Patients with low back pain and asymptomatic individuals were evaluated while performing controlled and free-dynamic lifting tasks in a laboratory setting. To evaluate how low back pain influences spine loading during lifting tasks. An important, yet unresolved, issue associated with low back pain is whether patients with low back pain experience spine loading that differs from that of individuals who are asymptomatic for low back pain. This is important to understand because excessive spine loading is suspected of accelerating disc degeneration in those whose spines are damaged already. In this study, 22 patients with low back pain and 22 asymptomatic individuals performed controlled and free-dynamic exertions. Trunk muscle activity, trunk kinematics, and trunk kinetics were used to evaluate three- dimensional spine loading using an electromyography- assisted model in conjunction with a new electromyographic calibration procedure. Patients with low back pain experienced 26% greater spine compression and 75% greater lateral shear (normalized to moment) than the asymptomatic group during the controlled exertions. The increased spine loading resulted from muscle coactivation. When permitted to move freely, the patients with low back pain compensated kinematically in an attempt to minimize external moment exposure. Increased muscle coactivation and greater body mass resulted in significantly increased absolute spine loading for the patients with low back pain, especially when lifting from low vertical heights. The findings suggest a significant mechanical spine loading cost is associated with low back pain resulting from trunk muscle coactivation. This loading is further exacerbated by the increases in body weight that often accompany low back pain. Patient weight control and proper workplace design can minimize the additional spine loading associated with low back pain.

The effects of psychosocial stress on muscle activity and spinal loading were evaluated in a laboratory setting. To evaluate the influence of psychosocial stress, gender, and personality traits on the functioning of the biomechanical system and subsequent spine loading. Physical, psychosocial, and individual factors all have been identified as potential causal factors of low back disorders. How these factors interact to alter the loading of the spine has not been investigated. Twenty-five subjects performed sagittally symmetric lifts under stressful and nonstressful conditions. Trunk muscle activity, kinematics, and kinetics were used to evaluate three-dimensional spine loading using an electromyographic-assisted biomechanical model. A personality inventory characterized the subject’s personality traits. Anxiety inventories and blood pressure confirmed reactions to stress. Psychosocial stress increased spine compression and lateral shear, but not in all subjects. Differences in muscle coactivation accounted for these stress reactions. Gender also influenced spine loading; Women’s anterior–posterior shear forces increased in response to stress, whereas men’s decreased. Certain personality traits were associated with increased spine loading compared with those with an opposing personality trait and explained loading differences between subjects. A potential pathway between psychosocial stress and spine loading has been identified that may explain how psychosocial stress increases risk of low back disorders. Psychosocially stressful environments solicited more of a coactivity response in people with certain personality traits, making them more susceptible to spine loading increases and suspected low back disorder risk.