Pelvic unlevelness in chronic low back pain patients - Biomechanics and EMG time-frequency analyses

Background and significance: A controversy persists about the role of pelvic unlevelness and leg length inequality (LLI) as etiologic and aggravating factors in low back pain (LBP), and the diagnostic approach to the use of heel lifts. A question arises: does LLI causes LBP, or is pelvic unlevelness a result of LBP? If the latter, why would we try to change the posture by unilateral heel lift, presumably to something more painful in LBP patients? Purpose: The aims of this study was: a) to investigate the external forces by means of force plate analysis, and the internal forces of the spine by erector spine surface EMG during isometric contraction and sudden load, and b) to define how these responses were modified with or without expectancy and before and after fatigue, when artificial LLI was created in normals and in chronic LBP patients. Subjects and methods: In the first study, 10 patients with chronic LBP (age 41.4, SD 9.6) and 10 matched healthy subjects (age 41.1, SD 9.4) were investigated. The patients participated in a well established 2-week rehabilitation program. The pain degree was quantified by Visual Analogue Scale (VAS). The subjects stood on a force platform with extended knees, their arms along the body and pelvis against a board to push off. In this position they extended their back against a mechanical resistance at 20% of the individual maximum voluntary contraction MVC for 4 s. Surface EMG activity of the multifidus and longissimus were recorded. An artificial LLI was created by placing different boards of 12 mm and 25 mm respectively under the left and right foot in random order. After the 2-week rehabilitation course the same investigation was repeated for the LBP patients. The same procedures were repeated for the controls after 2 weeks. In the second study, 11 chronic LBP patients (7 males 38.4 years SD 9.8, and 4 females 37.2 years SD 3.6) and 11 age and sex matched controls (7 males with mean age 39.5 SD 9.8, and 4 females with mean age 36.2 SD 3.7) were recruited. The experimental setup was the same as in the first study. Expected and unexpected load and unloads were applied before and after fatigue at the level of T4 by weight of 2 kg attached via a load cell to a harness around the subject's shoulder. The weight was dropped from a height of 45 cm, applying a sudden forward bending moment. Results: The results from the first study showed that in healthy subjects EMG activity increased proportionally on the side with a board and decreased correspondingly on the contralateral side. In LBP patients the EMG increment is higher when the artificially elongated leg corresponds to the more painful side. In healthy subjects COP is close to the middle line, and shifts proportionally away from the longer leg side. In LBP patients COP is shifted initially away from the more painful side even without artificial LLI. COP shifts significantly more in patients when the longer leg corresponds to the more painful side. The displacement of COP is significantly smaller at the end of the rehabilitation program for chronic LBP. The healthy subjects did not show electrophysiologic signs of muscle fatigue, detected by median frequency shift, after 45 s of isometric contraction at 20% MVC. The LBP patients not only were fatigued under the same conditions, but showed asymmetric erector spine fatigue, higher on the side corresponding to the longer leg. The fatigue index decreased significantly after the rehabilitation program. The most fascinating result in the second study was a 5-level interaction of LId together with expectation, fatigue, group, and axis, defined from the force plate parameters. The results of the EMG reaction time and magnitude to sudden load were expressed mostly by a 3-level interaction of LLI together with expectation and group. An obvious discrepancy between the ground reaction forces (expressing the external forces) and the EMG activity (showing the internal forces) comparing normals versus LBP patients were found. The EMG magnitude to sudden load was smaller but the magnitude of the ground reaction forces were larger in LBP patients versus normals. EMG reaction time was slower but the latency of the ground reaction forces are faster in LBP patients versus normals. This internal/external force discrepancy increased after placing a heel lift under the foot, corresponding to the painful side in LBP patients. Conclusion: This study proved only the short term beneficial effect of this frontal plane postural correction in chronic LBP patients. Further studies are necessary to verify a longer term effect of monitored frontal plane posture correction in chronic LBP.