The relation between the flexion relaxation phenomenon onset angle and lumbar spine muscle reflex onset time in response to 30 min of slumped sitting

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Abstract

Viscoelastic creep of spine tissue, induced by submaximal spine flexion in sitting, can delay the onset of the flexion-relaxation phenomenon (FRP) and low back reflexes (LBR). Theoretically, these two outcome measures should be correlated; however, no studies have investigated this. This study aims to determine whether 30 min of near-maximal spine flexion will affect the onset of FRP and LBR in the lumbar erector spinae (LS) and lumbar multifidus (LM), and to examine the relation between these parameters. 15 participants were recruited (9F, 6M). Spine angle (between L1 and S2) was monitored synchronously with bilateral muscle activity in the LS (L1) and the LM (L4). FRP onset and LBR were measured in a randomized order before and after 30 min of slouched sitting. No significant difference was found for any muscle LBR onset time between pre and post-sitting (p > 0.05). A significant increase in FRP onset was found in the RLM (p = 0.016) following sitting. No significant correlation was found between the FRP and the LBR for any muscle. These results suggest that the LBR onset might not be as sensitive as an outcome measure to investigate shorter exposures of sitting as FRP.

Introduction

In a large number of industries, occupational tasks require workers to sit for long periods of their workday (Shin, D’Souza, & Liu, 2009). Prolonged flexed postures have been suggested to induce changes in the tissue properties of the low back, which can lead to a possible low back injury (Sánchez-Zuriaga, Adams, & Dolan 2010). For instance, creep, a change in the length of viscoelastic structures, can result in biomechanical dysfunction by reducing the functional stiffness and responsiveness of reflexive activation of important postural muscles (Solomonow, Baratta, Banks, Freudenberger, & Zhou, 2003a). Sustained postures have been observed to induce these viscoelastic changes in the passive structures surrounding the spine, such as ligaments and tendons (Bazrgari et al., 2011, McGill and Brown, 1992, Larson et al., 2020). The increase in joint laxity due to creep can increase the trunk flexion range of a worker, heightening their risk of a hyperflexion injury (McGill & Brown, 1992). These injuries may cause or further exacerbate pain, which can limit or alter the daily functioning of an individual.

The changes that are induced by creep have been observed to influence the flexion-relaxation phenomenon (FRP) of the low back muscles as well as the onset of low back reflexes (LBR) (Howarth et al., 2013, Hendershot et al., 2011, Radebold et al., 2001). The FRP is a biomechanical phenomenon where back muscle activity turns off at a certain point in the flexion range (Howarth et al., 2013). This cessation in muscle activity is due to the passive tissues being able to withstand the load from the external low back moment of the torso without help from the muscles (Li et al., 2019). This typically occurs near the end range of flexion. It has been previously shown that prolonged spine flexion reduces the passive support of the spine (McGill & Brown, 1992), increasing the point during the flexion range where the low back muscles turn off (FRP onset) (Solomonow et al., 2003a, Howarth et al., 2013). The LBR is the activation of the trunk muscles in response to a sudden perturbation (Reeves, Cholewicki, & Milner, 2005). In cats, these reflexes have been shown to have a delayed response after the low back tissues have been passively stretched in a prolonged flexion posture, where creep is induced (Solomonow et al., 2003b). This delay in trunk reflexes is often found in people who experience low back pain (Radebold, Cholewicki, Panjabi, & Patel, 2000). Both the FRP and LBR provide important insights into the stiffness and function of the spine. In theory, if any lengthening occurs in spine tissues, this change should be reflected in increases in both the FRP onset point and LBR onset time.

Currently, the FRP and LBR measures are used in laboratory-based studies. Since they can indirectly assess viscoelastic changes in the low back, there may be potential to use them in the field to monitor injury risk in workers. If these measures are highly correlated, one could be used as a surrogate measure for the other. To our knowledge, this has yet to be investigated. Therefore, the primary objective of this research study was to determine if there is a positive correlation between the changes in the onset of the FRP and LBR following a 30-minute near-maximal flexion exposure. A secondary objective was to determine if there is a significant change in the onset of these measures in the lumbar multifidus (LM) and lumbar erector spinae (LS) muscles following the sitting exposure.

Section snippets

Participants

Based on sample size calculations using previously published LBR and FRP data (and choosing the larger sample) we aimed to recruit 42 healthy participants from the general population for this study. Inclusion criteria were adults aged 18–69 with no low back impairments. Low back impairments were classified as a history of spinal tumors, back pain in the past 6 months, surgery, fracture, infection, or inflammatory arthritis (i.e., rheumatoid arthritis or ankylosing spondylitis). Participants

Participants

15 participants were recruited and included in the study. Participant characteristics were as follows: 6 males (20.83 ± 1.57 years, 184.49 ± 6.82 cm, 77.70 ± 12.11 kg) and 9 females (21.89 ± 1.59 years, 161.73 ± 7.19 cm, 60.64 ± 7.12 kg). The age range of these participants was 18–26 years.

Sitting exposure

During the sitting exposure, participants adopted an average lumbar angle of 87.53 ± 22.95% of maximum lumbar spine flexion.

FRP

Following the sitting exposure, a statistically significant increase in FRP onset

Discussion

The current investigation examined whether or not the change in the onset of the FRP and LBR are correlated following 30 min of near-maximal lumbar flexion. Additionally, the study aimed to determine whether the sitting exposure would significantly change the onset of the FRP and LBR compared to baseline measures. This study found no statistically significant correlations between the FRP and LBR measures for each muscle, a statistically significant increase in the angle of FRP onset in the RLM

Acknowledgements

The authors would like to thank Mona Frey for her assistance with the statistical analysis. This study was funded by an NSERC Discovery Grant (#20161771).

Sarah Mackey is a graduate student in the Faculty of Medicine at Memorial University of Newfoundland. She earned her Bachelor's Degree (Honours) in Kinesiology from the School of Human Kinetics and Recreation in 2020. Her research interests are the biomechanics of the low back, low back pain and physician postures during clinical procedures.

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Sarah Mackey is a graduate student in the Faculty of Medicine at Memorial University of Newfoundland. She earned her Bachelor's Degree (Honours) in Kinesiology from the School of Human Kinetics and Recreation in 2020. Her research interests are the biomechanics of the low back, low back pain and physician postures during clinical procedures.

Jason Barnes earned his Bachelor's Degree (Honours) in Kinesiology from the School of Human Kinetics and Recreation in 2020. He is currently a physiotherapy student at Dalhousie University in Halifax, Canada.

Kristen Pike earned her Bachelor's Degree (Honours) in Kinesiology from the School of Human Kinetics and Recreation in 2020. She is currently a physiotherapy student at Dalhousie University in Halifax, Canada.

Dr. Diana De Carvalho is an Assistant Professor in the Faculty of Medicine at Memorial University of Newfoundland. She obtained her doctorate in 2015 from the Department of Kinesiology in the Faculty of Applied Health Sciences at the University of Waterloo. Dr. De Carvalho's research program focuses on the spine biomechanics of prolonged spine flexion, clinical low back pain and ergonomic/treatment interventions.

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