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A software tool to assess injury risk and maximum allowable exertions for repetitive, forceful one hand and two hand shoulder push/pull motions


OBJECTIVE: Develop injury criteria, an assessment methodology, a risk analysis software tool and design criteria for repetitive, forceful one and two hand shoulder push/pull motions performed for variable (brief to long) durations while operating military equipment. The injury criteria, assessment methodology and analysis software will be used to evaluate injury risk from man-machine interactions performed during routine use of military systems. The design criteria will estimate both the maximum strength capacities and the maximum allowable exertion limits that should be imposed on Soldier operators'shoulders. DESCRIPTION: Military jobs impose heavy demands upon shoulders and contribute to musculoskeletal injury. For example, a RAND Corporation study (Kirin, 1992) revealed that 43% of Army Military Occupational Specialties were classified as"very heavy", requiring Soldiers to occasionally lift more than 100 pounds and constantly lift more than 50 pounds. Besides heavy exertion, Soldiers often perform highly repetitive jobs that involve stressful postures. Postural stresses are an important, underappreciated risk factor for shoulder injury. Generally speaking, when the arm moves up and away from the body, pressure increases inside the shoulder joint and soft tissues are sometimes traumatized as they are pinched between the humerus and acromion. These traumatic events are not associated with pain initially. But with repetition and over time, episodic inflammatory reactions cause rotator cuff and articular cartilage degeneration. Severe degeneration produces profound disability. The best prevention for severe adverse health effects is to reduce ergonomic risk factors through proper equipment and job design. Two strategies can be used to achieve that end: provide System Developers with design criteria that reduce biomechanical shoulder stress and conduct health hazard assessments of equipment that identify and mitigate hazardous designs. An assessment tool is needed to characterize the hazards posed by the biomechanical forces evident inside the shoulder while pushing and pulling in different arcs of trajectory using different intensities of exertion for a range of durations. From these injury criteria a tool will be built to characterize the exertional hazards to which Soldiers are exposed while performing tasks and interacting with equipment. Design guidance is also needed that prescribes maximum allowable exertions for pushing and pulling with one or two hands, by either male or female Soldiers. The weakness in current guidance is illustrated by the advice that MIL-STD-1472G gives to System Developers for hatch design. That instruction advises that no more 50 pounds of force be required to open or close an overhead hatches. This guidance fails to recognize that performing repetitive exertions at this intensity may be hazardous, particularly when performed in some postures (especially end range shoulder abduction) or by small-framed individuals. Another issue with the MIL-STD-1472G hatch guidance is that it does not specify reach distance or hand placement. Most Soldiers are not capable of generating 50 pounds of force while reaching far or when hatch location requires that operators assume an awkward shoulder postures. Besides the potential hazard for musculoskeletal injury, the current design standard may allow equipment to be fielded that is difficult to egress. It is imperative that problematic design standards such as this one are revised as the military services open jobs more jobs to take full advantage of the capabilities of women. PHASE I: Develop injury criteria based upon the relationship between changes in intra-articular biomechanical stress related to shoulder posture and intensity of push-pull exertions. Use these criteria and the exposure schedule (hours per day, days per week, weeks per year) to determine hazard severities and probabilities for key adverse outcomes: load intolerance from shoulder muscle fatigue and increased risk (relative to a comparable normal population) of developing near term acute rotator cuff disorders (such as supraspinatus tendonitis and subacromial bursitis) and long term chronic degenerative tissue diseases of the shoulder (such as rotator cuff tears and degenerative joint disease). Build and demonstrate a proof-of-concept exposure assessment model that estimates and displays a biomechanical measure of adverse mechanical stress from a set of data that are either readily measurable or obtainable from the System Developer's specification (joint angle of the shoulder during exertion and force of exertion) or from the use scenario. This model will determine injury risk relative to the full range of male and female anthropometries and display risk using hazard severity and hazard probabilities as defined in AR-40-10. This model shall be limited to a simple vertical overhead push. PHASE II: Build a software application that integrates all of the features specified in Phase I. Additional exposure assessment criteria will be developed and integrated into the software that will estimate biomechanical forces and predict injury risk for pushing at other locations in the 3D work envelope described by the range of motion of the shoulder joint. Additional data will be collected to develop an analogous exposure assessment model for pulling exertions. The software will allow the user to enter key task parameters such as shoulder joint angle and push or pull force and exposure data such as rate or number of exertions and other frequency and duration data. The model will estimate the sum of the biomechanical energy to which musculoskeletal tissues are exposed based upon the task parameters and exposure data entered, determine hazard severity and hazard probability in accordance with AR-40-10 definitions and display these data on the graphical user interface. Develop a design standard that specifies maximum allowable one and two hand push and pull forces for various locations throughout the 3D work envelope based upon muscular strength capacity (fatigue modeling) and separate data for the same 3D work envelope locations that predict maximum push and pull forces based upon tolerance to adverse biomechanical force. PHASE III DUAL USE APPLICATIONS: The contractor will provide a working software application. The US Army Public Health Command (USAPHC) Ergonomics Program will develop at least five hypothetical scenarios that include equipment and user scenarios that require physically demanding pushing or pulling in different directions within a 3D work envelope. The USAPHC Ergonomics Program will use the assessment methodology to conduct analyses, write reports and submit the results through the USAPHC Health Hazard Assessment Program's report review process. USAPHC will collect comments and appraisals from Ergonomics and Health Hazard Assessment Personnel and analyze them. Results will be discussed with the US Army Medical Research and Material Command to identify deficiencies and develop a plan for reconciliation. The injury criteria, assessment software and design criteria will serve as a sentinel reference for ergonomists to describe strength capacity and upper extremity injury risk for dynamic exertions performed at various reach distances throughout the full range the work envelope. This will replace the very meager compilation of static strength reference values that have very limited applicability to equipment design. The normative values from this study will be vital to human resources professionals who need to write accurate job descriptions and occupational health professionals prescribing post-injury return-to-work recommendations. Vehicle and heavy equipment designers will find these data useful for determining maximum exertion levels for manipulating vehicle controls within a driver"s compartment. Ship and aircraft designers will also be able to use the data to estimate force requirements for hatches and doors. The injury criteria will find extensive application by all branches of the armed services as well as others who must perform physically demanding tasks that require pushing or pulling and individuals who design or work in work stations that require frequent reaching or forceful exertion. REFERENCES: 1. Chow A and Dickerson CR. Shoulder strength of females while sitting and standing as a function of hand location and force direction. Applied Ergonomics. 40:3003-8, 2009. 2. Gold GE, Pappas GP, Blemker SS, Whalen ST, Campbell G, McAdams TA and Beaulieu CF. Abduction and external rotation in shoulder impingement: An open MRI study on healthy volunteersinitial experience. Radiology. 244(3):815-822, 2007. 3. Hoffman SG. Whole-body postures during standing hand-force exertions: development of a 3D biomechanical posture prediction model. Doctoral dissertation, University of Michigan, 2008. 4. Hoozemans MJ. Van der Beek AJ. Frings-Dresen MH. Van Dijk FJ. Van der Woude LH. Pushing and pulling in relation to musculoskeletal disorders: a review of risk factors. Ergonomics. 41(6):757-81, 1998. 5. Kirin SJ and Winkler JD. The Army Military Occupational Specialty Database. Report No. N-3527-A. RAND Corporation, Santa Monica, CA, 1992.
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