Ergonomic Evaluation 

of Loaders and 

Hashers at a Postal 

Processing and 

Distribution Center

Jessica Ramsey, MS

Ming-Lun (Jack) Lu, PhD

Health Hazard Evaluation Report

HETA 2007-0170-3070

United States Postal Service 

Processing and Distribution Center

Seattle, Washington

September 2008

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention 

Workplace

Safety and Health

 National Institute for Occupational 

Safety and Health

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific 

facility evaluated and may not be universally applicable. Any recommendations made are not to be 

considered as final statements of NIOSH policy or of any agency or individual involved. Additional HHE 

reports are available at http://www.cdc.gov/niosh/hhe/
The employer shall post a copy of this report 

for a period of 30 calendar days at or near 

the workplace(s) of affected employees. The 

employer shall take steps to insure that the 

posted determinations are not altered, defaced, 

or covered by other material during such 

period. [37 FR 23640, November 7, 1972, as 

amended at 45 FR 2653, January 14, 1980].
Health Hazard Evaluation Report 2007-0170-3070 Page i

Contents

Report Abbreviations.......................................................................i

Highlights of the NIOSH Health Hazard Evaluation............ iii

Summary .............................................................................v

Introduction..........................................................................1

Assessment.........................................................................6

Results & Discussion...........................................................6

Conclusions.........................................................................9

Recommendations...............................................................9

References ........................................................................11

Appendix A Ergonomic Evaluation Criteria...........................................12

Acknowledgments Acknowledgments and Availability of Report.....................1
Page ii Health Hazard Evaluation Report 2007-0170-3070

Abbreviatio

3DSSPP University of Michigan’s 3-dimensional static strength prediction program

DCF Disc compression force

GPMC General purpose mail container

HDCS Hash delivery conveyor system

HETAB Hazard Evaluations and Technical Assistance Branch

HHE Health hazard evaluation

LI Lifting index

MSD Musculoskeletal disorder

NAICS North American Industry Classification System

NIOSH National Institute for Occupational Safety and Health

OSHA Occupational Safety and Health Administration

P&DC Processing and distribution center

RNLE Revised NIOSH lifting equation

RWL Recommended weight limit

TMS Tray management system

USPS United States Postal Service

WMSD Work-related musculoskeletal disorder 
Health Hazard Evaluation Report 2007-0170-3070 Page iii

Highlights of 

NIOSH 

Hazard Eval

The National Institute for 

Occupational Safety and 

Health (NIOSH) received 

a management request 

for a health hazard 

evaluation at the United 

States Postal Service 

Seattle Processing and 

Distribution Center 

(P&DC) in Seattle, 

Washington. NIOSH 

investigators evaluated 

potential ergonomic 

hazards among 

employees on the East 

Dock Hash Delivery 

Conveyor System. A site 

visit was performed in 

November 2007.

What NIOSH Did

● We watched and videotaped workers loading and hashing 

mail.

● We used special equipment to re-create the jobs to see if the 

tasks could injure workers.

What NIOSH Found

● The current hashing job does not pose as much risk to 

workers as the old hashing job. 

● The current hashing job could be improved to further reduce 

the risk. 

● The general-purpose mail containers loading job had a high 

risk of workers developing low back pain.

What Seattle P&DC Managers Can Do

● Managers should move shelving in the hashing area to make 

space for empty containers, eliminating 180° twists. The top 

chute of the hashing station should be moved to eliminate 

mail being dumped at or above shoulder level.

● Managers should move the tray management system (TMS) 

conveyor in front of the hashing worker to eliminate 180° 

lifts.

● If the TMS conveyor cannot be moved, managers should 

lower the TMS conveyor height to 32 inches to eliminate 

elevated shoulder postures.

● Managers should install cushioned mats on the floor in the 

loading area. Non-slip material should be installed on the 

hashing area footrests.

● Managers should increase the number of workers in the 

loading and hashing area to allow for rotation through the 

jobs. If workers are moved from other areas, managers 

should make sure they are properly trained.

