Transient Trunk Flexion: The Potential to Alleviate Low Back Pain During Prolonged Standing

Introduction: The flexion-relaxation phenomenon (FRP) represents myoelectric silencing of the erector spinae musculature during peak trunk flexion. In individuals with chronic low back pain (LBP), the FRP response is absent. Acute LBP development is commonly induced by prolonged static postures such as standing and tends to be temporary in nature. If FRP is present in people who develop acute LBP, it is plausible that intermittent peak trunk flexion may reduce discomfort development by inducing periods of muscle rest experienced during FRP. The purpose of this study is two-fold: 1) to determine if transient trunk flexion mitigates pain development induced by prolonged standing; and 2) to determine if FRP occurs and changes over time due to prolonged standing. Methods: Sixteen participants free of LBP in the previous 12 months were recruited to complete two separate periods (one week apart; randomized) of a 2-hour standing protocol. On one of the days participants stood statically for the full 2 hours (control day); on the other day participants were instructed to bend forward to full spine flexion for 5 seconds every 15 minutes to illicit FRP (flexion day). During the full spine flexion periods, erector spinae and gluteus medius muscle activation was recorded using surface electromyography (EMG). Lumbar flexion angles were recorded to determine the degree of flexion when FRP occurred. During flexion days, participants reported low back discomfort prior to and following each flexion trial using a 100mm visual analogue scale (VAS). On the control day, participants reported discomfort every 15 minutes, again using the VAS. Based on control day VAS scores, a cut-off of 8mm of discomfort was used to categorize participants as low back discomfort developers (LBD developers) or non-low back discomfort developers (non-LBD developers). These two groups Running head: TRUNK FLEXION AND STANDING INDUCED PAIN ii were then statistically compared using two-way mixed model ANOVAs to determine any differences in the FRP response between discomfort and condition. Results: Twelve of the sixteen participants were classified as LBD developers (16.19mm VAS (SE 1.78)) and four participants were considered non-LBD developers (1.64mm VAS (SE 0.4)). FRP was present in all participants regardless of discomfort development. No notable differences were seen in the FPR response over time, though LBD developers experienced significantly greater muscle activation during FRP than non-LBD developers. The transient flexion trials significantly decreased the level of low back discomfort (LBD) after 75 minutes of standing (p=0.01) when VAS scores during control days were compared to those recorded immediately after flexion; LBD decreased by approximately 10mm of discomfort on the VAS after flexion. Further, in an exit survey, 81% of participants reported that flexing forward was beneficial and helped to alleviate any discomfort development that occurred during the two hour standing protocol. An additional and unexpected finding of this study was the presence of FRP not only in the erector spinae but also in the gluteus medius. Discussion and Conclusion: People who developed acute LBD during prolonged standing continued to experience erector spinae muscle FRP. The FRP response in acute LBD developers seems to more closely resemble healthy populations than chronic LBP populations, for which it has been shown to be absent. However, the increased muscle activation during FRP seen in LBD developers suggests that they may be considered a sub-clinical population prone to developing chronic LBP. An interesting and novel finding of this work is the occurrence of the FRP in the gluteus medius muscle. Previous research has identified that coactivation of the gluteus medius is likely a predisposing factor to LBP development. It Running head: TRUNK FLEXION AND STANDING INDUCED PAIN iii is possible that during the flexion trials, FRP of the erector spinae and gluteus medius muscle mitigated LBP development. This study has shown that transient trunk flexion significantly reduces LBD experienced during prolonged standing. Running head: TRUNK FLEXION AND STANDING INDUCED PAIN iv