Effects of Trunk Angle in the Sitting and Standing Postures on the Static Flexion Relaxation Phenomenon

[Purpose] The effects of different postures on the static flexion relaxation phenomenon were compared using electromyography, to clarify whether the angles for the activation and relaxation of the spinal muscles are different. [Subjects and Methods] Thirty healthy males maintained several different trunk inclination angles while standing and sitting. The muscle activities were measured and compared among the different postures and the different trunk angles: 0, 10, 20, 30, 40, 50, 60, and maximum forward leaning. The thoracic and lumbar erector spinae, multifidus, gluteus maximus, and biceps femoris were the target muscles. [Results] Muscle activities increased with increasing forward inclination angle, but at low inclination angles in the sitting position, the activities of the multifidus and erector spinae were constant. Between the postures, trunk muscle activities became significantly lower in the sitting position than in the standing position with increasing inclination. [Conclusion] Compared to standing, muscle activities at low inclination angles have low, constant values.