● Managers should look at the system that feeds tubs/trays 

down the gravity conveyor lines as these stations were loaded 

unevenly at times.

● Managers should replace the roller conveyors on hash lines 1, 

3, and 4 with ones similar to those used on hash line 2. This 

will help the movement of tubs/trays and eliminate extended 

reaches and push/pull forces for stuck containers.
Health Hazard Evaluation Report 2007-0170-3070 Page v

Summary

Both loaders and 

hashers are exposed to 

a combination of risk 

factors for developing 

musculoskeletal 

disorders. Risk factors 

include heavy lifts and 

awkward postures. 

Improved reach heights 

and distances as well as 

station reorganization 

would reduce the physical 

stress associated with 

these jobs.

On March 16, 2007, NIOSH received an HHE request from the 

management at the USPS Seattle P&DC to evaluate potential 

ergonomic hazards among workers at the Seattle, Washington 

P&DC. The redesign of the East Dock Hash Delivery Conveyor 

System area and a previous HHE request by employees prompted 

the request. 

During November 27–28, 2007, NIOSH investigators held an 

opening conference with representatives from USPS management 

and the National Postal Mail Handlers Local 316. NIOSH 

ergonomic specialists toured the HDCS area to observe loading 

and hashing (sorting) tasks specified in the request. 

The ergonomics evaluation indicated that workers are exposed to 

risk factors for developing musculoskeletal disorders due to heavy 

lifts and awkward postures. The employees were at elevated risk of 

injury while performing loading tasks, as measured by the RNLE 

and 3DSSPP. Recommendations for reducing the risk of injury 

include improving the heights and horizontal reach distances of 

the lifts during loading and reorganization of the hashing stations.

Keywords: NAICS 491110 (Postal Service), repetitive motions, 

awkward postures, lifting, sorting/hashing, ergonomics, USPS
Page iv Health Hazard Evaluation Report 2007-0170-3070

Highlights of 

NIOSH 

Hazard Eval

(continued

What Seattle P&DC Employees Can Do

● Employees should work safely and lift properly. For example, 

prior to lifting, disengage the door or drop the netting on the 

general purpose mail containers to avoid bending over at the 

waist when lifting. 

● Employees should not lift tubs/trays over the front conveyor 

onto the back conveyor in the loading area. This increases 

the reach distance, which increases the hazard of the loader 

job.

● Employees should not push tubs or trays down the gravity 

conveyor lines. Additional weight from the tubs/trays 

increases the force required to pull the container to be 

hashed.

● Employees should actively participate on safety and 

ergonomic committees.

● Employees should report injuries and unsafe work conditions 

to the union and management.
This page intentionally left blank
Health Hazard Evaluation Report 2007-0170-3070 Page 1

Introduct

On March 16, 2007, NIOSH received an HHE request from 

management at the USPS P&DC to evaluate potential ergonomic 

hazards among workers in Seattle, Washington. The redesign of the 

East Dock HDCS area and a previous HHE request by employees 

prompted the request.

During November 27–28, 2007, NIOSH investigators conducted 

a site visit at the USPS P&DC in Seattle, Washington. The site 

visit was delayed due to scheduling conflicts for both NIOSH and 

Seattle P&DC. On November 27, 2007, NIOSH investigators held 

an opening conference with management and union officials from 

the National Postal Mail Handlers, Local 316. NIOSH ergonomic 

specialists observed loading and hashing (sorting) tasks in the East 

Dock HDCS area. Investigators also recorded video of workers 

performing job tasks for use in subsequent analyses. On November 

28, 2007, NIOSH investigators held a closing conference and 

provided preliminary recommendations to management and union 

officials.

Process Description

The HHE request specified evaluation of redesigned work tasks 

in the East Dock HDCS area. Previously, when workers received 

mail tubs/trays in GPMCs and hampers, they sorted the mail 

into other GPMCs and hampers based upon mail type (Figure 1). 

