Biomechanics of standing posture

"Posture is a composite of the positions of all the joints of the body at any given moment..." (Kendall, McCreary, & Provance, 1993, p.71)

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An ideal posture:

• is stable; postural alignment maintains the body's mass over its base of support.

• minimizes stress and strain on tissues
  • statically; at rest
  • dynamically; during movement

• minimizes energy cost

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To judge how well a posture maintains stability, we analyze the body's alignment with regard to the gravity's line of application, the "gravity line."

In the sagittal plane, the gravity line is located:

1. anterior to the ankle joint's lateral axis, producing an ankle dorsiflexion moment, necessitating activity in the ankle plantar flexors.

2. anterior to the knee joint's lateral axis, producing a knee extensor moment, necessitating no muscle activity, just passive tension in posterior knee ligaments.

3. posterior to the hip joint's lateral axis, producing a hip extensor moment, necessitating no muscle activity, just passive tension in anterior hip ligaments (iliofemoral ligament).

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When we must maintain the center of gravity aligned over the base of support, certain joint movements indirectly affect other joints. For instance, in a closed chain:

• ankle dorsiflexion causes knee flexion
• ankle plantar flexion produces knee extension
• hip extension produces knee extension
• hip flexion produces knee flexion

In closed-chain postures like standing, changes in one joint's alignment often require changes at other joints. Here are several examples:

1. While standing, turn your kneecaps inward, then outward. Notice how rotating the lower extremity affects the subtalar joint's alignment in a closed chain:
   • tibial internal rotation -> flattening of arch -> subtalar pronation

   • tibial external rotation -> raising of arch -> subtalar supination

   Notice how rotating tibia with respect to the femur affects knee motion, including the screw-home mechanism:

   • tibial internal rotation -> reversal of screw-home mechanism -> knee flexion

   • tibial external rotation -> screw-home mechanism -> knee extension

2. You can produce similar sets of complementary lower extremity movements by moving the pelvis in the transverse plane. While standing with weight on both feet, "twist" so that:
   • the right side of pelvis moves forward and
   • the left side of pelvis moves backward.

   Notice how, in the RLE:

   the knee flexes with relative tibial internal rotation and subtalar pronation

   While in the LLE:

   the knee extends with relative tibial external rotation and subtalar supination

3. Moving the pelvis in the sagittal plane, while standing with the legs fixed, also requires complementary movements in the hip joints and the lumbar spine.

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Frontal plane alignment

If the pelvis is not level in the frontal plane, postural compensations are necessary to maintain alignment of the body's center of gravity over the base of support.

For example, if the pelvis' left side is lower than its right side, the hip joints and the spine assume postures that maintain the body's stability:

the spine sidebends to the right. The sidebending induces coupled motion. the right hip joint is chronically adducted. the left hip joint is chronically abducted. In Control of Human Movement II, students investigate this posture's long-term effects on muscle length and tension.

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Coupled motion in the spine

Sidebending and rotation are automatically coupled in a curved rod like the spine. In situations where the spine sidebends to the right: The thoracic spine rotates to the left at the same time that it sidebends to the right. The thoracic spine's leftward rotation causes the ribs to protrude posteriorly more on the left (the convex side of the thoracic curve).

a more complete discussion of coupled motion in the spine.

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Last updated 4-2-01 ©Dave Thompson PT
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