Quantification of Cervical Musculature Using Magnetic Resonance Imaging in Military Pilots

Cervical spine musculature contributes to stability and load carrying capacity of the human head-neck. As muscle morphologies influence segmental neck biomechanics and physiological responses to operational activities, their accurate description in computational musculoskeletal models is needed to better estimate osteoligamentous column loads, and determine the mechanisms of internal load transfer, and segmental and local component load sharing. Cervical muscle morphologies specific to fighter jet pilot populations have not been included in the current musculoskeletal injury models. The objective of the current preliminary descriptive study was to obtain muscle morphological data from a group of pilots from the US Air Force.

Supine magnetic resonance images (MRIs) were taken from nine experienced pilot subjects. Cross-sectional areas of the flexor (longus capitis, longus colli, and sternocleidomastoid) and extensor (semispinalis and spinalis cervicis, semispinalis capitis, and multifidus) muscles of the sub-axial spine were obtained. Muscles were segmented on the axial view images at the inferior vertebral endplate (the inferior axial section of the vertebral body) for each spinal segment.

The mean age, stature, total body mass, and body mass index of two female and seven male pilots were 39 ± 5 years, 1.8 ± 0.1 m, 83 ± 8 kg, and 27 ± 2 kg/m2, respectively. These data for the females were 3 ± 5 years, 1.7 ± 0.01 m, 75 ± 4 kg, and 27 ± 1 kg/m2, and for the males were 41 ± 5 years, 1.8 ± 0.1 m, 85 ± 8 kg, and 26 ± 2 kg/m2, respectively. In the flexor group, the area of the sternocleidomastoid muscle decreased cranially from the lower cervical spine (C5 and C6 levels), although the longus colli muscle did not demonstrate such tendency. The longus capitis muscle increased from the lower (C5-C7) to the upper cervical levels, with a peak at the C3. In the extensor group, the area of the semispinalis capitis increased from the inferior to the superior direction although the spinalis cervicis muscles areas showed an inverse pattern. In contrast, the areas of the semispinalis cervicis and multifidus muscles did not show a pattern from the upper to lower or from the lower to upper cervical levels. Level specific data are given in the body of the main paper.

The areas obtained from MRIs at different levels of the cervical column from a group of military pilots showed that their variations depend on muscle type and action. As fighter pilots were the subjects, the primary finding about the patterns in the muscle cross-sectional areas is related to the use of head supported mass, cockpit ergonomics, posture, head accelerations from high-g forces that the pilot experiences, and other factors. From this perspective, the present results are unique. Although the study had more males, an evenly matched group is necessary to elicit male-female differences. It would be a future study to enroll more female fighter pilots to delineate males-to-female differences. The current data can be used in musculoskeletal models to predict realistic neck loads, internal stresses and strains, and anterior and posterior column load sharing under military loading scenarios.