Workers were required to walk around carrying the tubs/trays and 

sort the mail. Due to the inefficiency of this system, management 

redesigned this area into loading and hashing stations. 

Figure 1. Computer-aided design of old hashing (sorting) area. 
Page 2 Health Hazard Evaluation Report 2007-0170-3070

Introducti

(continued) Loadin

In the redesigned system, loaders receive GPMCs and hampers 

from the East Dock. They remove the tub/trays from the GPMCs 

and hampers and load them onto two conveyor systems—one 

lower and one upper (Figure 2). The lower, powered conveyor 

transports the mail to the hashing stations. The upper, powered 

conveyor transports the mail to a worker who transfers it to the 

TMS. The two ball transfer sections in front of the conveyors 

have systems that stop the conveyor to allow the loaders to insert 

tubs/trays without being hit by tubs/trays loaded from preceding 

workstations. 

upper conveyor 

lower conveyor 

Figure 2. Loading station powered conveyor lines. 

Hashing (Sorting)

The East Dock HDCS area has four hashing stations (Figure 3). 

Each hashing workstation has a gravity-fed conveyor on the left 

side that is connected to the lower powered conveyor from the 

loading stations. Workers receive tubs/trays from the loading 

stations via the gravity-fed conveyor. Seattle P&DC engineers 

designed the system to evenly distribute the tubs/trays to the 

different stations, as well as limit the number of tubs/trays on the 

gravity-fed conveyors. Limiting the number of tubs/trays decreases 

the amount of weight on the bottom tub, thus reducing the force 

required to slide the tub from the conveyor to the sorting area. 

Workers slide a tub/tray from the conveyor, sort through the 
Health Hazard Evaluation Report 2007-0170-3070 Page 3

Introductin  (continued) contents, and distribute to other tubs, trays, and chutes based u

mail type. Directly in front of the worker are two chutes where 

workers dump mail. On the left and right side of the worker are 

various size tubs/trays where the worker places mail. Once a tub/

tray is full, the worker must lift the tub and turn around to place it 

on another powered conveyor system that transports it to the TMS.

Powered 

conveyor

Gravity

conveyor

Figure 3. Hashing (sorting) station showing the powered conveyor and the gravity 

 conveyor. 

Equipment Used in Mail Handling

GPMCs 

GPMCs are used to transport mail to different areas within the 

P&DC. Workers in the HDCS area receive some mail from the 

East Dock in GPMCs. All GPMCs have some sort of netting or 

cage to keep tubs and trays inside the cart during transportation. 

The two types shown in Figure 4 have drop-down netting that can 

be removed as the worker removes layers of tubs/trays to eliminate 

reaching over the netting or cage. The USPS has a rule that no tubs 

or trays should be stacked higher than the sides of the GPMC. This 

rule is to limit overhead reaching.
Page 4 Health Hazard Evaluation Report 2007-0170-3070

Introducti

(continued

GPMC

GPMC

Dropdown 

netting

Figure 4. Two types of GPMCs with drop-down netting and a cloth hamper 

Hampers

Hampers are also used to transport tubs/trays of mail within the 

P&DC. Workers in the HDCS area also receive mail from the East 

Dock in hampers. Two types of hampers were observed during our 

evaluation: cloth and plastic (Figure 5). The plastic hamper has 

black netting on the sides that allows the work to bend into the 

hamper when performing a lift. The cloth hampers do not have 

this feature. Workers in the HDCS area showed the investigators a 

hamper tilter that can be used with either cloth or plastic hampers. 

However, it was not being used because the workers stated that 

there was no space for it in the current Hash layout. 
Health Hazard Evaluation Report 2007-0170-3070 Page 5

Introductin  (contin

Plastic

Hamper

Cloth

Hamper

Figure 5. An example of a plastic hamper with netting 

 (left) and a cloth hamper 

Tubs/Trays

Tubs and trays are used to move mail throughout the P&DC 

(Figure 6). Tubs are used for oversized mail, and trays are used for 

letter-size mail. The USPS has a rule that no mail should be placed 

in tubs above the handles of a tub, to keep the handles available for 

use when lifting.

