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PHED 386 Biomechanics of the Spine Today’s objectives… Analyze structure vs. function of the spine Identify factors influencing mobility & stability of regions of the spine Identify the relationship between muscle location & its effectiveness on the trunk List mechanical contributions to common injury Structure of the spine Vertebral column Curved stack of 33 vertebrae Movements: motion segments Joints of the movement segment Symphysis: bodies & discs Gliding (diarthroses): facet joints Structure of the Spine Vertebrae Body, neural ring, processes Processes provide MA for mms Body size-function relationship Spines & facet joint relationship Facet joints assist in loading Facet joint compression Structure of the spine Intervertebral discs Composition Annulus fibrosus: collagen bands cross vertically at 30° to each other Nucleus pulposus: 90% water + collagen & proteoglycans Function Bind vertebrae together Allow for movement Bear & distribute loads Structure of the spine Mechanics of discs Flexion, extension & lateral flexion forces Rotation forces Structure of the spine Compression forces Water loss & sodium /potassium gain Results in loss of ~2cm of height Within first 30 minutes Increased volume stiffness Theory: risk of disc injury early in day Disc nutrition Blood supply to age 8 Pumping action from position change Negative aspect of fixed position Affect of injury & aging Discs and aging • • • • Reduced water absorption Decreased shock absorbing Abnormal movements between vertebrae Loads assumed by other structures Reduced spinal column height Postural alterations Ligaments of the spine Anterior longitudinal ligament Posterior longitudinal ligament Supraspinous ligament (ligamentum nuchae) Short ligaments Interspinous Intertransverse Ligamentum flavum** Spinal curves Primary Secondary Thoracic & sacral Cervical & lumbar Life changes Postural & mechanical changes ?? Wolff’s law Lordosis Associated w/weak abdominal mm’s & anterior tilt Causes Kyphosis Cause (Scheuermann’s disease) Population/s Scoliosis Lateral/rotational deformity Causes Neck Normal Lower Back Hypolordotic Military Kyphotic S-Shaped Movements of the spine Flexion, extension, hyperextension Greatest in cervical & lumbar regions Lateral flexion & rotation Greatest in cervical Least in lumbar Muscles of the spine Complex arrangement Naming Pairs Unilaterally or bilaterally Muscles of the spine Anterior (cervical region) Pre-vertebral muscles Rectus capitus anterior & lateralis Longus capitus Longus colli Hyoid muscles – 8 pairs Bilateral tension: flexion of the head Unilateral: lateral flexion or rotation Muscles of the spine Anterior Abdominals Rectus abdominus External obliques Internal obliques Bilateral tension: flex spine, reduce anterior tilt Unilateral tension: lateral flexion, rotaion Abdominal wall function Muscles of the spine Posterior Cervical Spenius capitus Primary extensors Splenius cervicis Assisted by smaller/deeper mms Thoracic & lumbar Errector spinae Sacrospinalis, semispinalis, Spinalis, longissimus, iliocostalis Deep spinal mms Muscles of the spine Lateral Sternocleidomastoid (antagonistic) Scalenes Lumbar region Quadratus lumborum Psoas major Loads on the spine Forces acting on Body weight Tension in ligaments Muscles tension Intra-abdominal pressure External loads Axial loading Spinal compression Loads on the spine Effect of center of gravity Role of spinal extensor mms Effect of actual forward bending or arm position Spinal mms have very small moment arms must generate large forces Increased compression forces on lumbar spine Loads on the spine Shear forces on spine On lumbar spine in standing position Increases during flexion & hyperextension Contributor to disc herniation Loads on the spine Flexion relaxation phenomenon Tension in extensors increases w/flexion At full flexion – extensor tension disappears Flexion torque supported by posterior spinal ligaments But increased shear from tension in interspinous ligaments Also increased facet loading Loads on the spine Lateral flexion and rotation Role of movement speed Common injuries Low back pain 75-80% of population Second only to common cold Most frequent/expensive of workman’s comp claims More prevalent in males Causes: Children – strains & sprains Adults – Sitting for prolonged periods/unable to sit at all Unnatural postures Sudden unexpected motions Weak abdominal muscles Common injuries Soft tissue Contact or overload Spasm response Acute fractures: forceful contraction of mms Contact: direct or indirect Large compressive loads Common injuries Stress fractures Spondylolysis Spondylolisthesis From repeated axial loading while hyperextended Common injuries Disc herniations Bulging, slipped, ruptured Protrusion of pulposus from annulus Common locations cervical & lumbar