According to anatomical textbooks [9, 18], the PM
1. Flexes the lumbar spine (LS) and bends it laterally
2. Powerfully flexes the femur and rotates it a little outwards,and
3. If the femora are fixed, flexes lumbar spine and pelvis.
Abstract Psoas muscle (PM) function with regard to the lumbar spine (LS) is disputed. Electromyographic studies attribute to the PM a possible role as stabilizer. Anatomical textbooks describe the PM as an LS flexor, but not a stabilizer. According to more recent anatomical studies, the PM does not act on the LS, because it tends to pull the LS into
more lordosis by simultaneously flexing the lower and extending the upper region, but due to the short moment arms of its fascicles, this would require maximal muscular effort and would expose the LS motion segments to dangerous compression and shear. The findings of the present study indicate that the described opposite action of the PM on upper and lower LS regions, performed
passively and requiring minimal muscular effort, may serve to stabilize the LS in an upright stance. It was demonstrated that a vertically placed elastic metal strip, modelled into a lordotic configuration to imitate the LS, will be brought into more lordosis, with maintenance of vertical position, if a string fastened at its upper end is pulled downward in a very specific direction. Conversely, any increase of lordosis of the strip brought about by vertical
downward pushing of its top, will be stabilized by tightening the pulling string in the same specific direction. As this direction corresponded with the psoas orientation, the experiments
show that the PM probably functions as a stabilizer of the lordotic LS in an upright stance by
adapting the state of contraction of each of its fascicles to the momentary degree of lordosis imposed by factors outside the LS, such as general posture, general muscle activity and weight bearing. The presence of multiple PM fascicles, all of about equal length, and attaching to all LS levels, facilitates this function.
The textbooks do not describe the PM as having a stabilizing function with respect to the LS.
According to anatomical textbooks [9, 18], the PM1. Flexes the lumbar spine (LS) and bends it laterally2. Powerfully flexes the femur and rotates it a little outwards,and3. If the femora are fixed, flexes lumbar spine and pelvis.Abstract Psoas muscle (PM) function with regard to the lumbar spine (LS) is disputed. Electromyographic studies attribute to the PM a possible role as stabilizer. Anatomical textbooks describe the PM as an LS flexor, but not a stabilizer. According to more recent anatomical studies, the PM does not act on the LS, because it tends to pull the LS intomore lordosis by simultaneously flexing the lower and extending the upper region, but due to the short moment arms of its fascicles, this would require maximal muscular effort and would expose the LS motion segments to dangerous compression and shear. The findings of the present study indicate that the described opposite action of the PM on upper and lower LS regions, performedpassively and requiring minimal muscular effort, may serve to stabilize the LS in an upright stance. It was demonstrated that a vertically placed elastic metal strip, modelled into a lordotic configuration to imitate the LS, will be brought into more lordosis, with maintenance of vertical position, if a string fastened at its upper end is pulled downward in a very specific direction. Conversely, any increase of lordosis of the strip brought about by verticalกดลงด้านบนของ จะมีเสถียร โดยขันสาย pulling ในทิศทางเดียวกันเฉพาะ ตามทิศทางนี้ corresponded กับแนว psoas การทดลองดูที่ PM คงเป็นโคลงของ LS lordotic ในท่าทางตรงตามดร.รัฐหดตัวแต่ละ fascicles ที่ระดับอับของ lordosis กำหนด โดยปัจจัยภายนอก LS ท่าทั่วไป กิจกรรมของกล้ามเนื้อทั่วไป และแบกน้ำหนัก ของหลาย PM fascicles ทั้งหมดยาวเท่า และแนบทุกระดับ LS อำนวยความสะดวกนี้ฟังก์ชัน ตำราอธิบาย PM มีฟังก์ชันเกี่ยวกับการ LS สมัยมีเสถียรภาพ
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