Spine Biomechanics of Prolonged Sitting: Exploring the Effect Chair Features, Walking Breaks and Spine Manipulation have on Posture and Perceived Pain in Men and Women.

Abstract

Purpose: Prolonged sitting while working in an office has become a standard function in our society. While seated postures do conserve energy and permit a worker to focus on a task, the posture also involves a significant amount of spine flexion. Held for a prolonged period of time, this non-neutral posture has the potential to generate low back pain through the loading, strain and compression tissues of the low back and buttocks. Indeed, literature points to an association between prolonged occupational sitting and back pain: an expensive problem in terms of direct and indirect health care costs. Two factors involved in this problem, the flexed posture of the low back and the prolonged constrained nature of sitting, could be manipulated in order to reduce their respective contributions to pain and injury pathways. Specifically, decreasing low back flexion could be achieved with various office chair design features and the quasi-static loading scenario could be broken up with movement. The purpose of this thesis was to explore the effect of these two strategies on biomechanical parameters and perceived pain during prolonged sitting. The first part of this thesis explores the effect office chair design features including lumbar support, forward seat pan tilt and a scapular relief backrest have on low back posture, muscle activity and pain compared to a control chair configuration. The second part of this thesis explores the effect active (walking) and passive (lumbar spine manipulation) movement interventions have on those same biomechanical factors. Methods: Twenty-eight participants (14 male and 14 female) were recruited for a radiographic study of low back and pelvic postures adopted in standing, maximum flexion and each of the four office chair conditions: control configuration, lumbar support, anterior seat pan tilt and backrest with scapular relief. Measures of lumbar lordosis, intervertebral joint angles and sacral tilt were taken from lateral lumbar radiographs and compared between conditions. To assess these chair features in a more realistic way, this radiographic study was followed by an in-vivo laboratory study allowing for prolonged exposures to each condition. In this study, 31 (15 males and 16 females) completed a standardized word processing task while sitting in each of the four chair conditions (control, lumbar support, forward seat pan tilt and scapular relief, presented in a random order) during four 30-minute blocks. Measures of spine posture (upper back, lower back and pelvis), torso muscle activity (abdominal, back and gluteal), seat pan pressure and perceived pain were collected throughout this two-hour exposure and compared between conditions. To assess the impact of walking breaks on biomechanical parameters and perceived pain throughout a two-hour sitting exposure, 32 subjects (16 males and 16 females) were recruited for two data collections. In a random order, the subjects experienced either a control experiment that consisted of completing a standardized word processing task while sitting for two-hours on a neutral office chair seat pan (backrest removed) or an intervention experiment that was identical to the control session with the exception of two, two-minute, self-paced walking breaks at 40 minute intervals. Measures of spine posture (upper back, lower back and pelvis), back and pelvic muscle activity, seat pan pressure and perceived pain were collected throughout these two-hour exposures and compared between conditions. The effect of a lumbar spine manipulation, a passive form of movement imparted to the body, on biomechanical parameters of muscle activity, back and pelvic posture and perceived pain was examined in a shorter intervention study. Twenty subjects (10 male and 10 female) received either a lumbar spine manipulation or a control maneuver (lumbar spine manipulation set-up with preload but no thrust) in a random order after 40-minute blocks of sitting on an office chair seat pan (backrest removed) completing a standardized word processing task. Surface muscle activity for the low back and pelvis, indwelling activity of a deep back muscle, spine and pelvic postures and perceived pain were compared between conditions. Results: The radiographic study confirms the extensive lumbar spine flexion in sitting compared to upright standing and maximum flexion. Sitting in an office chair, regardless of design features to reduce spine flexion, results in postures of approximately 70 % of maximum range of low back flexion. No significant differences in low back posture were found between the chair features or control configuration, however; significantly more anterior rotation of the pelvis was found with the lumbar support and forward seat pan configurations. In the prolonged sitting experiment, Study 2, use of the lumbar support and seat pan tilt features were again found to impart significant anterior rotation of the pelvis but these features also resulted in significantly more upright spine postures as well. These improved postures were maintained actively by muscles in the seat pan tilt condition and passively by the backrest in the lumbar support condition. Chair conditions had minimal impact on seat pressure variables. Despite the improvements in posture with two of the chair features and regardless of muscle activity levels, perceived back pain steadily increased to clinically significant levels throughout the two-hour exposure. Analysis of the pain scores revealed the presence of statistically different sub-groups: non-pain developers, subclinical pain developers and pain developers. Reassessing the effectiveness of each chair condition in light of these groups revealed that pain developers demonstrated a clear intolerance for the seat pan tilt configuration. In the third study, brief walking breaks of self-selected intensity had no effect on most biomechanical factors with the exception of reduced seat pressure and seat pressure area. The walking breaks were able to provide a significant, but short-lived, reduction in perceived pain, however, they were not able to reduce the level of perceived pain that develops by the end of a two-hour exposure to prolonged sitting. Similar to the walking breaks examined in Study 3; lumbar spine manipulation does not appear to effect postures or ultimate perceived pain levels during prolonged sitting. However, the results from Study 4 show an immediate reduction in perceived pain following both the manipulation and control maneuvers and a significant reduction in muscle activity following spine manipulation. Conclusions: Both posture and movement interventions are important to consider when addressing the issue of low back pain associated with sitting. However, it does appear that altering seated posture through chair design features alone is not enough to solve this problem. Indeed, while features such as lumbar supports and forward seat pan tilt have been shown reduce the flexion of the low back and pelvis, there is the potential for these features to add to the problem as opposed to reducing it. Specifically, forward seat pan tilt without appropriate back support will likely increase pain in a portion of the population. Movement interventions appear to be more promising in solving this problem, however, the ratio of work/break and intensity, frequency and duration parameters need to be explored further. Brief walking breaks at 40-minute intervals can provide significant immediate relief of sitting associated back pain, however, this intervention is not able to alter biomechanical parameters or ultimate perceived pain in prolonged sitting. Similarly, there is evidence that lumbar spine manipulation may provide short term relief from sitting induced pain as well as reduced muscle activity in sitting, but future work needs to determine the implication of reduced muscle activation as well as the intervention dosage required to obtain longer lasting relief from pain.