Tub

Tray

Figure 6. Examples of tubs and trays used to distribute mail through the P&DC 
Page 6 Health Hazard Evaluation Report 2007-0170-3070

Assessment

The NIOSH ergonomics evaluation consisted of walking through 

the facility to observe job tasks specified in the HHE request: 

loading and hashing (sorting) tasks in the HDCS area. NIOSH 

investigators recorded digital videos, lift frequencies, working 

heights, and reach measurements to document the tasks performed 

by the workers. The ergonomics tools used to evaluate the physical 

risk factors for WMSDs were biomechanical outputs obtained from 

lab-simulated postures evaluated using 3DSSPP [Chaffin 1995] 

and recommendations for acceptable lifting weights as determined 

by the RNLE [Waters 1994]. Appendix A contains a description 

of the ergonomics evaluation criteria the investigators used, as 

well as a detailed description of the analysis methods. NIOSH 

investigators reviewed OSHA Form 300 Logs of Work-Related 

Injuries and Illnesses from 2003–2007. NIOSH investigators also 

held confidential interviews with a random sample of workers in 

the HDCS area.

Results & Discussi

Job Analyses

Table 1 shows the DCF obtained from the 3DSSPP and the LI 

values obtained from analyzing tasks using the RNLE. The weight 

used in the analyses was the average weight as estimated by the 

workers (22 lbs). A duration of 2 hours was used for calculating LIs 

due to observations that the workers seemed to work continuously 

for about 2 hours without a break. 

One of the workers performed a simulation of the old hashing 

job. The management had set up an area similar to the layout of 

the previous conditions. NIOSH investigators used this worker’s 

simulation video for the analysis of this task. 

Table 1. Disc Compression Forces (DCF) and Lifting Index (LI) 

results for lab-simulated East Dock HDCS tasks 

Task DCF (lbs) LI

Old Job 726 3.0

Loader – GPMC 506 3.6

Loader – Hamper 173 2.2

Hash – Worker A 271 1.6

Hash – Worker B 235 1.6
Health Hazard Evaluation Report 2007-0170-3070 Page 7

Results and Discussi   (continued) Two loading tasks were recorded for comparison of the vari

loading conditions. NIOSH investigators were informed that a 

hamper tilter was available in the area but was not used because 

there was not enough space for it. NIOSH investigators observed 

workers not removing the sides of the GPMCs and hampers and 

leaning over into them to retrieve tubs/trays.

NIOSH investigators recorded video and performed analyses on 

two workers performing hashing tasks for comparison of work 

style. The main difference in style was where workers placed the 

empty tubs after hashing the mail. NIOSH investigators observed 

Worker A placing the empty tub right beside the hashing station 

and Worker B placing the empty tub on the tub cart behind the 

hashing station. Also, when placing tubs/trays on the TMS after 

hashing, workers had to turn 180° and lift the container onto the 

roller ball table. 

Every lifting job task evaluated exceeded the recommended LI of 

1.0, and two job tasks surpassed a LI of 3.0, which is considered 

to be hazardous for nearly all workers. LIs between 1.6 and 2.2 

indicated an elevated risk of low back injury as compared to jobs 

that have a LI ≤ 1.0 [Waters 1994]. DCFs followed a similar trend 

as the LIs, but none exceeded 770 lbs. Any DCF exceeding 770 

lbs is above the back compression design limit and indicates an 

elevated risk of low back injury [Chaffin 1995]. 

An average weight of 22 lbs and a duration of 2 hours were 

estimated and used for the above calculations. Workers lifting 

containers that weigh more than 22 lbs, with the postures analyzed, 

may produce back compression forces that exceed the NIOSH 

recommended back compression upper limit of 1,430 lbs [NIOSH 

1981]. Also, if workers perform these job tasks for more than 2 

hours continuously, then the LI for the hashing jobs would be 

higher, posing a higher risk of developing low back injury. 

