UTAH ASSESSMENT OF BACK COMPRESSIVE FORCES (BLOSWICK ESTIMATION OF BACK

Daniel Focht

UTAH ASSESSMENT OF BACK COMPRESSIVE FORCES (BLOSWICK ESTIMATION OF BACK

UTAH ASSESSMENT OF BACK COMPRESSIVE FORCES (BLOSWICK ESTIMATION OF BACK COMPRESSIVE FORCE)

The Utah Assessment of Back Compressive Forces is a relatively simple and concise tool to evaluate the compressive forces encountered by the lower lumbar spine during various lifting efforts. As can be seen by reviewing Figure 11-6,

FIGURE 11-5 Fast-food worker handing food tray to customer.

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the examiner determines the values listed on the worksheet and addresses the variables of body weight, load, horizontal distance, and back posture. These then become the components of an equation that can predict the potential or actual existence of excessive lower lumbar compressive forces. At this juncture the therapist can mani-pulate any of the variables to provide the neces-sary ergonomic abatement. This can be done on site and in many cases affect the situation immediately.

Although the issue of asymmetry is not ad-dressed in this quick screen, all of the most fre-quently encountered risk factors are identifi ed and measured. If, again, the activity is deemed

poten-tially hazardous by this preliminary assessment tool, then a more elaborate biomechanical analy-sis taking into account multiplanar movement pat-terns can be applied.

Let’s review our case study of Boston Packag-ing, Inc. The fi rst step for the therapist was to assess, via a quick screen, the risk potential of the job. At the time of the ergonomic assessment, two employees were working the line, one male and one female. The man weighed 210 pounds, and the woman weighed 130 pounds.

The Bloswick Estimation of Back Compressive Force was chosen as the quick reference guide.

To illustrate the differences between potential and actual compressive forces, both individuals Estimation of Back Compressive Force

A representation of the model by Donald S. Bloswick at the University of Utah.

Job Analyst

Task Date

Measure Symbol Value

Body weight [lb] BW

[lb]

Load [lb] L

[lb]

Horizontal distance [in]

(Hands to lower back {L5-S1 joint}) HB

[in]

Back posture (angle from vertical) Θ [°] sinΘ

Vertical 0 0.0 Θ

[°]

Bent ¹/4 of the way 23 0.4

Bent ¹/2 of the way 45 0.7 sinΘ

[–]

Bent ³/4 of the way 67 0.9 Horizontal 90 1.0

Contributor Computation Value

Back posture

A = 3(BW) sin Θ 3 * ( ) * ( )

Load moment

B = 0.5(L * HB) 0.5 * ( ) * ( )

Direct compression

C = 0.8[(BW)/2 + L] 0.8 * {( ) / 2 + ( )}

Estimated compressive force Fc = A + B + C

Sum computed values in last column.

Comparison value: 700 lb

If the estimated compressive forces exceeds 700 lb, consider a more detailed analysis or make changes. Note: This is just an estimate and its accuracy varies with posture, especially as the hands move forward of the shoulders.

FIGURE 11-6 Bloswick’s revised estimation of back compressive force. (From Bloswick DS: E rgonom-ics. In Harris RL, editor: Patty’s indus-trial hygiene and toxicology, ed 5, vol 4, New York, 2000, John Wiley &

Sons.)

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were assessed. Figure 11-6 has the following com-putations:

Male Employee Female Employee Body weight 210 lb 130 lb

Load 20 lb 20 lb

Horizontal 50 inches 24 inches distance

(hands to lower back)

Trunk angle 0.9 (³/4 bend) 0.4 (¹/4 bend) The reason for the considerable differences between the male and female employees’ horizon-tal and trunk angle distances was the male’s ten-dency to stack the pallets back row fi rst with a front-oriented approach. This resulted in his adop-tion of a stooped posture while lowering the load to the pallet, thus increasing the lever and moment arm and angle of trunk fl exion (Figure 11-7, A).

Conversely the female employee chose a differ-ent approach. She preferred to walk behind the pallet to load the back row. This allowed for the box to be maintained closer to the body and for her to employ the squat lift, one more appropriate for heavier loads (Figure 11-7, B). This in turn reduced the lever and moment arm and decreased

the angle of trunk fl exion. The comparative analy-sis of the two approaches, using the equation in Figure 11-6, demonstrated the following.

Male Employee Female Employee Back posture 567 156

Load moment 500 240

Direct 100 68

compression

Sum 1167 lb 464 lb (2042 N) (5134 N)

It is quite clear that the male employee’s ex-tremely forward bent posture, in addition to the excessive moment arm created by reaching over the length of the pallet, created forces to the lower lumbar structure far in excess than has been deemed safe by NIOSH’s lifting guidelines. In con-trast, the female employee handled the same object in a much more effi cient manner, exposing her to signifi cantly less compressive force.

The therapist’s fi rst recommendation would be to train all employees assigned to this packaging line in the proper loading of the pallet, in addition to demonstrating the proper lifting technique.

A B

FIGURE 11-7 A, Male worker—stoop lift. B, Female worker—squat lift.

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Further considerations would include recommen-dations regarding pallet height assistive devices such as scissor or other hydraulic lifts that would limit the horizontal distance that would have to be negotiated. These, again, are examples of knowing what variables contribute to the risks and how the variables can be manipulated to mitigate the risks.

