Posture Biomechanics 3 Sagittal plane analysis

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Biomechanics of theposture : 3sagittal plane analysis : 

Biomechanics of theposture : 3sagittal plane analysis Dr. Dibyendunarayan Bid [PT] The Sarvajanik College of Physiotherapy, Rampura, Surat

Sagittal Plane Alignment and Analysis : 

Sagittal Plane Alignment and Analysis 2

Ankle : 

Ankle In the optimal erect posture, the ankle joint is in the neutral position, or midway between dorsiflexion and plantarflexion. The LoG passes slightly anterior to the lateral malleolus and, therefore, anterior to the ankle joint axis. The anterior position of the LoG in relation to the ankle joint axis creates an external dorsiflexion moment that must be opposed by an internal plantarflexion moment to prevent forward motion of the tibia. 3

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In the neutral ankle position, there are no ligamentous checks capable of counterbalancing the external dorsiflexion moment; therefore, activation of the plantarflexors creates the internal plantarflexion moment that is necessary to prevent forward motion of the tibia. 4

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The soleus muscle contracts and exerts a posterior pull on the tibia and in this way is able to oppose the dorsiflexion moment (Fig. 13-9). If the force that the muscle can exert is less than the gravitational moment, the tibia will move the ankle into dorsiflexion and the soleus muscle will undergo an eccentric contraction while trying to oppose the forward motion of the tibia. 5

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EMG studies have demonstrated that soleus and gastrocnemius activity is fairly continuous in normal subjects during erect standing. This activity suggests that these muscles are exerting a minimal but constant internally generated plantarflexion torque about the ankles to oppose the normal external gravitational dorsiflexion moment. 6

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Ankle joint muscles that have shown inconsistent activity in EMG recordings during standing are the tibialis anterior, peroneal, and tibialis posterior muscles. It is possible that these muscles may be helping to provide transverse stability in the foot during postural sway rather than acting to oppose the external dorsiflexion at the ankle joint. 7

Knee : 

Knee In optimal posture, the knee joint is in full extension, and the LoG passes anterior to the midline of the knee and posterior to the patella. This places the LoG just anterior to the knee joint axis (see Figs. 13-8 and 13-9). The anterior location of the gravitational line in relation to the knee joint axis creates an external extension moment. 8

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The counterbalancing internal flexion moment created by passive tension in the posterior joint capsule and associated ligaments is usually sufficient to balance the gravitational moment and prevent knee hyperextension. 9

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However, a small amount of activity has been identified in the hamstrings. Activity of the soleus muscle may augment the gravitational extension moment at the knee through its posterior pull on the tibia as it acts at the ankle joint. In contrast, activity of the gastrocnemius muscle may tend to oppose the gravitational extension moment because the muscle crosses the knee posterior to the knee joint axis. 10

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Hip and Pelvis : 

Hip and Pelvis In optimal posture, according to Kendall and McCreary, the hip is in a neutral position and the pelvis is level with no anterior or posterior tilt (Fig. 13-11A). In a level pelvis position, lines connecting the symphysis pubis and the anterior-superior iliac spines (ASISs) are vertical, and the lines connecting the ASISs and posterior-superior iliac spines (PSISs) are horizontal. In this optimal position, the LoG passes slightly posterior to the axis of the hip joint, through the greater trochanter. 13

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However, during postural sway, the LoG may pass anterior to the hip joint axis, and contraction of the hip exterior may be required. The posterior location of the gravitational line in relation to the hip joint axis creates an external extension moment at the hip that tends to rotate the pelvis (proximal segment) posteriorly on the femoral heads (see Fig. 13-11B). 14

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EMG studies have shown activity of the iliopsoas muscle during standing, and it is possible that the iliopsoas is acting to create an internal flexion moment at the hip to prevent hip hyperextension. If the gravitational extension moment at the hip were allowed to act without muscular balance, as in a so-called relaxed or swayback posture, hip hyperextension ultimately would be checked by passive tension in the iliofemoral, pubofemoral, and ischiofemoral ligaments. 15

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In the swayback standing posture, the LoG drops farther behind the hip joint axes than in the optimal posture (Fig. 13-12). 16

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Therefore, the swayback posture does not require any muscle activity at the hip but causes an increase in the tension stresses on the anterior hip ligaments, which could lead to adaptive lengthening of these ligaments if the posture becomes habitual. 17

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Also, because of the diminished demand for hip extensor activity, the gluteal muscles may be weakened by disuse atrophy if the swayback posture is habitually adopted. The relaxed standing or sway posture may also increase the magnitude of the gravitational torque at other joints in the body. 18