Walk-through Observations and 

Employee Concerns

Loading 

Toward the end of the shift, to process mail by the designated 

deadline, a worker crosses over the conveyor line in the loading 

area and presorts some of the tubs/trays before reaching the 

hashing (sorting) stations. In addition, when the front conveyor 
Page 8 Health Hazard Evaluation Report 2007-0170-3070

Results and Discussi   (continued) is backed up, the workers lift tubs/trays over the front conve

line onto the back conveyor. These two actions increase the lift 

frequency and reach distance, respectively; therefore further 

increasing the LI for the loader job.

NIOSH investigators noted loading stations with no mats for 

workers to stand on. The hashing stations had built-in footrests; 

however, the surface was worn and slippery and did not allow 

the workers to utilize them to change body positions. Prolonged 

standing, without mats and/or the ability to change postures, can 

cause pain in the muscles of the legs, back, and neck.

Hashing

NIOSH investigators observed the gravity conveyor lines in the 

HDCS area over-loaded (more than 6–7 tubs/trays) due to workers 

pushing mail down the gravity conveyors to clear up the line for 

loaders to add more. At other times, the hashing stations seemed 

to be unequally loaded, resulting in more tubs/trays on certain 

lines than on others. Also, it was noted that hash station 2 had 

a new roller conveyor. The new roller conveyor had more rollers, 

which facilitated the movement of tubs/trays down the gravity 

conveyor line, thereby reducing the number of stuck tubs/trays that 

workers had to reach to pull down to the sorting area.

NIOSH investigators observed a large amount of mail thrown or 

dumped in the top chute of the hashing station, which is located 

at or above shoulder level for some workers. The mail included 

usually large packages and envelopes and required forward flexion 

of the back and extended arm postures to throw into the top chute. 

Review of OSHA 300 Logs 

Review of the OSHA Logs indicated that most entries occurring in 

East Dock workers were due to MSDs (not including acute injuries 

such as fractures, laceration, punctures or contusions). OSHA 

Logs documented three MSDs for East Dock workers in 2007 (two 

lower extremity, one chest); none in 2006; one in 2005 (one upper 

extremity); none in 2004; and five in 2003 (two upper extremity, 

two lower extremity, one chest).
Health Hazard Evaluation Report 2007-0170-3070 Page 9

Conclusion

On-site assessments and interviews at the Seattle P&DC are 

the basis for the following conclusions and recommendations. 

The new hashing job had lower LI and DCF values than the old 

job. Although the old job was simulated for the purposes of job 

analysis, NIOSH researchers believe that the actual old job may 

have posed greater musculoskeletal disorder risks to workers. The 

loading job from GPMCs had the largest LI among all the current 

jobs analyzed. This indicates a high risk of workers developing low 

back pain and injury. The current hashing jobs had the lowest LIs 

of all the current jobs analyzed. However, some improvements to 

the HDCS area could be made to further reduce the LI to below 

1.0. 

It is important to restate that an average weight of 22 lbs and a 

duration of 2 hours were estimated and used for the calculations. 

Workers lifting containers that weigh more than 22 lbs, with the 

postures analyzed, may produce DCFs that exceed the NIOSH 

recommended limit. Also, if workers perform these job tasks for 

more than 2 hours continuously, then the LI for the hashing 

jobs would be higher. General recommendations, as well as 

recommendations for reducing the DCF and LI, follow. 

The following recommendations are offered to help reduce the 

risk for development of work-related musculoskeletal disorders for 

employees in the East Dock HDCS area. The preferred method 

of controlling ergonomic hazards is to provide engineering 

controls that redesign the workstation and/or job task to reduce 

or eliminate the risk of WMSDs. Recommendations 1 through 

5 provide suggestions for engineering controls. Administrative 

controls or policies designed to limit workers’ exposures to 

hazardous conditions can be used temporarily until engineering 

controls are implemented. Recommendations 6 and 7 provide 

suggestions for administrative controls. In addition, NIOSH 

investigators recommend training employees to recognize 

ergonomic hazards and asking them to participate in the 

process of identifying hazards and making job modifications. 