After refl ecting on the case study, answer the following questions:

1. Identify the most critical aspects of the sub-ject’s work characteristics as outlined in this case study that predispose him or her to injury.

2. Which risk factors can be addressed imme-diately, and which can be minimized through work site design modifi cation?

3. How are you, as the consulting therapist, going to justify to the company’s adminis-trators the expenditure of nonbudgeted monies to rectify the potential risks?

CONCLUSION

The analysis of lifting, and the inherent risk factors associated with it, is an ongoing process.

The review of evidence literature reveals that there is no particular lifting technique that is supe-rior to another, but there are a number of princi-ples that need to be observed when an individual is exposed to a potentially diffi cult and injury-producing endeavor. To reiterate, these include the following:

• Maintain the load as close to the body as possible

• Ensure adequate hand couple

• Maintain the lumbar spine in as much of a lordotic curve as possible

• Lift in the sagittal plane, and avoid extrane-ous multiplanar movement patterns

• Ensure proper footing

• Lift slowly

• Use the lifting technique (stoop, squat, semi-squat) best suited for the situation

These principles can easily be applied to any number of occupational scenarios that require moderate or heavy lifting. From the shipping and receiving dock fl oor to the nursing unit, common

everyday stressors can be mitigated if these crite-ria are followed.

Occupational and physical therapists possess a breadth of knowledge relative to the factors that infl uence human performance. It is our responsi-bility as therapists to incorporate this knowledge into common processes to prevent the maladies associated with aberrant lifting practices through education, early intervention once an injury has

Learning Exercise Overview

Can the biomechanical analysis of a potentially injurious lift be captured by observation alone?

Purpose

The purpose of this exercise is to determine, through observation, what happens at various joint structures (particularly the low back, hips, knees, neck) at and approaching an individual’s stated or perceived maximum lifting effort. If your observations are reliable and can be repli-cated, they can serve as means by which a work situation can be identifi ed as potentially hazard-ous. From that point a more empirically based analysis can be performed.

Exercise

Observe classmates during a fl oor-to-waist lift using a standard milk crate as the container.

Increase the load to be lifted by regular incre-ments (5 to 10 pounds) until the individual reaches his or her safe lifting maximum. Observe for the following:

1. At what point does the subject’s lifting style change?

2. What lifting style (squat, stoop, semi-squat) does the subject adopt at the outset, and what style is employed at the maximal effort?

3. What changes in joint angulation (estimate only) occur at the hip versus the low back, knees, and head and neck as the subject advances from an easily managed load to one that is subjectively perceived as diffi cult?

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occurred, and postinjury maintenance program-ming.

A critical component of our involvement is to become familiar with the nuances of our patients’

lifestyles, be it at work or at home, and what practices may put them at risk for lift-related low back injury or the aggravation of an ongoing con-dition. This requires observation not only in the clinic but also at the work site and at times at home. We must become mobile therapists, because practice demands may require that we treat offsite, at the work place, or in the clinic and that we follow the patient once he or she has returned to full duty. As a result I have been able to devote this comprehensive care model for the worker with an injury. It is through this ongoing involve-ment with our clients and the commitinvolve-ment to further research that we can make a positive impact on the occurrence and prevalence of lift-related injuries.

Multiple Choice Review Questions 1. The spinal motion segment consists

of:

A. the apophyseal joint and lumbar paraspinals.

B. adjacent vertebral bodies and the intervertebral disc.

C. the junction of the sacrum and the ilium.

D. interspinous ligaments and vertebral endplates.

2. The motion segment is exposed to which force vectors(s)?

A. Compression, torsion, shear B. Flexion, extension, sidebending C. Sagittal, transverse, frontal D. Ascending, descending, lateral 3. The three most researched lifting

techniques include:

A. squat, semi-squat, and stoop.

B. stoop, kinetic, and crouch.

C. squat, quad, and astride.

D. golfer’s, lateral, and semi-squat.

4. The lifting technique that requires greatest aerobic cost is:

A. the stoop.

B. the squat.

C. the semi-squat.

D. the kinetic lift.

5. Per the NIOSH lifting guide, the

maximum allowable force that the L5/S1 segment can withstand is:

A. 2000 N.

B. 3400 N.

C. 5000 N.

D. 1000 N.

6. Which of the following lifting techniques would be ideal for a task that requires lifting frequent, light loads from the fl oor?

A. Stoop B. Squat C. Semi-squat D. Kinetic

7. Which is the preferred lift of those who are experiencing chronic or acute low back pain?

A. Stoop B. Squat C. Semi-squat D. Kinetic

8. The optimal height range from which to overcome inertial forces while pushing is:

A. 90 to 115 cm.

B. 50 to 65 cm.

C. 100 to 125 cm.

D. 70 to 90 cm.

9. The variable that was used in the 1991 NIOSH lifting guide that set it apart from its predecessor (i.e., the 1981 lifting guide) was:

A. vertical displacement.

B. horizontal distance from the load.

C. type of hand couple.

D. frequency of the lift.

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10. The Bloswick Evaluation of Low Back Compressive Forces uses what benchmark as its maximum allowable aggregate?

A. 500 pounds B. 800 pounds C. 1000 pounds D. 700 pounds

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