Background and Aim: Several biomechanical and clinical studies have examined the apparent myoelectric silence of the lumbar spine extensor musculature during lumbar flexion from a neutral upright standing posture.

Objective: The purpose of this study was to investigate the relationship between trunk flexibilitiy and flexion relaxation response of the the lumbar erector spinae

Material and Methods: 30 female healthy students were participated in this study which They were arranged in three groups on toe touch test (low flexible,  normal and high flexible).The surface electromyographic from the lumbar erector spinae muscles as well as flexion angle of the trunk, hip and lumbar were recorded. The angle in which electromyographic diminished during flexion and initiated during extension was determined and subjected to one way Anova to determine any significant differences  in the flexion-relaxation response between three groups.

Results: The erector spinae muscles were relaxed in  larger angles during flexion and initiated activity significantly earlier during extension in high flexible group (p<0/001). pearson correlation indicate that there were high correlation between trunk and hip angle during full flexion (r=0/88 p<0/001) and no correlation between trunk and lumbar angle (r=0/1 p< 0.3).

Conclusion: The results indicated that individuals with more flexible trunk , the erector spinae muscles gets relaxed in further angles of the trunk and the hip, furthermore while returing to the upright posture ,this muscle is recruiting sooner than two other groups .the biomechanical basis of this finding relies in ligamentous and passive tissues properties of individuals with different Flexibilities of the trunk which can effect there recruitment pattern of active tissues.