Sport is built on pushing limits. But one limit that science is making increasingly clear is that we must respect is the brain’s tolerance for repeated rapid movement, impacts, blows, jolts, and rotational forces. Head impact sensors exist to make those forces visible – to turn an invisible, silent risk into objectively measured data that coaches, athletes, and parents can actually use.
This guide explains what head impact sensors are, how they work, what the data means, and how the ACT Head Impact Tracker head sensor Pro takes the technology a step further than most products on the market today.
The problem sensors are trying to solve
Before explaining what a head impact sensor is, it helps to understand why one is needed at all.
The brain sits inside the skull, surrounded by cerebrospinal fluid. When the head moves rapidly – through a collision, a tackle, a fall, a heading duel, or a hard jolt to the torso – the brain lags slightly behind the skull before catching up. That lag causes the brain tissue to stretch, compress, and twist against the inside of the skull. This is the mechanical basis of traumatic brain injury, including concussion.
Here is what makes this so difficult to manage without measurement tools:
There are no pain receptors in brain tissue. Unlike a broken bone or a torn ligament, the brain cannot signal distress directly. An athlete can sustain a significant head acceleration event and feel nothing in the moment.
Symptoms may take up to 48 hours to appear. An athlete can walk off the pitch apparently fine and develop symptoms – headache, confusion, sensitivity to light or many more – later that day, the following day, or even day after that.
Cumulative effects become visible only years or decades later. Repetitive concussive and sub-concussive events, smaller impacts and events that cause no immediate symptoms, can contribute to progressive conditions including chronic traumatic encephalopathy (CTE), dementia, Parkinson’s disease, and ALS – often only becoming apparent long after an athlete’s playing career is over.
You cannot manage what you cannot measure. Without data on the number, magnitude, frequency, and proximity of events acting on an athlete’s head, decisions about training loads, return-to-play, technique modification and more, are made in the dark.
What is a head impact sensor?
A head impact sensor is a wearable device that measures the acceleration, deceleration and other forces acting on an athlete’s head during sport. It contains one or more accelerometers and gyroscope – precision instruments that detect motion – and captures data each time the head moves, accelerates or decelerates rapidly.
Most modern head impact sensors record some or all of the following:
Peak linear acceleration (g-force): How powerfully the head accelerated or decelerated in a straight-line direction, measured in multiples of gravitational acceleration (g).
Angular acceleration (rad/s²): How fast the head rotated during the event, which science now identifies as the primary driver of brain tissue deformation and injury.
Angular velocity (rad/s): The speed of rotation of the head during an impact.
Event timing: When each event occurred, enabling frequency and proximity analysis across a session.
Accumulated load: Metrics that combine magnitude and duration to give a picture of cumulative brain load over time.
The sensor transmits this data – typically via Bluetooth – to a smartphone app and cloud dashboard, where in just seconds it becomes readable, trackable, and comparable across sessions and seasons. There are also sensors available which do not send the data in real-time, when the events are occurring, but make the data available only after the session. Study the options carefully and ensure you buy the sensor type which the best meets your requirements and needs.
How does a head impact sensor actually work?
Inside the sensor is an accelerometer chip – micro-electro-mechanical device (MEMS) that detects changes in velocity along one or more axes. When the head moves suddenly, a tiny mass inside the chip is displaced; the chip measures this displacement and converts it into an acceleration value. More advanced sensors also have gyroscopes micro-electro-mechanical device (MEMS) used to measure orientation and angular velocity. ACT Head Impact Tracker head sensor Pro combines a linear accelerometer with a gyroscope to capture both linear and rotational motion simultaneously. This is significant, because the two types of force act on the brain in different ways and contribute to injury through different mechanisms.
Recent technological advances have enabled the development of head impact sensors to estimate head impact exposure in vivo, giving the ability to track how many events an athlete experiences, how forceful they were, and how often they occur.
The sensor must be mounted correctly for the data to be meaningful. Attaching it firmly to the head – via a tight-fitting headband, or purpose-built dock for example, or on a mouthguard – ensures that the sensor is measuring what the head actually experiences, not the motion of loose equipment moving around it.
Why rotational acceleration matters so much
For many years, head impact research focused primarily on linear acceleration – how hard the head was hit. But a growing body of science is shifting this understanding significantly. Rotational head motion appears to be the primary contributor to brain injury risk, due to the unique mechanical properties of the brain and its location within the body. The brain has a high bulk modulus relative to its shear modulus – meaning it deforms most easily under shear forces, which are generated by rotation rather than straight-line acceleration. When the head spins, even briefly, brain tissue experiences shear strain that can damage axons, disrupt neural connections, and in repeated instances contribute to progressive disease.
Researchers have theoretically shown that angular acceleration determines brain strain, and validated this through numerical simulations using a finite element head model – while also demonstrating that the brain strain caused by linear acceleration is small compared to that caused by angular acceleration.
Research in American football makes the scale of the difference vivid. The average sub-concussive impact recorded in a large study of football players had a rotational acceleration of 1,230 rad/s², while the average concussive impact had a rotational acceleration of 5,022 rad/s². The data from rotational kinematics provides far more discriminating information about injury risk than linear acceleration alone.
We at ACT Head Impact Tracker are relentlessly working towards enabling studies, research and development towards ever better, accurate and relevant data to help the sports to safeguard athletes’ brain health and safety more effectively. A great example of this is the development of Impact Feature Space (IFS, patent pending). Impact Feature Space is a multidimensional representation of head-motion events. Instead of relying only on peak linear acceleration, it describes each event using multiple mechanical domains, including magnitude, timing, rotational components, cumulative measures, and waveform structure. This supports more detailed comparison of events in research and advanced product analytics. Do not present Impact Feature Space as a single injury-risk score unless such use has been separately validated.”