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Lumbosacral and Sacroiliac Joints : 

Lumbosacral and Sacroiliac Joints The average lumbosacral angle measured between the bottom of the L5 vertebra and the top of the sacrum (S1) is about 30° but can vary between 6° and 30°. Anterior tilting of the sacrum increases the lumbosacral angle and results in an increase in the shearing stress at the lumbosacral joint and may result in an increase in the anterior lumbar convexity in standing (Fig. 13-13A). 21

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In the optimal posture, the LoG passes through the body of the fifth lumbar vertebra and close to the axis of rotation of the lumbosacral joint. Gravity therefore creates a very slight extension moment at L5 to S1 that tends to slide L5 and the entire lumbar spine down and forward on S1. 23

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This motion is is opposed primarily by the anterior longitudinal ligament and the iliolumbar ligaments. Bony resistance is provided by the locking of the lumbosacral zygapophyseal joints. When the sacrum is in the optimal position, the LoG passes slightly anterior to the sacroiliac joints. 24

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The external gravitational moment that is created at the sacroiliac joints tends to cause the anterior superior portion of the sacrum to rotate anteriorly and inferiorly, whereas the posterior inferior portion tends to move posteriorly and superiorly (see Fig. 13-13B). Passive tension in the sacrospinous and sacrotuberous ligaments provides the internal moment that counterbalances the gravitational torque by pre-venting upward tilting of the lower end of the sacrum. 25

The Vertebral Column : 

The Vertebral Column There is considerable variation among individuals, as can be seen in Table 13-1, but the average values are fairly close to one another in the studies presented. In the optimal configuration, the curves of the vertebral column should be fairly close to average or normal con-figuration described in Chapter 4. The optimal position of the plumb line LoG is through the midline of the trunk (Fig. 13-14). 26

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Although not confirming either Cailliet’s or Kendall and McCreary’s hypotheses, EMG studies have shown that the longissimus dorsi, rotatores, and neck extensor muscles exhibit intermittent electrical activity during normal standing. This evidence suggests that ligamentous structures and passive muscle tension are unable to provide enough force to oppose all external gravitational moments acting around the joint axes of the upper vertebral column. 30

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In the lumbar region, where minimal muscle activity appears to occur, passive tension in the anterior longitudinal ligament and passive tension in the trunk flexors apparently are sufficient to balance the external gravitational extension moment. 31

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Head : 

Head The LoG in relation to the head passes slightly anterior to the transverse (frontal) axis of rotation for flexion and extension of the head and creates an external flexion moment (Fig. 13-15). This external flexion moment, which tends to tilt the head forward, may be counteracted by internal moments generated by tension in the ligamentum nuchae, tectorial membrane, and posterior aspect of the zygapophyseal joint capsules and by activity of the capital extensors. 33

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Ideally, a plumb line extending from the ceiling should pass through the external auditory meatus of the ear, and the head should be directly over the body’s CoM at S2. 34

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Alignment of Body Segments in the Sagittal Plane : 

Alignment of Body Segments in the Sagittal Plane Table 13-3 shows the relationship of the LoG to various body segments in the sagittal plane. However, the reader must realize that the swaying motion that occurs in the normal erect posture will change the position of the LoG in relation to individual joint axes. The CoP also will move during swaying. 36

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For example, if the amount of forward sway is large enough, the LoG may move from the optimal posterior location in relation to the hip joint axis to a position anterior to the hip joint axis. The CoP will move anteriorly toward the toes. 37

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The resulting external flexion moment at the hip created by the change in position of the LoG may be counteracted by activity of the hip extensors, which will move the LoG and CoP posteriorly. On the other hand, increased activity in the soleus muscles rather than in the hip extensors might be used to bring the entire body and thus the LoG back into a position posterior to the hip joint axis. 38

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Some independent motion may occur in each leg, and relative motion may occur between body segments in response to postural sway. 39

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If your body is suddenly thrust forward, either by someone bumping into you or by a sudden backward movement of the supporting surface, a large and forceful movement of the LoG will occur. Consequently, flex-ion moments will be created at the neck and head; cervical, thoracic, and lumbar spines; hip; and ankle. To counteract these moments, the neck, back, hip extensor, and ankle plantarflexor muscles may have to contract. 40

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The CNS responds with activation of a muscle or pattern of muscles that will counteract the inertial and flexion moments, bring the LoG back over the CoM, and reestablish static erect equilibrium and stability. Furthermore, individual variations in the curves and apices will cause changes not only in external gravitational moments but also in the amount of internal counterforce that is necessary. 41

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End of part - 3 43

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