Review by Courtney Zoschke
Study: The influence of psychosocial stress, gender, and personality on mechanical loading of the lumbar spine
In recent years, the cause of low back pain disorders in the workplace has been actively debated. Prior research has focused primarily on identifying physical characteristics of the workplace that may influence biomechanics and spinal loading and subsequent injury risk. Little work, however, has been done to assess how psychosocial stress may affect muscle activity and spinal loading. This study sought to investigate the impact of psychological stress on variables that influence spinal loading while controlling for biomechanical effects. The study also aimed to identify particular personality traits as a predictor for low back pain under psychosocial stressors.
Marras, W. S., Davis, K. G., Heaney, C. A., Maronitis, A. B., & Allread, W. G. (2000). The influence of psychosocial stress, gender, and personality on mechanical loading of the lumbar spine. Spine, 25(23), 3045-3054.
Twenty-five male and female subjects that were asymptomatic for low back pain performed sagittally symmetric (bilateral) lifts under stressful and non-stressful conditions. Subjects’ personalities were characterized using the Myers-Briggs Type Indicator questionnaire prior to the test. Trunk kinematics, torso kinetics, and electromyographic (EMG) activities of 10 trunk muscles were used to evaluate three-dimensional spine loading under both stressful and non-stressful conditions. Subject anxiety, blood pressure and heart rate were also recorded for both conditions to verify the level of stress manipulation.
Subjects were required to complete five controlled lifts under both stressed and unstressed conditions, while moving at varying trunk extension velocities. They were to maintain their position within a specified tolerance while lifting a 13.6kg mass. Activities of the right and left latissimus dorsi, erector spinae, external and internal obliques, and rectus abdominis were collected using standard EMG techniques. Goniometers were utilized to measure the position of the L5/S1 relative to the centre of the force plate. Blood pressure and heart rate were measured with each lift.
In the unstressed situation, experimenters used encouraging language and provided positive reinforcement, creating a positive environment for the subject. When the unstressed session was completed, the experimenters left the room. Upon returning to the room, the experimenters added stress to the situation by appearing distraught and using non-supportive language. Further, the visual feedback provided to subjects during their lifts was manipulated to fall outside the prescribed tolerance, providing a reason for the experimenters to criticize the subject’s performance.
Statistically significant changes in spine loadings, muscle responses, and kinetic responses were observed under conditions of psychosocial stress manipulation. Compressive loads on the spine increased under psychosocial stress by approximately 7%, with even larger increases seen for certain types of individuals (e.g., introverts (13.7%)). Increases in anterior-posterior shear of 1.5-5% and in lateral shear of 12% were seen in the subjects when stress was present (the latter result was unexpected because the task involved only sagittally symmetric lifting).
Kinematic data indicated a significant reduction in hip motion in female subjects during stress conditions. Further, findings that muscle activity increased by 3.5-6.5% in several trunk muscles during psychosocial stress indicated an increased coactivation in response to stress and therefore increases in spinal loading. There were significant personality-stress interactions, indicating a difference in response to stress between individuals with differing personality traits. Interestingly, introversion and intuition characteristics were associated with large increases in lateral shear and thinker/feeler traits were associated with anterior/posterior shear.
It is possible that the impact of psychosocial stress in this study may be underrepresented for a few reasons. First, these results represent short-term responses to stress. The literature suggests long-term stress and associated increases in muscle activity, and therefore spinal loading, may prove to be a significant mechanism by which stress leads to chronic low back pain; however, there remains work to be done in this area. Second, the correlation between psychosocial and biomechanical changes in the development of low back pain appears to be dependent on gender, as differences in loading patterns of the spine in response to stress were noted between males and females. Third, this study was limited to evaluation of only one external load and movement, though previous literature supports an increase in muscular coactivation in antagonist muscles surrounding both the elbow and shoulder under stressful conditions.
This study provides an indication that there is a biomechanical pathway to spine loading associated with psychosocial stress. Results suggest that the pathway is complex, in that spinal loading is dependent upon a combination of physical work demands, one’s reaction to the psychosocial environment and the unique characteristics of an individual. One recognized mechanism by which stress increases spine loading is through alterations in muscle coactivity and trunk kinematics. Collectively, these results suggest that one should consider these factors’ effects on the biological system when attempting to evaluate etiology of lower back pain in the workplace.
The findings from this study highlight the importance of stress reduction in the management and prevention of chronic low back pain by identifying key biomechanical changes caused by acute and chronic stress. While reducing stress may seem to be a simple task, it is hardly ever easy for those dealing with chronic conditions. That said, identifying potential stressors and working to reduce or alter the physical response associated with stressful events by techniques such as mindfulness, proper breathing exercises, and restful sleep can provide hope in reducing long term pain.