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What Is Rotational Motion?

Many Aren’t Aware of the Common Cause of Concussions, a Significant Workplace Safety Threat

By Peter Halldin and Sofia Hedenstierna, Contributors

Many of us are at risk of head injuries at work, but for the construction, industrial and manufacturing industries, the risks are far greater. In fact, according to the Centers for Disease Control and Prevention (CDC), the construction industry has the highest absolute number of both fatal and nonfatal traumatic brain injuries of any U.S. workplace. The same authority reports that, between 2003 and 2010 alone, a total of 2,210 construction workers died due to a traumatic brain injury, representing 25% of all construction fatalities.

Although safety helmets and hard hats are mandatory – as the Occupational Safety and Health Administration (OSHA) requires hard hats and head protection when workers are at risk for head injuries – construction and industrial workers still face significant risks on the job. The main causes of traumatic brain injury, both fatal and non-fatal, are slips, trips and falls, and contact with objects and equipment.

One of the reasons these accidents may cause traumatic brain injuries is that when the head impacts the ground in connection with fall, or if a falling object glances the head, this likely occurs at an oblique angle. When your head hits something at an angle, it may expose your head to rotational motion. Conventional hard hats are designed to protect against injuries like skull fractures, caused by linear motion, but oftentimes are not developed to address rotational motion.

Understand Rotational Motion

Rotational motion is a combination of rotational energy (angular velocity) and rotational forces (angular acceleration) that may result from oblique impacts to the head. As the rotational motion is transferred to the brain, it starts to rotate. If the rotation is large enough, this can cause shearing of the axons, the cable transmitters of neurons, in the brain.

During most impacts, both linear and rotational motions occur and can cause injury. However, there are some differences regarding the types of injuries the different motions could cause. While linear motions primarily cause focal injuries, such as fractures and contusions, rotational motion may lead to diffuse injuries such as diffuse axonal injury and subdural hematoma[1],[2]. Additionally, it has been shown in both experiments and numerical computer simulations[3] that the brain is more sensitive to rotational motion than linear motion when it comes to concussions. In other words, a concussion caused by rotation occurs at lower levels of energy than in cases of linear motion. This means that even seemingly light impacts can cause concussions when rotation is introduced.

Protect Against Rotational Motion

First of all, you should wear a helmet, and make sure that it fits comfortably and securely on your head. You should also consider a rotational motion mitigation system. Thanks to many years of research in the field of rotational motion, there are ways to add protection into a helmet to potentially help mitigate its effects.

For example, some rotational mitigating systems allow material in the helmet to move and slide relative to the head, with the intention to redirect rotational energy to linear energy and thereby help to reduce the rotational energy otherwise transmitted to the brain.

Today’s Helmets & PPE

Historically, helmets were designed to protect the wearer’s head from linear forces and most helmets available today protect against a linear impact at the crown, which is mirrored in the regulatory standards. The standard ANSI/ISEA Z89.1 requires five performance tests that must be met to assign a safety helmet its classification, including force transmission, impact energy attenuation, apex penetration, flammability, and electrical insulation.

The force transmission tests, which determine whether a helmet can reduce the force of a linear impact from a falling object to the top of the wearer’s head, are a stark reminder of the necessity to wear helmets on the job. Also, the additional impact energy attenuation test, necessary for an ANSI Z89.1 type 2 helmet assignment, tests the linear accelerations transmitted to the head, although the impacts are located on the side, front, and rear of the helmet.

Although angled or off-center impacts to the head are seen as common accident scenarios and proved to be able to cause brain injuries, the regulatory standards don’t certify helmets regarding their ability to help mitigate against rotational motion from an angled or off-center impact. However, more and more standards in the sport and moto sectors, for example, are introducing testing of angled impacts, illustrating the importance of taking rotational motion and the harmful effect it can have on one’s head into account.

Why it Matters

On the job, most traumatic head injuries that occur are not caused by a pure linear impact to the crown of the safety helmet. When objects fall or a person falls on an object, they typically strike the safety helmet from an angle or off-center. For example, if a brick falls on someone’s head, it most likely will glance off scaffolding or a beam before hitting the safety helmet. Both falling at an angle and an oblique strike to the head by a falling object create rotational motion.

Critical to keeping workers safe is increased awareness of the potential causes of traumatic brain injuries and rotational motion, as well as access to appropriate PPE equipment, including helmets. When it comes to PPE, the first and most important step is for workers to wear a helmet. In fact, a recent survey from PPE provider J.J. Keller Safegear found that 72% of respondents didn’t wear PPE because they didn’t want to, while 50% thought it wasn’t necessary. Educating workers about the risk of rotational motion and traumatic brain injuries is an important step in increasing PPE usage.

The types of helmets being worn matters as well. Workers should ensure their helmets meet current safety standards. In addition, they can consider a helmet equipped with a rotational mitigating system, intended to help reduce rotational motion to the wearer’s head during certain angled impacts.

As workers continue to face risks on the job, education around rotational motion and encouraging best practices around PPE and safety practices is critical to improve conditions.

Peter Halldin is Co-Founder and Chief Science Officer, Mips AB (

Sofia Hedenstierna is Safety and Biomechanics Expert, Mips AB (

[1] Gennarelli (1987). “Directional dependence of axonal brain injury due to centroidal and non-centroidal acceleration,” in Proceedings of the 31st Stapp Car Crash Conference (Warrendale, PA: Society of Automotive Engineers).

[2] Gennarelli et. al. (1972). “Pathophysiological responses to rotational and translational accelerations of the head,” in Proceedings of the 16th Stapp Car Crash Conference, 1972 (Warrendale, PA: Society of Automotive Engineers).

[3] Kleiven, S (2007). “Predictors for traumatic brain injuries evaluated through accident reconstructions,” Stapp Car Crash J, vol. 51, pp. 81–114, Oct. 2007.

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