Health Hazard Assessment (HHA)

 Hazard Category - Musculoskeletal Trauma

Last Updated: March 15, 2021
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Musculoskeletal Trauma

Coordinate with the Health Hazard Assessment (HHA) Program early in the acquisition process to eliminate or control health hazard exposures associated with musculoskeletal trauma. Subject matter experts from the Ergonomics Program provide input for HHAs related to musculoskeletal trauma.

Increased stress placed on the musculoskeletal system may cause fatigue, chronic degenerative changes, chronic spine and/or neck injuries, and degraded performance. Injuries may occur by direct trauma, a single exertion (overexertion), or as a result of multiple exertions (repetitive trauma). Specific health effects vary based on the type of exposure. For example, moderate levels of mechanical force often produce pain, superficial ecchymosis, or deep tissue hematoma at contact locations. Exposures to high levels of mechanical shock increase the depth of the shock wave’s penetration and the likelihood of injury to a critical organ. High dosages of recoil force directed at the anterior shoulder may produce soft tissue injury, tendonitis, focal bursitis, nerve injury, or fracture of the clavicle. 

Lift and Carry

The lift and carry of heavy components yields complex exposures producing levels of biomechanical stress that vary as a function of the load handled, postures used, and the frequency, duration, and periodicity of the physical activity employed. Other work-related factors may also lead to musculoskeletal trauma, such as pushing and pulling activities and non-neutral postures.  

Data Requirements
Provide the system description, including the weights and dimensions of all items weighing over 31 pounds  (lbs) that will be lifted. Provide a detailed use scenario (e.g., handling environment, required tasks and physical demands, frequency of lifts). 

Health Protection Criteria
MIL-STD-1472H contains design guidance for efficient handling. Maximum design weight limits based on lifting team size are provided.1 Reduction multipliers for lifting risk factors (e.g., lifting frequency, lifting height, load asymmetry) are considered in the determination of the safest load that a team is permitted to handle. Each item required to be manually lifted/carried should be labeled with their weight and lifting requirements.


Head-Supported Mass

A quantitatively descriptive property which represents the loading on the neck from wearing head protection systems that may affect Soldier performance and health. This term may also be used to refer to the helmet and helmet-mounted system components. 

Data Requirements
Provide the weight of the HSM component, center of mass offset, detailed use scenario (e.g., duration, frequency of use, vibration exposure), and operational baseline helmet configuration (e.g., MOS, aviation, ground).

Health Protection Criteria
Currently-approved health protection criteria are not available for health hazards associated with exposure to HSM for mounted or dismounted ground Soldiers. The U.S. Army Aeromedical Research Laboratory (USAARL) External Link developed performance and acute injury risk guidelines for Army aviation which describe acceptable ranges for mass properties of Army aviation-specific HSM.2,3 Medical and safety personnel, specifically those at the USAARL, are currently working to develop applicable health protection criteria. Completed studies establish preliminary performance guidelines; however, injury criteria are still being researched. 

As a result of the preliminary performance guidelines, it is recommended that the rear and forward offsets relative to the tragion notch be limited to -2 and 9.5 centimeters, respectively, and the preliminary maximum allowable helmet mass be limited to 2.5 kilograms.4


Load Carriage

The total loading on the body from carrying clothing and equipment. The load is typically carried over long distances or durations, and may be worn, affixed, or sometimes hand-carried. Load carriage has been associated with musculoskeletal trauma, and movements or exertions while carrying increased loads may elevate the risk of injury. Examples of systems requiring load carriage include body armor, backpacks, personal weapons, radio handsets, and other equipment attached to the body. 

Data Requirements

Detailed system and use scenario information are required to assess the risk of injury associated with load carriage (e.g., component size and weight, distribution of weight, diagram/picture, method of attachment to the body, backpack type or design (if applicable), other equipment worn and carried by the Soldiers using the system, distance and duration expected to carry load, required tasks while carrying load, environmental conditions).

Health Protection Criteria

MIL-STD-1472H provides design criteria for load carriage1. Individual portions of portable equipment shall not exceed 35 pounds, unless the individuals carrying the load do not need to maintain the pace of infantry movement. The total load carried (i.e., all equipment, clothing, and weapons) shall not exceed 30% of the user's body weight for close combat operations, or 45% of the user's body weight for marching. Additional MIL-STD-1472H design criteria provide suggestions for reducing the risk of musculoskeletal injuries (e.g., distribute and balance the load throughout muscle groups, design the center of gravity as close to the spine and waistline as possible, design the load to permit freedom of movement).


Mechanical Shock (Acceleration/Deceleration)

The delivery of a mechanical impulse transmitted to an individual or body part by the acceleration or deceleration of an inertial force. Potential exposures to mechanical shock include the opening force of a paratrooper’s parachute harness5,6 and the firing of large caliber weapon systems exhibiting whole-body recoil forces (e.g., howitzers).

