Biomechanics of Lifting andLower Back Pain

S.N. RobinovitchOutline

Epidemiology of lower backpain

Strength of spinal segments Lifting models:

muscle and intervertebral jointforces (compression and shear)

effect of abdominal pressure onlifting mechanics

Required Reading from CustomCourseware1. McGill, Stuart: Chapter 6: Lumbar Spine Stability

2. McGill, Stuart: Dynamic Low Back Models: Theory andRelevance in Assisting the Ergonomist to Reduce the Risk ofLow Back Injury

3. Norman, Robert and McGill, Stuart: Selection of 2-D and 3-D Biomechanical Spine Models: Issues for Consideration bythe Ergonomist

Also recommended:Nordin, Margareta and Frankel, Victor H.: Basic Biomechanics ofthe Musculoskeletal System, 3rd Ed. Lippincott Williams & Wilkins,Philadelphia, 2001. Chapter 10:Biomechanics of the LumbarSpine

Epidemiology of lowerback pain

Lower back pain: true or false?

I will probably experience at least oneepisode of back pain during my lifetime

slouching is bad for my back (i.e., increasesspinal forces)

sitting is harder on my back than standing

chairs should have lumbar supports

I should lift by flexing my knees instead ofmy trunk

I should use a back belt when lifting heavyobjects

Epidemiology of lower back pain

lower back pain (LBP) is the most costlymusculoskeletal disorder in industrial nations

80% of individuals will suffer BBP in theirlifetimes

severity ranges from completely debilitatingto temporary annoyance

10-17% of adults in the U.S. have an episodeof LBP in a given year

annual cost in U.S. of back-pain-related careand disability compensation is $50 billion

In Canada, LBP accounts for 18% of allinjuries, and more than 21 million disabilitydays annually

average sickness absence period is 21 days

80% to 90% ofpatients recover completelywithin seven weeks

others require special treatments such astraction, chemonucleolysis, or disk surgery

7% are left with permanent disability, which inabout 50% of cases permanently prevents thepatient from doing strenuous work

Epidemiology of LBP (continued)

Cause and prevention of LBP

LBP is caused by chemical or mechanicalirritation of pain-sensitive nerve endings instructures of the lumbar spine

exact cause of pain in 85% of cases isundiagnosed

therefore, considerable guesswork is ofteninvolved in selecting between treatmentoptions (e.g., ice, rest, gentle activity,manipulative therapies, therapeutic exercise,surgery)

Work-related risk factors for LBP

heavy physical work

static work postures

frequent bending or twisting

lifting, pushing and pulling

repetitive work

vibrations

psychological/psychosocial

Probability of case-group members reportingLBP increases with peak load during work(Norman et al. 1998)

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Peak Moment (Nm)

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Peak Compression (N)

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Spinal loads during daily activities:the problem with slouched sitting the graph on the rightshows compressiveforces between theL3/L4 vertebrae forvarious activities, as apercent of the L3/L4load during standing(shaded bar)

lying creates the leastforce, while slouchedsitting creates thegreatest force

Muscle forces augment spinalloads during standing

during standing, the COG of thetrunk is located ventral to the spine

this creates the need for the erectorspinae muscles to be active

this, in turn, creates the need forcompressive forces in individualvertebrae that equal ~twice theweight of the body above themeasured level (e.g., 700 N inL3/L4 for a 70 kg individual)

Pelvis tilt, reduced lordosis cause lumbarloads to be higher in sitting than standing

when compared tostanding, sittinginvolves backwardtilting of the pelvis andstraightening of thelumbar lordosis

these both act toincrease the momentarm (LW) of the trunkweight with respect tothe lumbar spine

Backrests and lumbar supportsreduce spinal loads during sitting

backrests reduce spinalloads by supporting aportion of the weight ofthe trunk

the farther back theinclination, the smallerthe load

lumbar supports (but notthoracic supports) reduceLW and spinal loads

Load geometry and body posture affectvertebral loads during carryingMoments at L5/S1 during lifting (shown on the topof the figure) are affected by (a) geometry of theload, (b) trunk angle, and (c) knee flexion

(a) (b) (c)

60 Nm 80 Nm 69 Nm 193 Nm 151 Nm 213 Nm

Review Questions Why are spinal forces non-zero during

standing?

Why are spinal forces higher during sittingthan standing?

What factors affect spinal forces duringlifting?

What lifting techniques should be used tominimize spinal loads?

Strength of spinalsegments

The L4/L5 and L5/S1 disks aremost vulnerable to herniation

85-95% of all diskhernias occur at theL4/L5 & L5/S1 levels

The frequency of herniasis similar at each site

Some lifting models useL4/L5, some L5/S1.