Recommendations 8 and 9 will help achieve these goals. While 

implementing all of these recommendations over a period of time 

is optimal, even implementing some of them along with other 

administrative controls would be beneficial in reducing workers’ 

risk of WMSDs.

Recommendatio
Page 10 Health Hazard Evaluation Report 2007-0170-3070

Recommendatio

(continued) 1. Readjust shelving in the hashing area to make space for

workers to place empty containers. This will eliminate 180° 

twisting that occurs when placing empty containers in their 

current location. Also, the top chute of the hashing station 

should be readjusted to eliminate dumping mail at or above 

shoulder level.

2. Relocate the TMS conveyor in front of the worker. This 

will eliminate 180° lifts to place full containers on the TMS 

conveyor. This would require raising the area, or using 

a platform to provide sufficient clearance for the TMS 

conveyor.

3. If recommendation #2 is not possible, lower the TMS 

conveyor height from 37" to 32". This will eliminate elevated 

shoulder postures when lifting full containers onto the TMS 

conveyor.

4. Replace the roller conveyors on gravity lines leading to hash 

stations 1, 3, and 4, similar to hash station 2. This will 

facilitate the movement of tubs/trays down the conveyors, 

thus eliminating reaching to push or pull tubs/trays on the 

conveyors.

5. Reevaluate the system that feeds tubs/trays down to the 

hash stations. The stations were occasionally unequally 

loaded.

6. Install antifatigue mats in the loading area and non-slip 

material on the footrests in the hashing area.

7. Add workers to both the loading and hashing areas to 

allow rotation of employees through all the job tasks (dock, 

loading, and hashing). 

8. Train employees to recognize WMSDs and instruct them in 

work practices that can ease the task demands or burden.

9. Management and employees should jointly address 

ergonomic issues routinely and periodically evaluate the 

effectiveness of implemented engineering and administrative 

controls.
Health Hazard Evaluation Report 2007-0170-3070 Page 11

Referenc

Chaffin DB [1995]. Users’ manual for the Michigan 3-D static 

strength prediction program, version 3.0. University of Michigan 

Software: Ann Arbor, Michigan.

NIOSH [1981]. Work practices guide for manual lifting. 

Cincinnati, OH: U.S. Department of Health and Human Services 

(NIOSH) Technical Report No. 81-122.

Waters TR, Putz-Anderson V, Garg A [1994]. Applications 

manual for the revised NIOSH lifting equation. Cincinnati, OH: 

Centers for Disease Control and Prevention, National Institute 

for Occupational Safety and Health, DHHS (NIOSH) Publication 

94-110.
Page 12 Health Hazard Evaluation Report 2007-0170-3070

Appendix A: Ergonomic Evaluation

The term MSD refers to conditions that involve the nerves, tendons, muscles, and supporting structures 

of the body. WMSDs are a major component of the cost of work-related illness in the United States. A 

substantial body of data exists providing strong evidence of an association between MSDs and certain 

work-related factors (physical, work organizational, psychosocial, individual, and sociocultural). The 

multifactorial nature of MSDs requires a discussion of individual factors and how they are associated 

with WMSDs. There is strong evidence that working groups with high levels of static contraction, 

prolonged static loads, or extreme working postures involving the neck/shoulder muscles are at increased 

risk for neck/shoulder MSDs [NIOSH 1997]. There is also strong evidence that job tasks that require 

a combination of risk factors (highly repetitious, forceful hand/wrist exertions) increase risk for hand/

wrist tendonitis [NIOSH 1997]. Lastly, there is strong evidence that low-back disorders are associated 

with work-related lifting and forceful movements [NIOSH 1997]. A number of personal factors can also 

influence the response to risk factors for MSDs including: age, gender, smoking, physical activity, strength, 

and anthropometry. Although personal factors may affect an individual’s susceptibility to overexertion 

injuries/disorders, studies conducted in high-risk industries show that the risk associated with personal 

factors is small compared to that associated with occupational exposures [NIOSH 1997].

