The FRP is examined by performing dynamic trunk flexion/extension, and the movement is then traditionally divided into trunk flexion, maximal voluntary flexion (MVF) (also known as the relaxation phase) and extension phases 4. This phenomenon has been extensively reported in asymptomatic individuals but can be absent in individuals with low back pain (LBP) 1, 2, 3. The flexion relaxation phenomenon (FRP) refers to the myoelectrical silence of the superficial paraspinal muscles which is observed during trunk flexion 1.

This study provides novel insights into changes in lumbar muscle behavior in individuals with LBP. They also showed reduced normalized amplitude compared to those without the FRP, but still greater than pain-free controls (significant mean difference of 27.4% and 11.6% respectively). Individuals with LBP and the FRP exhibited a delay of its onset compared to pain-free controls (significant mean difference of 13.3% of trunk flexion). All pain-free controls and 64.3% of the individuals with LBP exhibited the FRP. Smoothing spline analysis of variance models and the contrast cycle difference approach using the Bayesian interpretation were used to determine statistical inference. The root mean square of the HDEMG signals was computed to generate the average normalized amplitude and the spatial FRP onset was determined and expressed as percentage of trunk flexion. To evaluate this, HDEMG signals were detected with grids of electrodes (13 × 5) placed bilaterally over the lumbar paraspinal muscles in individuals with and without LBP as they performed repetitions of full trunk flexion. Recent research using high-density electromyography (HDEMG) has provided a more precise understanding of the behaviour of the paraspinal muscles in people with low back pain (LBP) but so far, HDEMG has not been used to investigate the flexion relaxation phenomenon (FRP).

How to use emg test for lower back pain