Health Hazard Assessment (HHA)

 Hazard Category - Shock

Last Updated: June 30, 2022


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

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. ​

​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 harness1,2 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 contant 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.


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:3

Recoil-based Firing Limitations for Test Weapons

Computed Recoil EnergyLimitations 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.4,5

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


(1) 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

(2) 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

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

(4) 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

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