Relationships between lumbar inter-vertebral kinematics and paraspinal myoelectric activity during sagittal flexion: a quantitative fluoroscopy and surface electromyography study

Introduction. Previous investigations that have attempted to relate mechanical parameters to NSLBP groups are often contradictory of each other, and currently clear mechanical markers for LBP remain elusive. In order to move forward in this area, it may be necessary to take a step back, and improve understanding of ‘normal’ spinal biomechanics (i.e. in low back pain free populations). Indeed, Peach et al. (1998) stated “By knowing what is “normal” and what is “abnormal” it may be possible to provide objective evaluation of rehabilitation protocols, and possibly classify different low back pathologies” (Peach et al. 1998). Therefore, an improved understanding of biomechanical behaviours in groups of back pain free people is desirable, particularly at an inter-vertebral level, an area where clear knowledge gaps still exist. Control of the spine during voluntary movement requires finely-tuned coordination of numerous trunk muscles. This dynamic control is believed to be achieved via communication between three sub-systems, the passive (vertebrae, discs and ligaments), the active (muscles and tendons) and the control (central and peripheral nervous system) systems. Investigating the interplay between these sub-systems however is difficult, as the spine is a complex structure with a hidden kinematic chain. Quantitative fluoroscopy (QF) is an imaging technology capable of measuring continuous spinal kinematics at the inter-vertebral level, and surface electromyography (sEMG) provides a non-invasive means of objectively quantifying muscle activity. This study used QF and sEMG technologies concurrently to investigate relationships between and amongst lumbar kinematic (QF determined) and muscle activity (sEMG determined) variables, during weight-bearing active forward flexion. This was the first time such technologies have been combined to investigate the biomechanics of the lumbar spine in vivo. An improved understanding of normal lumbar kinematic and myoelectric behaviour, will assist in the interpretation of what is abnormal in terms of inter-vertebral spinal mechanics. Methods. Contemporaneous lumbar sEMG and QF motion sequences were recorded during controlled active flexion of 60° in 20 males with no history of low back pain in the previous year. Electrodes were placed adjacent to the spinous processes of T9, L2 and L5 bilaterally, to record the myoelectric activity of the thoracic and lumbar erector spinae (TES and LES) and lumbar multifidus (LMU) respectively. QF was used concurrently to measure the maximum inter-vertebral rotation during flexion (IV-RoMmax) and initial attainment rate for the inter-vertebral levels between L2 and S1, as well as each participant’s lordotic angle. The sEMG amplitude data were expressed as a percentage of a sub-maximal voluntary contraction (sMVC). Ratios were calculated between the mean sEMG amplitudes of all three muscles examined. Each flexion cycle was also divided into five epochs, and the changes in mean sEMG amplitude between epochs were calculated. This was repeated to determine changes between all epochs for each muscle group. Relationships between IV-RoMmax and all other kinematic, morphological (i.e. lordosis) and muscle activity variables were determined using correlation coefficients, and simple linear regression was used to determine the effects of any significant relationships. The reliability and agreement of the IV-RoMmax, initial attainment rate, and normalised RMS sEMG measurements were also assessed. Results. The reliability and agreement of IV-RoMmax, initial attainment rate and sEMG amplitude measurements were high. There were significant correlations between the IV-RoMmax at specific levels and the IV-RoMmax at other lumbar motion segments (r = -0.64 to 0.65), lordosis (r = -0.52 to 0.54), initial attainment rate (-0.64 to 0.73), sEMG amplitude ratios (r = -0.53) and sEMG amplitude changes (r = -0.48 to 0.59). Simple linear regression analysis of all significant relationships showed that these variables predict between 18% and 42% of the variance in IV-RoMmax. Conclusion. The study found moderately strong relationships between kinematic, morphological and muscle activity amplitude variables and the IV-RoMmax of lumbar motion segments. The effects of individual parameters, when combined, may be important when such inter-vertebral levels are considered to be sources of pain generation or targets for therapy. This is an important consideration for future non-specific low back pain (NSLBP) research, as any attempts to associate these parameters with low back pain (LBP), should also now take in to account the normal biomechanical behaviour of an individual’s lumbar spine. Indeed, consideration should be given to the interactions that exists between such parameters, and they should not be considered in isolation. Multivariate investigations in larger samples are warranted to determine the relative independent contribution of these variables to the IV-RoMmax.