Effect of individual flexibility and knee posture on the lumbar curvature and back muscle flexion-relaxation phenomenon

Back muscle flexion–relaxation phenomenon (FRP) studies have focused on patients with low back pain but little on individual flexibility. This study recruited 18 female participants (nine each with low and high flexibility) and analyzed the lumbar curvature and activation of the lumbar erector spinae, quadriceps, and hamstrings when the trunk flexed forward from 0° to 45°, 60°, 75°, and 90° under two knee postures (with and without the screw-home mechanism). The results show that individual flexibility significantly influenced FRP in the erector spinae during trunk forward flexion, whereas knee posture only slightly affected quadriceps activation. When trunk angle was 75° and 90°, the participants with low flexibility experienced lower erector spinae activation than did the participants with high flexibility. The participants with low flexibility also exhibited less lumbar curvature (by approximately 7°) during trunk forward flexion. This study inferred that flexible participants have a larger range of motion for trunk flexion; when the trunk flexes forward to 90°, greater lumbar curvature is maintained, resulting in a lower degree of FRP. The lumbar spine is the part most prone to injury at work and in daily life. The current results indicate that individual flexibility may be a crucial factor when considering FRP during clinical assessment and ergonomic applications (e.g., tasks with full trunk flexion). •Back muscle activity reduces during trunk flexing means flexion-relaxation phenomenon (FRP).•This study was to determine the effects of flexibility and knee posture on back muscle FRP.•Eighteen females with low and high flexibility (9 each) performed a trunk flexion test from 0° to 90°.•Flexible group revealed a lower degree of FRP and a greater lumbar curve during full trunk flexion.•Knee posture (screw-home mechanism of knees) only slightly affected the quadriceps activation.