May 2010 Newsletter from Malik Slosberg, DC, MS

De Carvalho, DE, DC, MSc, et al. Lumbar spine and pelvic posture between standing and sitting: A radiologic investigation. JMPT 2010;33(1):48-558.

O'Sullivan P, PhD, et al. Evaluation of the flexion relaxation phenomenon of the trunk muscles in sitting. Spine 2006;31(17):2009-16.


De Carvalho, DE, DC, MSc, et al. Lumbar spine and pelvic posture between standing and sitting: A radiologic investigation. JMPT 2010;33(1):48-558.

Healthy males were radiographed standing and sitting in an automobile seat to measure lumbar spine and pelvic posture differences between standing and sitting.

Results: Lumbar lordosis in standing (using Cobb’s method: nl=50°-60°) average was 63° and sacral inclination average was 43°. Both decreased by 43° & 44° in sitting. Lumbar intervertebral joint angles in sitting underwent substantial flexion (L1/L2 -5°, L2/L3 -7°, L3/L4 -8°, L4/L5 -13°, & L5/S1 -4°. Measures of lumbar lordosis; intervertebral disk angles between L2/L3, L3/L4, & L4/L5; lumbosacral lordosis; lumbosacral angle and sacral tilt were all significantly decreased between standing and sitting. Intervertebral joints, which are in extension in lumbar lordosis, became flexed throughout the spine in sitting. From L1/L2 to L4/L5, intervertebral disk angles became neutral or reversed.

Discussion: In automobile sitting, the lumbar spine flattens completely except at L5/S1 and is highly suggestive of large strains at the posterior aspect of IVDs at L4/L5, one of the most common levels of lumbar disk herniation.

Conclusion: Interventions to return motion segments to a less flexed posture in sitting should be investigated because they may play a role in preventing injury and LBP. In sitting, the pelvis rotates posteriorly and the lumbar lordosis flattens increasing the strain on the posterior ligaments and other passive elements potentially contributing to injuries of the posterior passive elements. Prolonged sitting likely aggravates pre-existing LBP, instigates new LBP and may be a source of increased risk of low back injuries from disc herniations to musculoskeletal strains and sprains. Intradiscal pressure is found to be inversely proportionate to the degree of lumbar lordosis. Maintaining lumbar lordosis has a protective effect on the spine in different postures.

Commentary: This 2010 study is one of a series of recently published papers which document the loss or reversal of the normal lumbar lordotic curve when in a slump sitting posture. The contrast between the erect standing lordosis and the loss or reversal of this curve when sitting in a car seat is amply demonstrated in this study. Maintaining a neutral spine minimizes bending stresses, reducing both posterior tensile forces on back muscles and ligaments and anterior compressive forces on the lumbar intervertebral discs. It is important, in terms of patient management, to consider how much a patient sits daily at work in an office or behind the wheel of a car or truck because prolonged slump sitting needs to be identified, addressed, and managed in order to minimize recurrences and produce successful, long-term and enduring benefits in patients’ function and pain reduction.


O'Sullivan P, PhD, et al. Evaluation of the flexion relaxation phenomenon of the trunk muscles in sitting. Spine 2006;31(17):2009-16.

This study investigates the flexion relaxation phenomenon (FRP) using surface electromyography (sEMG) in the thoraco-lumbopelvic muscles: superficial lumbar multifidus (SLM) at L5, thoracic erector spinae (TES), internal obliques (IO) among 23 pain-free subjects (Ss) in upright sitting (maintaining neutral lumbar lordosis/anterior pelvic tilt), slump sitting (posterior pelvic tilt, semi-flexed lumbars), & while moving from one posture to the other.

Results: A significant decrease in both SLM & IO activity was found when moving from an erect to a slump sitting posture indicating the presence of the flexion relaxation phenomenon (FRP). TES activity was highly variable: 13 Ss exhibited an increase & 11 a decrease in TES activity. FRP occurred in the mid-range of spinal flexion for the SLM, IO, and TES when present. Dysfunction of the spinal stabilizing muscles such as the IO & SLM is common in CLBP Pts.

Discussion: The findings suggest that habitually sustaining not only fully flexed but also semi-flexed sitting spinal postures (that result in relaxation of these spinal stabilizing muscles) for long periods may result in deconditioning of the muscles, leaving Ss at greater risk of back injury or provocation of existing BP. Neutral lordotic postures facilitate tonic activation of the spinal stabilizing muscles, which supports the concept of postural retraining in the management of LBP disorders.

Conclusion: SLM & IO sEMG activity is facilitated in neutral lordotic sitting & exhibits the FRP in mid-and end-range flexion. Sustaining a mid- to end-range flexed sitting posture results in relaxation of the spinal stabilizing muscles. Findings support the importance of specific sitting postures to facilitate reflex activation of lumbar spine stabilizing muscles.

Commentary: I just retrieved this study in the past month after reading a 2010 article investigating the effects of long-term sitting which referenced this important study. Why is the study important? Its implications are enormous. Basically, the study demonstrates, for the first time, that the flexion relaxation phenomenon occurs not only in standing full sagittal flexion, but also is a result of sitting in a semi-flexed position or slump sitting, which is precisely how people sit a work. A 2009 study by Mork & Westgaard in Clinical Biomechanics concludes that the sustained stretch of passive lumbar structures in combination with essentially silent muscles while sitting in a slump posture seems exacerbate LBP throughout the course of the day and be a large part of the biomechanical risk for LBP in sedentary workers. In people who sit in a slump posture many hours a day, their back extensor muscles are being reflexively inhibited throughout the entire course of the day as a result of the flexion relaxation phenomenon (FRP). This may leave the lumbar spine biomechanically vulnerable due to deficient stabilization caused by inhibited and atrophying back extensor muscles (multifidus & erector spinae) and help explain LBP recurrences with minimal or no predisposing incidents.