Disk compression forces are thoughtto affect risk for LBP

disk compression is thought to be largelyresponsible for vertebral end-plate fracture, diskherniation, and resulting nerve root irritation

data exists on the compressive strength of thelumbar vertebral bodies and intervertebral disks

however, the relative importance to LBP of diskshear forces and torsional moments (versuscompressive forces) is not well understood

NIOSH suggests a maximum diskcompressive force of 3.4 kN In arriving at this estimate, NIOSH reviewed

data from cross-sectional field studies, andused biomechanical models to estimate Fcomp inlifting tasks associated with lower back injury

Herrin et al. 1986 studied 55 jobs (2934potentially stressful MMH tasks), and tracedmedical records for 6912 workers. Jobsinvolving a peak Fcomp between 4.5-6.8 kN hada 1.5-fold greater risk for back problems thanjobs where Fcomp was less than 4.5 kN

LBPincidence(freq. rate

per 200,000man-hours

worked)

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predicted compressive force,L5/S1 disk (kg)Chaffin & Park, 1973

Cadaver studies show that 3.4 kN is aconservative estimate of lumbarcompressive strength Jager & Luttman (1989) found that the mean

compressive strength of lumbar segments was 4.4 1.9 (SD) kN

30% of segments had a strength less than 3.4 kN

Brinckmann et. al. (1988) found compressivestrength ranged from 2.1-9.6 kN, with less than21% of specimens failing below 3.4 kN

Porter, Hutton and Adams (1982, 89) found anaverage compressive strength of approx. 10 kN formales specimens aged 20-40 yrs

Cadaver strength data: best estimates,which should be interpreted with care

strength estimates vary widely betweenstudies

depend on doner age, gender, occupation,history of injury and disease

also depend on method of testing: Method of specimen support Method of load application

spinal unit is usually stripped of supportingmuscles and ligaments (reducing strenght)

Lifting models

Lifting Models

Analytic models: HAT model of lifting Cantilever low back model of lifting Link segment static models

Computer models: 4D WATBAK computer program

(Univeristy of Waterloo)

3D Static Strength Prediction Program(University of Michigan)

HAT Model of lifting

upper torso is modeledas a single massrepresenting thecombined mass of thehead, arms, and trunk

referred to as the HATmodel

simple to analyze, butof limited accuracy

WHAT

WHAT 1/2 BW

HAT Model of lifting (continued)

back muscles mustproduce a moment(MM) to counteractmoments due to handforce (Fhand) andweight of HAT (WHAT)

Otherwise personwould bend or fallforwards.

Fhand

MM

WHAT

Fd

Moment is force times distance

the moment that a forceF produces about a fixedpoint 0 is equal to theforce times theperpendicular distancefrom 0 to the line ofaction of the force

MO = Fd

d = "moment arm"

0

Muscle force can be estimated frommuscle moment and moment arm

Once the musclemoment (MM) has beencalculated, the muscleforce (FM) can beestimated by dividingMM by the moment armof the erector spinaemuscle

This force contributesto the resultant jointcompressive force

L5

L4

5-6 cm

Single Equivalent Muscles

In most joints (other than the spine), morethan one muscle or muscle group contributesto MM

In shoulder flexion, there are two primemovers and two assistors

In forearm flexion, there are three primemovers

We often lump such muscle groups togetherand refer to the lumped muscle as a singleequivalent muscle

Examples: Vertebral moments dueto load only.

Examples: Vertebral moments dueto load and weight of HAT.

Calculate the requiredmuscle moment (MM)to stabilize L5/S1 (redsquare) when Lw =0.25 m and LP = 0.4 m

Answer: Answer: MM = 192.5 Nm = 192.5 Nm

If the erector spinaemoment arm is 5 cm,what is the magnitude ofthe muscle force FM?

Answer: Answer: FM = 3850 N = 3850 N

Sample problem

Sample problem

Lw = 0.18 mLp = 0.35 m

Lw = 0.25 mLp = 0.5 m

1 2 Find themusclemoment (MM)and themuscle force(FM),assuming theerector spinaemoment armis 5 cm

Need to consider trunk angle to determinedisc compressive and shear forces

Once you have calculated the muscle forceyou can calculate the compressive andshear forces across L5/S1.

However, you cannot do this for thequestions just given. Why? Think aboutwhat information you would need and howyou would go about calculating thesevalues.

You need to know the alignment of thesegment in question (i.e. trunk).

Sample problem

For the loading conditionshown, calculate theerector spinae muscleforce FM and thecompressive and shearcomponents of jointreaction force (FJC and FJS)at the L5/S1 vertebrae (redsquare).

Is FJC greater than themaximum safe value of 3.4kN recommended by theU.S. National Institute forOccupational Safety andHealth (NIOSH)?

FM

0.4 m

0.25 m

200 N

450 N0.05 m

FJC

35 deg

Y

X

35 deg

FJSO

Effect of intra-abdominal pressureand back belts on lifting mechanics

Back belts claim to increase IAP and reduce risk for injuryduring lifting. But are they really protective? In 1992, NIOSHconcluded that the effectiveness of using back belts to lessenthe risk of back injury among uninjured workers remainsunproven.

Compare the stiffness of a fulldrink can with that of an emptyone.We similarly can stiffen the trunkby contracting the abdominalmuscles to increase intra-abdominal pressure (IAP).

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