Spinal Biomechanics & Lifting

Transcription

1 Spinal Biomechanics & Lifting Mechanics of Lifting Flex with hips to avoid full spine flexion Minimising ligament & other passive tissue tension muscles responsible to generate the moment Pars thoracis extensors Large moment arm very effective lumbar spine extensors Mechanics of Lifting Flex with hips Upper body is flexed Large reaction shear force (R S ) due rib cage trying to shear forward on the pelvis R S supported by pars lumborum extensor muscles Abdominal muscles active Produce negligible contribution to moment-posture support Stabilise spinal column Produces compression

2 Mechanics of Lifting Flex with hips Posterior R S on superior vertebrae Direction of pars lumborum fibres of the longissimus thoracis & iliocostalis lumborum muscles 34.1 Interspinous & Supraspinous Ligaments Interspinous ligaments Supraspinous ligament Connect adjacent posterior Aligned parallel relative spines to compression axis Aligned oblique relative to Connects tips of compression axis posterior spine Control vertebral rotation Provides resistance through flexion against excessive assists facet joints to forward flexion remain in contact, gliding with rotation Protect against posterior shearing of superior vertebrae on it inferior vertebrae Lifting & Injuries Forces Compressive: tolerate 10 kn From weight of head, arms, trunk & other loads, & contractions of supporting muscles (eg. abdominals) Shear forces: 1 kn cause injury with cyclic loading More likely to cause injury than compressive load 2

3 Incorrect Lifting: flexing lumbar spine Flex spine Posterior ligament strain R S on superior vertebrae lift posterior R S lift anterior R S as interspinous ligament generates force that opposes obliquity No muscle activity in extensors Recruited ligament contributes to anterior shear force 1,000 N injury Lifting Flex with hips Neutral spine Allows trunk extensors to exert a posterior pull to oppose anterior R S from body weight & any added load 34.1 Loads in Cervical Spines Location COM of head external flexion moment on cervical vertebrae (AO & C7-T1) Moment arm C7-T1 2x weight head extensor muscles being at a disadvantage joint reaction force Cervical disc degeneration Why is it more common in the lower cervical? Will altering the head posture change the loading? 3

4 Loads in Cervical Spines Neutral zone Head & neck moves with little passive resistance from ligaments, joints & muscles = 130 N C7-T1 & 70 N AO 30 cervical flexion = 250 N C7- T1 & 750 N AO Cervical Spine Injuries Whiplash (rear-end) Driver accelerates forward with car Flexible cervical spine allows the head to lag behind the trunk cervical hyperextension neck structures stretches anterior & compression posterior Simultaneously, trunk rises towards head compression lower cervical Airbag bags hyperextension What does the relationship between kyphosis & moments? Static equilibrium: balance between external (weight) & internal (muscle & ligament) moments T = 0 Weight external flexion moment on thoracic vertebrae T1 < T4 due to moment arm Balanced by internal moments produced by extensor muscles & posterior ligaments 4

5 What does the relationship between kyphosis & moments? Thoracic kyphosis weight moment arm flexion moment compressive force on vertebral bodies compression failure (anterior) wedge fracture Cycle repeats Frame of reference Analysis focuses on the forces & moments that act in the sagittal plane around the L5 S1 joint Woman weighs 100 lbs & lifts 45 lbs. 5

6 How much tension must be developed by the erector spinae with a moment arm of 6 cm from the L5-S1 joint centre to maintain the body in a lifting position with segment moment arms as specified? (segment weights are ~600 N person.) Segment Weight Moment Arm Head 55 N 20 cm Trunk 275 N 11 cm Arms 60 N 26 cm Box 150 N 41 cm F m 6 cm Segment Weight Moment Arm Head 55 N 20 cm Trunk 275 N 11 cm Arms 60 N 26 cm Box 150 N 41 cm F m 6 cm T L1,S1 = 0 0 = (F M x d M ) [(wt head x d head ) + (wt trunk x d trunk ) + (wt arms x d arms ) + (wt box x d box )] 0 = (F M x 6 cm) [(55 N x 20 cm) + (275 x 11 cm) + (60 N x 26 cm) + (150 N x 41cm)] 0 = (F M x 6 cm) [(1100) + (3025) + (1560) + (6150)] 0 = (F M x 6 cm) 11,835 Ncm F m = N Manual Lifting Analysis Weight of the object Position of the load with respect to the body Measure both start & end points of lift in both vertical & horizontal location 3. Frequency of the lift = Average lifter per min for each task 4. Period/duration = total time in lifting 5. Grip capability: good, fair or poor 6

7 Classifying & Evaluating manual Work Work Selection, Training & Personal Protective Device Manual Material- Handling Limits Occupational Biomechanics Whole-Body & Segmental Vibration Sitting Posture Hand-Tool Design Guidelines Machine Control & Workplace Design lstu.edu/mccaw/hpr34 2/hpr342.htm 7