This study provided preliminary results demonstrating small differences in task performance between people with neck pain and those without neck pain performing standardized tasks; and the feasibility of assessing differences in task performance using two-dimensional video-based movement analysis. Although the current gold standard for kinematic analysis is three dimensional tracking of angles, Dartfish Video Software Solutions, a two dimensional video-based tracking system, has potential advantages for clinical assessment because it allows assessment of task performance in different contexts, is much more economical than a movement laboratory, and provides an option to review performance over time for retraining purposes. Others have suggested the usefulness of this approach for assessing lower limb movement [8–11]; however, this study adds that we were able to successfully monitor changes in neck flexion. This is important since neck motion is quite different than measurements that would be taken for the lower limb. Borel, Schneider, & Newman  identify that the software’s “ease of use” allows for studies to track movement for longer periods of time, as the software requires less time to complete tasks than other visual assessments of kinematics. This is particularly useful for studying problems aggravated by repetitive movements over extended periods of time, such as those that cause neck pain. Miller & Callister  identify that using the software to track movement results in high intra-rater reliability because even those new to Dartfish Video Software Solutions were able to accurately follow software instructions to create data.
Functional tasks are complex movements that are influenced by motor control, body size and shape, and many other factors. A variety of motor control theories exist to describe how movement takes place. The theory of abundance states that a limb has many degrees of freedom , and implies that there are many different strategies that can be used to accomplish a specific motor goal. In this study, we provided a standardized task that allowed some flexibility in how it was performed. However, both anthropometric factors and motor control factors undoubtedly contributed to the wide variation we saw between individuals and how these tasks were performed. When reaching overhead, individuals have an abundant number of potential strategies to lift their arms to reach and grasp the weight from the shelf. Every time the participant reached for the weight during the overhead reach task, the central nervous system employed its muscles and joints differently . Thus, there is some variation even within a single individual on how tasks are repeatedly performed.
People with neck pain may need to alter their strategies in performing tasks to accommodate their neck pain. Changes in strategies could influence the speed of motion, the path of motion taken, the arc of motion used, the coordination of the motion and other parameters. In this study, we focused on measuring the arc of motion used to perform the task. Thus, changes in motor strategies in other parameters such as speed of motion were not reflected in our analysis and may have been missed. Our study may not have captured different forms of compensation use by patients to minimize neck pain such as relying more heavily on upper limbs and postural muscles to achieve the reaching task.
Unfortunately, the variant of eye gaze previously identified in the results was not anticipated and was only noted once data collection began; so formal measurement of eye gaze was not performed in all subjects. A potential reason for older individuals to require more visual feedback to maintain their grip on an object is that they may have diminished sensory feedback about the nature of their grip ; they may have a lower grip strength and be gripping with a different operational range that younger individuals; or, they may have less confidence in their ability to maintain the activity and require visual feedback to increase their focus on the task .
Although motion analysis is often completed with small sample sizes, variability in task performance was greater than we had anticipated. Another potential source of variation in how people perform tasks is based on their anthropometric characteristics. Although the height of the reaching task was related to the person’s eye level, and hence had some compensation for height; it is likely that tasks vary by body size and gender. Future studies should consider matching on these factors to reduce this variation. The overall effect of variability between subjects whether due to anthropometric reasons, motor control reasons, or others contributed to variation between subjects beyond our initial estimates. The effect was reduced power in our statistical analyses. Hence, group differences of 10° in neck flexion which might be considered clinically significant by some, were not statistically significant.
There are also limitations in our study in relation to the use of video analysis. As Norris & Olson  have identified, a limitation to using Dartfish Video Software Solutions is that the number of studies tracking movements of participants with varying pathologies published today is small, and hence there is a lack of reference values or parameters for subject variability/error comparisons purposes. Although the software was able to track neck flexion in this study, a comparison with the gold standard of kinematics tracking would be useful to assess how accurately Dartfish Video Software Solutions was able to track differences in neck flexion over time. Obviously, two-dimensional analysis provides less information than three dimensional analysis. In this study, we restricted ourselves to a two-dimensional analysis because we are interested in neck flexion/extension arc of motion. Video analysis can include three-dimensional considerations when two cameras are used, as long as these are synced. Spinal motion is complex, and video analysis is not the optimal approach to investigate at what level motion was occurring. We know that flexion occurs differently at different levels of the spine. More specifically, upper cervical spine motion includes movement of the atlas (C1) and the odontoid process (C2) , and this study has referred to flexion at this C1-C2 craniocervical junction as short neck flexion. Furthermore, long neck flexion is associated with flexion at the lower cervical spine via the separation of spinous processes of C2-C7 vertebrae . A final limitation to this study is the fact that trunk movement was not restricted during the trials. However, it should be noted that excessive restriction placed on subjects could have led to unnatural task completion.
Left untreated, acute neck pain can become chronic and result in secondary health problems . For this reason, research into the causes and effects of neck pain is imperative, especially in today’s aging society where 90% of office workers use computers daily (computer work being cited as contributing to neck pain and postural muscle fatigue) . Gender, age, poor social support, job dissatisfaction, and high job demands all contribute to the development of neck pain . Driessen et al.  identify that research into prevention is “scarce”, and due to the “multifactorial origin” of neck pain, it is necessary to educate individuals on how to prevent neck pain. Although our study is preliminary, it does provide preliminary information on the amount of neck motion used, and the types of motor strategies employed during two standardized functional tasks. Understanding how functional and work tasks are performed, and how this might contribute to neck disorders is an important and understudied area. Larger studies of task performance in different contexts such as workplaces, and in larger groups of individuals considering personal and environmental factors are needed to fully understand the exposures which might be contributing to neck pain.