In all cases, the preferred method for preventing/controlling work-related musculoskeletal disorders is 

to design jobs, workstations, tools, and other equipment to match the physiological, anatomical, and 

psychological characteristics and capabilities of the worker. Under these conditions, exposures to task 

factors considered potentially hazardous will be reduced or eliminated.

The specific criteria used to evaluate the biomechanical and injury risk factors of job tasks at USPS were 

the biomechanical outputs obtained from the University of Michigan’s 3DSSPP [Chaffin 1995] and 

recommendations for acceptable lifting weights as determined by the RNLE [Waters 1994]. 

The 3DSSPP is a computerized model that can be used to evaluate the physical demands of a job. Typical 

inputs to the model are the magnitude and direction of forces at the hands, angles of body segments, and 

anthropometric selection such as gender and population size percentiles. The model outputs moments at 

the joints of the body, percentages of the chosen population able to sustain the inputted loads, and disc 

compression forces at the low back. For the purposes of this HHE, the 3DSSPP was used to evaluate the 

biomechanical demands of the loading and hashing tasks.

The RNLE is a tool for assessing the physical demands of two-handed lifting tasks. In brief, the equation 

provides a recommended weight limit and a lifting index for a lifting task, given certain lifting conditions. 

The RWL is the weight that can be handled safely by almost all healthy workers in similar circumstances. 

The LI is the ratio of the actual load lifted to the RWL. Lifting tasks with a LI ≤ 1.0 pose little risk of low 

back injury for most workers. Tasks with a LI > 1.0 may place an increasing number of individuals at risk 

of low back injury. The consensus opinion of an expert panel, described in the RNLE report, is that tasks 

with a LI > 3.0 pose a risk of back injury for most workers.
Health Hazard Evaluation Report 2007-0170-3070 Page 13

Appendix A: Ergonomic Evaluation          (

Analysis Method

To calculate the risk of injury posed by job tasks using the RNLE and the 3DSSPP, physical measurements 

must be taken from the worker and the task being performed. Use of the RNLE requires a measure of 

the position of the object with respect to the body at the beginning and end of the lift, the height to 

which the object is lifted, the frequency of the lift, and the angle of any twist the body makes while lifting. 

The 3DSSPP requires body segment angles as inputs so that the posture of the worker performing the 

task can be duplicated. In both cases, the weight of the object being lifted or moved during the job task 

must be recorded. Because obtaining these measurements can be tedious, time-consuming, and at time 

infeasible, NIOSH investigators now obtain injury risk outputs from the RNLE and the 3DSSPP through 

input of postural information from subjects simulating job tasks in a laboratory (see Figure A1) using a 

position and orientation measurement system. The system features commercially available body position 

and orientation measurement hardware (Flock of Birds, Motion Star, Ascension Inc., 1999) and Motion 

Monitor software (Innovative Sport, Chicago IL, 2003). 

Figure A1. Instrumented “simulator” assuming a work 

 posture on the simulation platform 
Page 14 Health Hazard Evaluation Report 2007-0170-3070

Appendix A: Ergonomic Evaluation          (

The motion capture system is capable of determining the position and orientation of the body segments 

by transmitting a pulsed direct current magnetic field that is simultaneously measured by all sensors 

located within 3 meters of the transmitter. From the measured magnetic field characteristics, the system 

independently computes the position and orientation of each sensor which is subsequently sent to the 

motion capture software program (Motion Monitor, Innovative Sports Inc.). The position and orientation 

data are further processed with a link segment model for calculating the posture variables needed for use 

by 3DSSPP and RNLE. The data are transmitted from 13 sensors attached with Velcro straps to various 

body segments of the person simulating the job of interest. The simulator stands on a platform that is 

surrounded by a magnetic field and assumes the various postures of the worker performing the job task, 

which are projected on a screen behind the platform. Figure A1 shows how the jobs are simulated in the 

lab. A NIOSH study has shown that the accuracy and reliability of the method is at least on par or better 

than manual posture specification method for using 3DSSPP and RNLE (Lu et al., 2007). 

