Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Industrial and Manufacturing Systems Engineering


Industrial Engineering

First Advisor

Gary A Mirka


The objective of this research was to develop a systems-level characterization of the biomechanical response of the neck in flexion. Several preliminary studies formed a strong foundation to explore this research objective. First, a systematic review of the literature was conducted (Chapter 2) to evaluate the relationship between neck flexion and neck problems and define appropriate angular thresholds for neck flexion as a risk factor for neck problems. A review of 21 papers revealed a consistent positive correlation between neck flexion and neck problems. This systematic review found a neck flexion angle of 20º with the greatest support as the cut-off angle separating high- and low-risk neck flexion postures. This systematic review also helped identify the gaps in the research area: How much is known about the importance of the frequency and duration of the neck postural exposure for the development of neck muscle fatigue? How accurate is the neck postural exposure defined in the previous studies and work assessment tools? What role do passive tissues (ligaments, fascia, etc.) play in the support of the neck in flexion postures? From this systematic review, two additional research questions were developed. The first research question was, “What are the effects of different work-rest cycles on neck muscle fatigue during static neck flexion tasks?” The second research question was, “What role do the passive tissues play in the support of the head/neck during flexion postures?” The first research question was evaluated in our second preliminary study (Chapter 3). The main goal was to investigate the impact of varied work-rest intervals and how they can affect the development of neck and shoulder muscular fatigue (evaluated using surface electromyography (sEMG)) during a simple standing task that required static neck flexion. Participants maintained a 45º neck flexion posture for a total of 60 minutes and were provided three minutes of rest distributed in different ways throughout the experiment [LONG (one, three-minute break), MEDIUM (two, 1.5-minute breaks), or SHORT (five, 36-second breaks)]. Results of the analysis of the EMG data revealed that the SHORT condition did not show increased activity, while LONG [21% increase] and MEDIUM [10% increase] did, providing objective data supporting the guidance of short, frequent breaks to alleviate fatigue. Our results may provide insights into the development of an optimal work-rest cycle strategy that minimizes fatigue but does not affect the work performance negatively. Chapter 4 was a paper that focused on an important methodological consideration in performing EMG-based studies. When performing an experiment across several days, normalization of EMG is an important procedure to control for the day-to-day variability. Performing maximum voluntary contractions on each of these days is problematic, particularly for sensitive regions of the body like the cervical spine. The study outlined in Chapter 4, provided a novel method for predicting maximum voluntary contraction EMG through the extrapolation of submaximal voluntary contraction EMG values. The results of this study showed promise for creating a margin of safety for those that conduct research on the cervical spine that requires multiple days of data collection. The final and primary contribution of this dissertation (Chapter 5) investigated the second research question that focused on the exploration of the role of passive tissues in the support of head/neck. The main goal was to explore the biomechanical differences (considering both active and passive tissues) between neck flexion when it is defined relative to the trunk vs. when it is defined relative to gravity. In the first experimental procedure, the flexion-relaxation phenomenon (FRP) of the cervical spine was investigated when the participant was standing upright with their trunk at a neutral posture and then when they leaned against a fixture that generated a 45º of trunk flexion posture. In the second experimental procedure, the fatigue of cervical musculature during a 30-minute static neck flexion task was studied. Two scenarios were defined and compared: 1) head and neck are flexed so that head is at 45º flexion relative to trunk while the trunk is standing upright (called “HN-45”), and 2) head and neck are not flexed relative to trunk, while trunk is flexed 45º (called “T-45”). The EMG activity of the cervical spine muscles were collected to investigate the pattern of the neck muscle activity during the FRP task and the neck muscles fatigue during the static neck flexion task. The EMG data during the first three minutes of the static neck flexion task were used as inputs into an EMG-assisted biomechanical model of the cervical spine to compute joint reaction forces at C4/C5 level. Also, a discomfort (neck, upper back, and lower back) and overall fatigue survey recorded the subjective evaluation of the static neck flexion task. The results showed that the cooperation between neck muscles and passive tissues will happen with and without 45º of trunk flexion. The findings also revealed that the T-45 condition causes higher neck muscle fatigue and neck subjective discomfort compared to the HN-45 condition during static neck flexion, indicating an important role of the passive tissues in this condition. The long-term effects of these two conditions is not clear as the difference in the role of passive tissues in holding these postures may have negative implications for neck health. The neck C4/C5 joint reaction compression force was the same for conditions HN-45 and T-45, while the joint reaction shear force was significantly higher in condition T-45 compared to condition HN-45. The importance of a system-level evaluation of the biomechanical response of the cervical spine to sagittal plane flexion was presented. The cervical spine was not investigated separately, but the combination of head/neck and trunk were explored in sagittal plane flexion postures. Also, active and passive tissues were both considered in our evaluation. The work assessment tools should note that the body segments shouldn’t be assessed separately. For example, trunk flexion can lead to neck muscle fatigue; however, the neck might be in a neutral posture relative to trunk. From the findings of this study, we recommend that the work assessment tools should consider both neck flexion relative to trunk and neck flexion relative to gravity in their assessment of neck postural exposure. Also, in implementing ergonomic interventions, the human body should be considered as a linkage. For example, if the height of a workstation is adjusted to decrease trunk flexion angles, it would affect neck postural exposure too.


Copyright Owner

Hamid Norasi



File Format


File Size

163 pages