Tablet computer use requires substantial head and neck flexion, which is a risk factor for neck pain. The goal of this study was to evaluate the biomechanics of the head–neck system during seated tablet computer use under a variety of conditions. A physiologically relevant variable, gravitational demand (the ratio of gravitational moment due to the weight of the head to maximal muscle moment capacity), was estimated using a musculoskeletal model incorporating subject-specific size and intervertebral postures from radiographs. Gravitational demand in postures adopted during tablet computer use was 3–5 times that of the neutral posture, with the lowest demand when the tablet was in a high propped position. Moreover, the estimated gravitational demand could be correlated to head and neck postural measures (0.48 < R 2 < 0.64, p < 0.001). These findings provide quantitative data about mechanical requirements on the neck musculature during tablet computer use and are important for developing ergonomics guidelin...

Gravitational demand on the neck musculature during tablet computer use

Degenerative changes adjacent to fused cervical vertebrae have been well documented1-7, and may occur as early as seventeen months following surgery7. The reported prevalence of these degenerative changes has ranged from 16% (of 112 patients)6 to 25% (of 374 patients)4 within ten years after the operation, and the changes require adjacent-level surgery in 6% to 10% of patients5,8-11. In a twenty-one-year follow-up study of fifty patients who had undergone anterior cervical spine arthrodesis, 32% developed recurrent pain at an average of 7.2 years after surgery, and 16% required surgery for disc disease at an adjacent level3. The most likely location for adjacent-segment degeneration is unclear, as one study suggested that progressive degeneration occurs with equal frequency superior and inferior to the fused segment12 while another indicated that adjacent-segment degeneration is level-dependent and most prevalent at the C5/C6 and C6/C7 discs4. The degeneration may progress as a result of underlying spondylosis4,13,14, increased motion in adjacent vertebrae2,15-18, or a combination of these factors1,5.

Investigations designed to identify mechanical factors that may expedite adjacent-segment degeneration have primarily focused on excessive motion adjacent to the fused segment during flexion-extension. To identify excessive motion, the normal range of motion in asymptomatic subjects must first be defined. This has been previously accomplished by manually identifying anatomic landmarks on two-dimensional lateral radiographs16,19-25. These measurements have inherent limitations, including potential differences between ranges of motion calculated from radiographs collected in static positions compared with dynamic muscle-driven movement26, the high measurement variability associated with manual digitization24,25,27-29, and the inability to assess motion that occurs out of the film plane. Numerous authors have noted the necessity for three-dimensional16,26,30-33, in vivo23,26,30,34 measurements of the cervical spine under dynamic load16,28,34,35.

The overall objective of the current study was to evaluate cervical intervertebral range of motion during dynamic flexion-extension in patients who had undergone single-level anterior cervical discectomy and fusion and asymptomatic control subjects. The first aim was to assess differences in intervertebral range of motion, in terms of all six degrees of freedom (three translations and three rotations), between control subjects and patients who had undergone single-level anterior cervical discectomy and arthrodesis. The second aim was to compare static range-of-motion measurements with those obtained during dynamic, functional movement.

/pdf/six-degrees-of-freedom-cervical-spine-range-of-motion-during-b079wzcskf.pdf

Six-degrees-of-freedom cervical spine range of motion during dynamic flexion-extension after single-level anterior arthrodesis: comparison with asymptomatic control subjects.

Cervical motion segment contributions to head motion during flexion\extension, lateral bending, and axial rotation

https://digitalcollection.zhaw.ch/bitstream/11475/8425/2/2015_Ernst_Extension_and_Flexion.pdf

Extension and flexion in the upper cervical spine in neck pain patients

http://www.cpaqv.org/mtpmh/artigos/cervicalspine.pdf

Cervical spine intervertebral kinematics with respect to the head are different during flexion and extension motions.

STUDY DESIGN A blind, repeated-measure design was employed in the study. OBJECTIVE To quantitatively measure the percentage contribution of segmental angular motion during different motion ranges of cervical flexion-extension for clinical applications and better understanding of cervical biomechanics. SUMMARY OF BACKGROUND DATA Restriction of cervical motion is a major symptom in patients suffering from neck injuries or pathologies. Although segmental angular motion alternation is a criterion for the detection of neck related impairments, the percentage contribution throughout cervical movements is not well understood. METHODS A total of 384 image sequences during cervical flexion-extension obtained from 48 healthy adult subjects were analyzed with a precise image protocol using dynamic videofluoroscopic techniques. RESULTS The middle cervical spines demonstrated significantly greater angular percentage contributions at C3/4 (29.89%) and C4/5 (37.14%) angles during the initial 1/3 flexion movement; whereas the lower cervical spines revealed statistically greater angular contributions (C5/6: 22.57% to 29.45%; C6/7: 28.80% to 37.42%) from the middle to final 1/3 ranges of flexion movement (P<0.001). With regard to cervical extension motion, the majority of segmental percentage contributions statistically shifted initially from C5/6 level (30.21%) to C4/5 (24.96%) and C5/6 (26.12%) levels, and finally to the C3/4 (27.55%) and C4/5 (29.77%) segments (P<0.001). CONCLUSIONS The segmental percentage contributions in this study might imply that the cervical flexion movement initially relied more on the middle cervical segments and later on the lower ones, whereas a motion pattern trend from lower to middle segments was observed during cervical extension.

Segmental percentage contributions of cervical spine during different motion ranges of flexion and extension.

The insufficient investigations on the changes of spinal structures during traction prevent further exploring the possible therapeutic mechanism of cervical traction. A blind randomized crossover-design study was conducted to quantitatively compare the intervertebral disc spaces between axial and anterior lean cervical traction in sitting position. A total of 96 radiographic images from the baseline measurements, axial and anterior lean tractions in 32 asymptomatic subjects were digitized for further analysis. The intra- and inter-examiner reliabilities for measuring the intervertebral disc spaces were in good ranges (ICCs = 0.928–0.942). With the application of anterior lean traction, the statistical increases were detected both in anterior and in posterior disc spaces compared to the baseline (0.29 mm and 0.24 mm; both P < 0.01) and axial traction (0.16 mm and 0.35 mm; both P < 0.01). The greater intervertebral disc spaces obtained during anterior lean traction might be associated with the more even distribution of traction forces over the anterior and posterior neck structures. The neck extension moment through mandible that generally occurred in the axial traction could be counteracted by the downward force of head weight during anterior lean traction. This study quantitatively demonstrated that anterior lean traction in sitting position provided more intervertebral disc space enlargements in both anterior and posterior aspects than axial traction did. These findings may serve as a therapeutic reference when cervical traction is suggested.

/pdf/comparison-of-the-intervertebral-disc-spaces-between-axial-2p77lbhzmi.pdf

Sen-Wei Tsai

Papers

Segmental percentage contributions of cervical spine during different motion ranges of flexion and extension.

Comparison of the intervertebral disc spaces between axial and anterior lean cervical traction.

The intervertebral translations during cervical flexion and extension