References 

Chaffin DB [1995]. Users’ manual for the Michigan 3-D static strength prediction program, version 3.0. 

University of Michigan Software: Ann Arbor, Michigan.

NIOSH [1997]. Musculoskeletal disorders and workplace factors: a critical review of epidemiologic 

evidence for work-related musculoskeletal disorders of the neck, upper extremity, and low back. U.S. 

Department of Health and Human Services, U.S. Public Health Service, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health, (DHHS) Publication No. 97–141. 

Lu M, Waters T, Piacitelli L [2007]. Accuracy and reliability of human postural simulation for assessing 

physical risk factors associated with low back disorders. PREMUS Sixth International Scientific 

Conference on Prevention of Work-Related Musculoskeletal Disorders, Boston, MA, Aug 27-30.

Waters TR, Putz-Anderson V, Garg A [1994]. Applications manual for the revised NIOSH lifting equation. 

Cincinnati, OH: Centers for Disease Control and Prevention, National Institute for Occupational Safety 

and Health, DHHS (NIOSH) Publication 94-110.
Health Hazard Evaluation Report 2007-0170-3070 Page 15

Acknowledgments a

Availability of Rep

The Hazard Evaluations and Technical Assistance Branch 

(HETAB) of the National Institute for Occupational Safety and 

Health (NIOSH) conducts field investigations of possible health 

hazards in the workplace. These investigations are conducted 

under the authority of Section 20(a) (6) of the Occupational Safety 

and Health (OSHA) Act of 1970, 29 U.S.C. 669(a) (6) which 

authorizes the Secretary of Health and Human Services, following 

a written request from any employers or authorized representative 

of employees, to determine whether any substance normally found 

in the place of employment has potentially toxic effects in such 

concentrations as used or found.

HETAB also provides, upon request, technical and consultative 

assistance to federal, state, and local agencies; labor; industry; and 

other groups or individuals to control occupational health hazards 

and to prevent related trauma and disease. Mention of company 

names or products does not constitute endorsement by NIOSH.

This report was prepared by Jessica Ramsey of HETAB, Division of 

Surveillance, Hazard Evaluations and Field Studies (DSHEFS) and 

Ming-Lun (Jack) Lu of Organizational Science and Human Factors 

Branch, Division of Applied Research and Technology. Desktop 

publishing was performed by Robin Smith. Editorial assistance was 

provided by Ellen Galloway. Health communication assistance was 

provided by Stefanie Evans.

Copies of this report have been sent to employee and management 

representatives at the United States Postal Service Seattle 

Processing and Distribution Center and the OSHA Regional 

Office. This report is not copyrighted and may be freely 

reproduced. The report may be viewed and printed from the 

following internet address: http://www.cdc.gov/niosh/hhe. Copies 

may be purchased from the National Technical Information Service 

(NTIS) at 5825 Port Royal Road, Springfield, Virginia 22161.
Below is a recommended citation for this report:

NIOSH [2008]. Health Hazard Evaluation Report: Ergonomic evaluation of loaders 

and hashers at a postal processing and distribution center, Seattle, Washington. 

Cincinnati, OH: U.S. Department of Health and Human Services, Public Health 

Service, Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health, NIOSH HETA No. 2007-0170-3070.

To receive NIOSH documents or information about 

occupational safety and health topics, contact NIOSH at:

1-800-CDC-INFO (1-800-232-4636)

TTY: 1-888-232-6348

E-mail: cdcinfo@cdc.gov

or visit the NIOSH web site at: www.cdc.gov/niosh.

For a monthly update on news at NIOSH, subscribe to 

NIOSH eNews by visiting www.cdc.gov/niosh/eNews.

Delivering on the Nation’s promise:

Safety and health at work for all people

through research and prevention.

 National Institute for Occupational 

Safety and Health