Difference in the Flexion Relaxation Phenomenon between Sitting and Standing

Purpose: We investigated differences in the flexion relaxation phenomenon (FRP) between sitting and standing using electromyography. Subjects and Methods: We measured the activities of the lumbothoracic spine muscles, and the multifidus, gluteus maximus, and biceps femoris of 10 healthy males. The activities of trunk flexion, at the initiation of extension to restore posture from maximal trunk flexion were measured in the standing, sitting, upright sitting and slump sitting positions. The appearance of FRP, the ratio of muscle activities (FRR), the ratio of the upright/slump muscle activities (Rest FRR), and the ratio of muscle activities excluding the activity at maximal flexion (Movement FRR) were calculated. Results: The appearance of FRP was high in the standing position, and low in the sitting position. Rest FRR was high in the sitting position, and Movement FRR was high in the standing position. Conclusion: The appearance of FRP in the sitting position is too infrequent for examination purposes; thus, Rest FRR in the sitting position, and Movement FRR in the standing position are the representative quantitative indices of FRP.