Impact Sensors for Helmets
Summary: Many brands of impact sensors are being sold to be added to helmets to detect hard impacts. They have limited usefulness for bicycle helmets, and limited accuracy. Most measure blows to the helmet, not the g's the head inside experienced after the helmet's energy management. The biggest risk is that they will fail to signal a concussive event or fail to signal damage to the helmet. Researchers believe they can signal hard impacts but can not diagnose concussions.
Impact sensors (usually accelerometers) attached to football players' heads made news about 2005, and devices to attach to helmets have been marketed by a number of companies since.
A sensor attached to a helmet can only record the g's that the helmet sees, not what the head inside experienced. Since the helmet is managing the energy, there will be a huge difference, with the head seeing far less shock. The original Simbex HIT system for football players mounts an array of accelerometers against the player's head, not on the outside shell of the helmet.
Accelerometers are linear, registering g's in only one direction. To capture all of the energy in an impact, seven is the minimum number of accelerometers to use, oriented in different planes. Eleven would be more accurate. Most of the devices marketed now use only one accelerometer, or at most three.
The Simbex product uses multiple accelerometers held against the head, and gathers the data on the sidelines of a game through a wireless connection. It works, and can alert the coach when a player takes a very hard hit. Simbex and the college football researchers using the system have gathered millions of hits in their data collecting. But correlating the hits and concussions has proven elusive. There is still no agreement on the exact profile of a concussive hit. Some players have experienced 140 peak g's without an injury that can be diagnosed clinically. Some have been concussed at levels below 80 g's. Results vary depending on the direction of the blow, area of the impact and other factors. There is no precise way yet to define the characteristics of the hit that produces a concussion. Peak g has to be important, and rotational energy is suspected to be another key element. Even a straight linear hit involves some rotational component. But what is happening inside the head as the brain moves, as different brain components with different densities and placement move in different ways, contributes to the nerve and blood vessel strain that is thought to be a big determinant of concussion.
Back in 2006 we bought some samples of a device called the Shok-SpotR and asked experts for an evaluation. The device is designed to be added to a hard shell helmet, and registers g's above a certain level by turning a spot red. We got mixed feedback from the lab techs and other experts, who saw it as a device to register helmet damage, since it did not register g's to the head. Some felt that even assuming it functioned correctly, the spot might not change in some crashes that would damage the helmet. Most believed that visual inspection and measuring for foam crush after a crash is the best way to determine if a helmet has been damaged. But since many consumers are not experienced in looking at damaged helmets and may not recognize damage under a shell, there may still be a place for this product if you wear a helmet with a rigid hard shell. Our samples were $25 plus shipping, from a supplier we found with Google.
As concern about concussions has grown in recent years other devices have come on the market, including Impact Alert, Shockbox, the Stabilizer First Alert (attaches to a hockey face mask) and many others. There is also the X2 Impact, with electronics in a mouth guard. That one at least is measuring g's to the head, not the helmet, but must assume that the jaw remains firmly clamped on the mouthguard through the impact sequence. We found all of those and more with this Google search. It will be more current than any list we post here would be. We know nothing of the effectiveness of any of the devices except the Simbex HIT system, a reliable indicator of hits on a sports field.
In 2016 the F08.53 Helmet and Headgear subcommittee of ASTM began developing a standard for the performance of sensors used to track head impacts. The task group will have a difficult job, and the development of the standard may take some time. But eventually you may be able to buy an impact device with confidence that it meets a performance standard.
Bottom line: many of the sensors are designed for use with hard shell helmets. Most will not tell you much about how hard your bike helmet was hit. We do not know of a consumer product that will tell you if you have a concussion, and for bike riders a medical diagnosis of the symptoms you are experiencing will always be your best indicator.
This page was updated or partially revised on: October 1, 2016.