Stress analysis and the impact on sports

Football Helmet

This week we will be discussing helmets as we move forward in our series regarding the impact of stress analysis on athletic equipment. Helmets are used most commonly in contact sports such as football and baseball to protect the most important part of the human body; the brain. These helmets must be strong enough to handle impacts from 250 pound linebackers or a 90 mph fastball as well as be lightweight and comfortable.

Testing and Technology 

Football helmets are tested and ranked on a scale known as the Summation of Tests for the Analysis of Risk value (STAR) rating system. The 1 to 5 scale ranks helmets on how likely they are to prevent a concussion or other serious damage to the head. The helmets are tested with a National Operating Committee on Standards for Athletic Equipment (NOSCAE) drop test which closely replicates the repeated impacts on the field. Each helmet is tested at four locations and at five drop heights from 12 to 60 inches. Many helmets in use today across the NFL and NCAA are only a 1 star rating and thus have a high injury rating whereas a three or four star helmet can provide as much as 55% reduction in risk of concussion which is very significant. One interesting fact noted in a 2011 study is that the price did not always follow the rating of the helmet. For example, a Schutt™ DNA Pro+ helmet was given a 4 star rating while being the most inexpensive helmet tested at only $169.95, while the Adams™ helmet was a 1 star rating with a price tag around $199.

[i]   Since then, the 2012 study from the same group notes that the three lowest rated helmets from the previous study are now “off the market”.[ii] This is a clear sign that engineers and scientists are taking note of these studies and working together to develop better and safer equipment. One issue with many football helmets, including the five star rated ones, is that while they do a great job preventing skull fractures, they may not do so well where concussions are concerned. While they they have the ability to absorb linear hits very well, angular impacts, which jerk the head and often cause more concussions than blunt force, don’t always fair as well.

In motorcycle stlye helmets, a different type of stress analysis can be seen. Many are poorly designed in that they only look at one factor of injury prevention; they are very well designed to reduce head form deceleration, but this alone does not make it optimized for injury prevention. When an average helmet was tested with a frontal impact, the brain pressure varied from 94 kPa to 350 kPa, which is high above the threshold. For a visible injury, the maximum stress that a brain can handle is 180 kPa in tension and 234 kPa in compression. It has been recently found that the elastic limit within the foam used, has the highest influence on head injuries. One way to measure the stiffness of an elastic material is known as Young’s modulus. To combat injury prevention  successfully, other factors such as the thickness of the shell, the Young’s modulus of the foam and the Young’s modulus of the shell must also be taken into account. Each of these factors will increase the injury prevention of any helmet  to a varying extent. [iii]

The Future of Helmet Engineering

New developments in helmet technology are coming onto the market and looks to address the problem of rotational acceleration. While no helmet on the market can completely eliminate this force, Peter Halldin, a biomechanical engineer at the Royal Institute of Technology is coming very close. He recently designed a system known as the Multidirectional Impact Protection System (MIPS). Instead of being a stationary helmet, a plastic layer sits snugly on the player’s head beneath the padding and is able to keep the head “floating”. In a circular acceleration test, the MIPS helmet performed about 55% better than that of a standard helmet.[iv] While it doesn’t appear to reach that magnitude of performance in every direction it is very plausible that a high reduction in concussions would be seen. While this technology is used sparingly already in snowboarding and cycling helmets, it is just beginning to be seen in hockey and football.

While some will argue that it is the athlete that makes the biggest impact on the future of sports, it can be said that the mechanical engineers behind the development of the equipment play a bigger role. Without these advances, the time on the field for these players would be greatly reduced.


[i] Virginia Tech Study “Football helmet ratings for reducing concussion risk.” ScienceDaily, 11May 2011

[ii] Virginia Tech (Virginia Polytechnic Institute and State University) 2012 May 1

[iii] Deck, C, Baumgartner B., Willinger R., 2003 “Helmet Optimization Based on Head-Helmet Modeling” .

[iv] Foster, Tom “The helmet that can save football”. Popular Science 18 December 2012