Data Requirements

The APHC has not established data requirements for assessing mechanical shock. It is anticipated that data requirements will include a method to measure acceleration either directly from an accelerometer or indirectly through calculation. Since most exposures occur while the body is in close contact with equipment, it may also be necessary to use a force gauge to measure forces transmitted to the body at those contact locations. Exposure is evaluated using a systems approach, and requires a description of all system components, including personal protective equipment, clothing, and other equipment.

Health Protection Criteria

Numerous biomechanical studies provide useful insights into the amount of mechanical shock that specific human tissues tolerate. Because most studies have either been conducted on cadavers or, most commonly, on isolated anatomical specimens, it is often difficult to apply the study data to the types of exposures encountered in dynamic military work environments. Mechanical shock interacts differently with intact, living subjects than with tissues studied in isolation. To ensure similarity between the research conditions and the military operation being targeted, caution should be observed when applying biomechanical injury criteria to military operations.

Typical recommendations may include implementing interventions to control the rate of velocity change or alter the forces transmitted through contact points with the human. Forces at contact locations may be moderated by increasing the surface area of the contact, adding cushioning, or incorporating a harness or suspension.


Recoil

Reactive force from the discharge of a firearm, often called “kick,” that propels the weapon backwards and imparts mechanical force to the point of contact with the Soldier's body (usually the shoulder or wrist). The recoil momentum balances the forward momentum of the projectile and propellant gases according to Newton’s Third Law, the conservation of momentum. The magnitude of recoil force delivered to the operator is dependent upon several factors including the design of the weapon as well as firing technique.

Data Requirements

Data requirements for recoil are currently being developed. It is anticipated that data requirements will necessitate conducting a weapon kinetics study. Specific data items required to assess the risk of injury from recoil will likely include measurements of weapon acceleration, weapon speed, and displacement along the axis of the weapon that aligns with the anatomical point of contact with the operator. Factors used to calculate recoil energy are also needed (e.g., weights of the gun, propellant, and bullet). Detailed use scenario information is needed, including the duration of exposure and anticipated number of rounds that may be fired on a typical training or operational day.

Health Protection Criteria

Currently, no Army-approved health protection criteria or medical models have been established for recoil exposures. TOP 03-2-504A contains the following design criteria and recommended test weapon firing limitations:7

Recoil-based Firing Limitations for Test Weapons


Computed Recoil Energy Limitations on Rounds

Less than 15 ft-lb (20.3 joules)
Unlimited firing
15 to 30 ft-lb (20.3 to 40.7 joules)200 rounds/day/man
30 to 45 ft-lb (40.7 to 61.0 joules)100 rounds/day/man
45 to 60 ft-lb (61.0 to 81.4 joules)25 rounds/day/man
Greater than 60 ft-lb (81.4 joules)No shoulder firing

The validity of these design criteria as a basis for health protection criteria for HHAs has not been substantiated. Studies have advised conducting additional research to obtain the data needed to develop a more definitive characterization of recoil exposure and health protection criteria.8.9


For more information and guidelines for assessing musculoskeletal trauma, see Technical Guide 351C, Health Hazard Assessor's Guide.


References

(1) DOD. 2020. MIL-STD-1472H. Department of Defense Design Criteria Standard: Human Engineering. External Link

(2) USAARL. 1998. Mass Requirements for Helicopter Aircrew Helmets (Report No. 98-14). Prepared by B.J. McEntire and D.F. Shanahan. Fort Rucker, Alabama. External Link

(3) AFRL. 1994. Vertical Impact Testing of Two Helmet-Mounted Night Vision Systems (AFRL Technical Report #AL/CF-SR-1994-0013). Prepared by C. Perry. Wright-Patterson Air Force Base, Ohio. External Link

(4) USAARL. 2019. Preliminary Head-Supported Mass (HSM) Performance Guidance for Dismounted Soldier Environments (USAARL Technical Memorandum No. 2019-11). Prepared by A. Madison. Fort Rucker, Alabama. (Note: Limited Release) External Link

(5) U.S. Army Public Health Center. 2019. Technical Information Paper No. 12-095-0219, Injuries Among Military Paratroopers—Current Evidence and Data Gaps. Aberdeen Proving Ground, Maryland. External Link

(6) U.S. Army Research Laboratory. 1995. AL-TR-926, Lower Extremity Assistance for Parachutist (LEAP) Program: Quantification of the Biomechanics of the Parachute Landing Fall and Implications for a Device to Prevent Injuries. External Link

(7) ATEC. 2013. TOP 03–2–504A, Safety Evaluation of Small Arms and Medium Caliber Weapons. External Link

(8) Blankenship K, Evans R, Allison S, Murphy M, Isome H. 2004. U.S. Army Medical Research Institute of Environmental Medicine, Report No. T04-05, Shoulder-fired weapons with high recoil energy: Quantifying injury and shooting performance. External Link

(9) Burns BP. 2012. U.S. Army Research Laboratory, Report No. ARL-CR-692, Recoil Considerations for Shoulder-fired Weapons. External Link