Full length articleInter-joint coordination and the flexion-relaxation phenomenon among adults with low back pain during bending
Introduction
People with Low Back Pain (LBP) move differently than healthy individuals [1]. These altered movement characteristics are often consistent with guarded behaviors (e.g. bracing) and may contribute to ongoing pain via suboptimal tissue loading [1]. Spinal flexion is a common movement in daily functioning and occupational activities [2]. Importantly, bending is often the mechanism associated with the initial onset of LBP, and a source of pain and re-injury for people with LBP [2]. Biomechanical investigation of spinal flexion can assist with assessing and quantifying LBP-related impairment [3].
Inter-joint coordination describes relative movement between adjacent joints and provides insight into neuromuscular control. Coordinated lumbo-pelvic coupling is required for functional movement, serving to help decrease load through the lumbar spine [4]. Inter-joint coordination can be determined using continuous relative phase analysis, an approach which quantifies the degree to which two adjacent joint or segments are moving in-phase (i.e. in-sync) or out-of-phase (i.e. out-of-sync) [5]. Patterns of inter-joint coordination differ between patients with and without LBP; however, findings are inconsistent between studies. For instance, patients with LBP display more in-phase lumbo-pelvic coordination during trunk flexion [6] and extension [6,7] than healthy individuals; a behavior associated with a guarded approach to movement. In contrast, more out-of-phase lumbo-pelvic coordination in patients with LBP has been reported during forward reaching [8]. These works, however, considered the lumbar spine as a functional unit, and overlooked regional differences in lumbar spine kinematics (i.e. upper vs. lower lumbar spine) [9]. Past work reported more out-of-phase coordination in both hip/lower lumbar and lower lumbar/upper lumbar joint pairs during sit-to-stand in people with LBP [10]. Regional analysis of inter-joint coordination could provide insight into differences in study findings, and this has yet to be examined during spinal flexion.
The Flexion-Relaxation Phenomenon describes myoelectric silencing of the erector spinae muscle group in full spinal flexion, and reflects a willingness to load posterior discoligamentous spinal structures [2]. While considered a functional response in healthy people, flexion-relaxation is often reduced or absent in patients with LBP [2]. Reduced flexion-relaxation may be an attempt to protect (i.e. guard) injured passive spinal structures through greater activity of the erector spinae muscle group [3,11]. While possibly helpful in the short-term, this strategy may increase spinal load and compromise spinal tissue in the long-term [1]. Clinically, reduced flexion-relaxation in people with LBP is linked to problematic outcomes such as increased pain and disability [2,3]. Recent work, however, found preserved flexion-relaxation in patients with recurrent LBP who were pain-free at the time of testing [3]. There is considerable variability in the methodology used to study flexion-relaxation (e.g. muscles under study), and these inconsistencies may contribute to differences in study findings [12]. Past work in healthy individuals has shown that flexion-relaxation was not affected by choice/region of muscle under study; however, it is unclear how this finding translates to people with LBP [13].
Altered inter-joint coordination and flexion-relaxation are observed in people with LBP, however, these observations remain variable. Additionally, while it is plausible that two behaviors consistent with guarding may co-occur (i.e. in-phase inter-joint coordination and greater trunk extensor activity), this relationship remains unexplored. Better understanding of movement characteristics in individuals with LBP can provide insight into this disorder and help guide treatment. Therefore, our primary objective was to investigate the extent to which patterns of regional lumbo-pelvic-hip coordination and flexion-relaxation of the erector spinae muscle group differ between adults with and without LBP during bending. A secondary objective was to explore the relationship between inter-joint coordination and flexion-relaxation in both groups. Compared to healthy participants, we hypothesized: (i) participants with LBP would demonstrate behaviors consistent with guarding, that is, more in-phase inter-joint coordination and reduced flexion-relaxation during bending; and (ii) that these two guarded behaviors would correlate.
Section snippets
Participants
Participants with sub-acute and chronic non-specific LBP (n = 16) and healthy adults (n = 21) were recruited from medical and rehabilitation clinics, and the community, via poster and newspaper advertising in Montréal, Québec. Individuals aged 18–49 years who met the criteria for non-specific LBP and were experiencing a pain episode were recruited. Specifically, inclusion criteria were: pain primarily in the low back (mean duration = 109.9 months, standard deviation (SD) = 113.5 months); no
Continuous relative phase (CRP)
Kinematic data were processed using a low-pass, fourth-order, bidirectional Butterworth filter (cut-off 10 Hz). To obtain joint angles, orientation data for each sensor were used to create a [3 × 3] rotation matrix for the thigh, S1, L3, and T12 segments [10,21]. Sagittal joint angles during the flexion task were extracted for the left hip (hip relative to S1), lower lumbar spine (L3 relative to S1), and upper lumbar spine (T12 relative to L3), by multiplying the rotation matrix of one segment
Results
Means and SDs for participant demographics are found in Table 1.
Discussion
This study examined regional differences in inter-joint coordination (MARP) and flexion-relaxation (FRR) during a bending task, in people with and without LBP. Contrary to our hypothesis, the LBP group demonstrated more out-of-phase coordination (higher MARP) of the hip/lower lumbar joint pair than healthy adults. Additionally, the LBP group showed reduced FRRs for all muscles, with multifidus showing the greatest difference. No significant correlations between coordination and FRR were found,
Conclusion
Our study demonstrated more out-of-phase coordination of the hip/lower lumbar joint pair in patients with LBP during bending, that was largely driven by three outliers characterized by a distinctive pattern of motor control. The LBP group also showed significantly less flexion-relaxation in three trunk extensor muscles than healthy adults; a response driven by multifidus. While some elements of movement guarding were observed, these behaviors did not present in all people with LBP.
Declaration of Competing Interest
None.
Acknowledgements
Patrick Ippersiel is supported by an Institut de Recherche Robert-Sauvé en Santé et en Sécurité du Travail (IRSST) doctoral scholarship and an OPPQ-REPAR project grant for clinical research (programme 4.2.1). The funding sources had no involvement in this study or preparation of this manuscript. The authors would like to thank Francois Thénault for his help with data processing and coding.
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