Trunk muscle responses to suddenly applied loads: do individuals who develop discomfort during prolonged standing respond differently?

Individuals with low back pain or injury (LBP/LBI) have been shown to display altered muscle responses to trunk perturbations; however it is unclear whether these observations are a cause or a result of the LBP/LBI. In this study, a 6.78 kg load was suddenly applied to the hands to perturb the trunk prior to and following a 2-h standing period, during which trunk and hip electromyography (EMG) and centre of pressure (CoP) at the feet were recorded. Seven of the 13 participants developed substantial low back discomfort (LBDiscomfort) during the standing period. These individuals, both pre- and post-standing, showed a greater average number of responsive extensor muscles (3.8 compared to 3.1 in those who did not develop discomfort) and a greater occurrence of extensor muscle response (95-100% of trials) as compared to those who did not develop LBDiscomfort (73-86% of trials). Also, after discomfort developed, these individuals displayed an increased response in their abdominal muscles. This overall increase in trunk musculature activity could either be detrimental by potentially increasing spinal loading leading to LBDiscomfort, or beneficial in that this increased musculature responsiveness may reduce one's likelihood of developing a future LBI through a pathway of increased spine stability. In either case, these responses indicate motor control characteristics that can distinguish the likelihood of an individual developing LBDiscomfort during common tasks such as prolonged standing.