Pelvic Motions and Muscle Function

The pelvis is the connecting link between the spine and lower extremities. Movement of the pelvis causes motion at the hip joints and lumbar spine articulations. The hip musculature causes pelvic motion through reverse action. Hip flexors cause an anterior pelvic tilt; hip extensors, a posterior pelvic tilt; and abductors and adductors, a lateral pelvic tilt. Rotators cause pelvic rotation. To prevent excessive pelvic motion when moving the femur at the hip joint, the pelvis must be stabilized by the abdominals, erector spinae, multifidus, and quadratus lumborum muscles.

Anterior Pelvic Tilt

The anterior superior iliac spines of the pelvis move anteriorly and inferiorly and thus closer to the anterior aspect of the femur as the pelvis rotates forward around the transverse axis of the hip joints. This results in hip flexion and increased lumbar spine extension.

• Muscles causing this motion are the hip flexors and back extensors.

• During standing, the line of gravity of the trunk falls anterior to the axis of the hip joints; the effect is an anterior pelvic tilt moment. Stability is provided by the abdominal muscles and hip extensor muscles.

Posterior Pelvic Tilt

The posterior superior iliac spines of the pelvis move posteriorly and inferiorly, thus closer to the posterior aspect of the femur as the pelvis rotates backward around the axis of the hip joints. This results in hip extension and lumbar spine flexion.

• Muscles causing this motion are the hip extensors and trunk flexors.

• During standing when the line of gravity of the trunk falls posterior to the axis of the hip joints, the effect is a posterior pelvic tilt moment. Dynamic stability is provided by the hip flexors and back extensors and passive stability by the iliofemoral ligament.

Pelvic Shifting

During standing, a forward translatory shifting of the pelvis results in extension of the hip and extension of the lower lumbar spinal segments. There is a compensatory posterior shifting of the thorax on the upper lumbar spine with increased flexion of these spinal segments. This is often seen with slouched or relaxed postures. Little muscle action is required; the posture is maintained by the iliofemoral ligaments at the hip, anterior longitudinal ligament of the lower lumbar spine, and posterior ligaments of the upper lumbar and thoracic spine.

Lateral Pelvic Tilt

Frontal plane pelvic motion results in opposite motions at each hip joint. Pelvic motion is defined by what is occurring to the iliac crest of the pelvis that is opposite the weight-bearing extremity (that is, the side of the pelvis that is moving). When the pelvis elevates, it is called hip hiking; when it lowers, it is called hip or pelvic drop. On the side that is elevated, there is hip adduction; on the side that is lowered, there is hip abduction. During standing, the lumbar spine laterally flexes toward the side of the elevated pelvis (convexity of the lateral curve is toward the lowered side).

• Muscles causing lateral pelvic tilting include the quadratus lumborum on the side of the elevated pelvis and reverse muscle pull of the gluteus medius on the side of the lowered pelvis.

• With an asymmetrical slouched posture, the person shifts the trunk weight onto one lower extremity and allows the pelvis to drop on the other side. Passive support comes from the iliofemoral ligament and iliotibial band on the elevated side (stance leg).

• When standing on one leg, there is an adduction moment at the hip, tending to cause the pelvis to drop on the unsupported side (hip or pelvic drop). This is prevented by the gluteus medius stabilizing the pelvis on the stance side.

Pelvic Rotation

Rotation occurs around one lower extremity that is fixed on the ground. The unsupported lower extremity swings forward or backward along with the pelvis. When the unsupported side of the pelvis moves forward, it is called forward rotation of the pelvis. The trunk concurrently rotates in the opposite direction, and the femur on the stabilized side concurrently rotates internally. When the unsupported side of the pelvis moves backward, it is called posterior rotation; the femur on the stabilized side concurrently rotates externally, and the trunk rotates opposite.

Lumbopelvic Rhythm

A coordinated movement between the lumbar spine and pelvis occurs during maximum forward bending of the trunk as when reaching toward the floor or the toes. As the head and upper trunk initiate flexion, the pelvis shifts posteriorly to maintain the center of gravity over the base of support. The trunk continues to forward-bend, being controlled by the extensor muscles of the spine, until at approximately 45°. At this point for an individual with relatively normal flexibility, the posterior ligaments become taut, and the facets of the zygapophyseal joints approximate. Both of these factors provide stability for the intervertebral joints, and the muscles relax. Once all of the vertebral segments are at the end of the range and stabilized by the posterior ligaments and facets, the pelvis begins to rotate forward (anterior pelvic tilt), being controlled by the gluteus maximus and hamstring muscles. The pelvis continues to rotate forward until the full length of the muscles is reached. Final range of motion (ROM) in forward bending is dictated by the flexibility of the various back extensor muscles and fasciae as well as hip extensor muscles.

The return to the upright position begins with the hip extensor muscles rotating the pelvis posteriorly through reverse muscle action (posterior pelvic tilt) then the back extensor muscles extending the spine from the lumbar region upward. Variations in the normal synchronization of this activity occur because of training (as with dancers and gymnasts), faulty habits, restricted muscle or fascia length, or injury and faulty proprioception.

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