loading test (CLT) (Study V). Skin temperatures were measured with a surface thermometer (Study II), and trampoline was used to simulate G-forces (Studies II-III) and as a training device (Study V). A six-week training consisted of strength training and trampoline exercises, for which purposes two training groups were formed.

All changes detected in the cervical spine during the follow-up were minor in both groups. Even though there were no significant differences between the groups, but changes in the pilot group seemed to concentrate in the lower cervical spine, i.e., C5-6 and C6-7, while degenerative changes in the control group were scattered more evenly. Both cold exposure and the extra mass of the helmet increased cervical muscle activity (EMG) under Gz. A regression model showed the increase of 2.6 % in muscle strain for every drop of one centigrade in skin temperature over the sternocleidomastoid muscles (SCM) during cold exposure. The results showed that the higher mass of the helmet had a more significant effect on cervical muscle loading than night vision goggles (NVG), which appeared to affect essentially those muscles that are inherently subjected to the highest loadings, i.e., SCM and cervical erector spinae (CES). There were indications of a tendency towards a lower muscle strain when a lumbar support was worn. Muscle activity (EMG) decreased in all measured erector spinae muscles in the cervical, thoracic, and lumbar regions;​ but these changes were not statistically significant. Training intervention improved the maximal force production in both groups. Training reduced in-flight muscle strain (%MVC) in both groups most significantly in cervical muscles and in the SCM in particular. CLT measurements yielded similar results. The positive effects of training period, i.e. decreased muscular loading, sustained in cervical area in both training groups (STG and TTG). No statistically significant differences between the groups were discovered.

In conclusion, high Gz exposures during over 1200 flight hours caused no significant radiological changes n the spinal column. Both cold exposure and the extra mass of helmet system increased cervical muscle activity (EMG) under Gz. It was hypothesised that Gz induced muscle strain could be reduced by improving the sitting posture with a lumbar support and/or by improving muscular capacity and performance through training. These two means seem to reduce muscle strain to some extent.