Abstract
The flexion-relaxation phenomenon (FRP) in standing is a specific and sensitive diagnostic tool for low back pain. Seated flexion as an alternative could be beneficial for certain populations, yet the behavior of the trunk extensors during seated maximum flexion compared to standing flexion remains unclear. Compare FRP occurrences and spine angles between seated and standing flexion postures in three levels of the erector spinae muscles. Thirty-one participants free of back pain performed seated and standing maximum trunk flexion. Electromyographical signals were recorded from the bilateral lumbar (L3), lower-thoracic (T9), and upper-thoracic (T4) erector spinae and assessed for the occurrence of FRP. Spine angles corresponding to FRP onset and cessation were determined, and FRP occurrences and angles were compared between posture and muscle. FRP occurrence was similar in standing and seated maximum flexion across all muscles, with the lumbar muscles showing the greatest consistency. Standing FRP onset and cessation angles were consistently greater than the corresponding seated FRP angles. Considering the similar number of FRP occurrences, seated maximum flexion may constitute an objective criterion for low back pain diagnosis. Future work should seek to confirm the utility of this test in individuals with low back pain.
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