Where and how are head impact sensors used?
Head impact sensors are used across a widening range of sports and settings:
In team sports – ice hockey, American football, rugby, football/soccer, cheerleading, and many more – coaches and physiotherapists use the data to monitor individual events occurring on the pitch, and cumulative brain load across a training session, a game, or a full season. This enables spotting athletes carrying unusually high loads, adjusting training intensity, and make more informed return-to-play decisions. Also the data may help in potentially hazardous events recognition by providing real-time data and more information on the events happening on the pitch.
In individual sports – equestrian, alpine skiing, cycling, kickboxing, boxing, martial arts, motor sports and more – sensors give athletes and their support staff objective data that simply did not exist before: how many events, and how many significant events, occurred in a session or season, how forceful the events were, and how today’s load compares to yesterday’s, this week’s to last week’s, this month’s to last month’s, this season’s to that of last. The data may also help in potentially hazardous events recognition by providing real-time data and more information on the events taking place.
With youth athletes – parents and junior coaches increasingly use sensors to build awareness of events on the pitch and cumulative load in young athletes, whose developing brains are particularly vulnerable. The data may also help in potentially hazardous events recognition by providing real-time data and more information on the events taking place.
In research – universities and sports medicine institutions use head impact sensors to study exposure patterns across different sports, age groups, and genders. The research is still in its early stages in many sports and for female and youth populations, but head impact research is improving our understanding of the acute and chronic effects of head impacts on neurological impairment and brain injury.
What does the data tell you – and what doesn’t it tell you?
Understanding the data is as important as collecting it.
g-force, linear acceleration. tells you the magnitude of linear acceleration. Higher values mean more forceful events. Research on adult male athletes has suggested thresholds around 70-100g as associated with increased concussion risk in some studies – but these values are not universally agreed upon and vary significantly by age, gender, concussion history, and individual biology. They should never be used as a definitive diagnostic threshold.
Angular acceleration and velocity capture the rotational forces that science identifies as most damaging to brain tissue. No universally accepted concussive threshold exists for these values either, but tracking them over time gives a more complete picture of cumulative risk than linear acceleration alone.
Event history is the record of how many events there are, when they occurred, how often, and how close together – is critical for understanding cumulative brain load.
What sensors do not do: ACT Head Impact Tracker is a measuring device. It is not a medical device, and does not diagnose, prevent, or treat concussion or any other brain injury. In suspected head injury, remove the athlete from play and seek assessment by a licensed medical professional.
ACT Head Impact Tracker head sensor Pro: the most complete picture available at affordable price
ACT Head Impact Tracker head sensor Pro is designed to give athletes, coaches, and other team personnel the fullest possible data set from a single, affordable, universally compatible sensor.
What it measures
It captures multiple distinct data streams simultaneously in real-time:
Maximum g-force (linear acceleration/deceleration) – the magnitude of each linear force event, in g.
Maximum initial maximum g-force – the magnitude of each linear force event in the beginning of the event, in g.
Impact g-load (AUC) – an accumulated metric that combines the magnitude and duration of linear forces, reflecting the total linear load of each event.
Initial impact g-load (AUC) – an accumulated metric that combines the magnitude and duration of linear forces, reflecting the total linear load of each event in the beginning of the event.
Maximum angular acceleration (rad/s²) – the rotational acceleration of the head, now widely considered the primary mechanism of brain tissue injury.
Maximum initial angular acceleration (rad/s²) – the rotational acceleration of the head in the beginning of the event.
Maximum angular velocity (rad/s) – the rotational speed of the head during each event.
Maximum initial angular velocity (rad/s) – the rotational speed of the head during each event in the beginning of the event.
Accumulated angular acceleration (AUC) – a cumulative measure of angular acceleration during an event.
Accumulated initial angular acceleration (AUC) – a cumulative measure of angular acceleration during an event in the beginning of the event.
Accumulated angular velocity (AUC) – a cumulative measure of angular velocity during an event.
Accumulated initial angular velocity (AUC) – a cumulative measure of angular velocity during an event in the beginning of the event.
Time of the event, frequency and proximity of the event
History of the events
No other consumer-available sensor at this price point combines all these measurements in a single head-mounted device.
What makes the Pro different
It measures what the science says matters most. The inclusion of angular acceleration and velocity means the Pro captures the rotational forces that linear-only sensors miss entirely – the forces that research increasingly identifies as the primary drivers of concussion and longer-term brain disease.
It is independently validated. In December 2023, the Machine Dynamics Laboratory at LUT University (Lappeenranta-Lahti University of Technology, Finland) independently tested ACT Head Impact Tracker sensors against calibrated precision reference accelerometers. The result: a maximum deviation of just 3.6% between ACT sensor readings and the reference instruments – confirming that the data ACT captures accurately reflects the linear acceleration that actually acted on the head.
It is sport-agnostic. ACT Head Impact Tracker head sensor Pro attaches via hook-and-loop (Velcro) to any tight-fitting head accessory – a headband, separated from the helmet liner, or ACT dock – and works across virtually all dry-land sports. This is in contrast to competitors that are designed exclusively for team sports, helmeted sports or specific equipment. Whether your athlete plays ice hockey, floorball, football, rugby, practices boxing or martial arts, or competes in equestrian sport, the same sensor works across all of them.
It works with both Android and iOS. ACT Head Impact Tracker app is available free on both Google Play and the App Store.
It is accessible. At €199,90, ACT Head Impact Tracker head sensor Pro sensor is a fraction of the cost of institutional-grade research systems, and unlike enterprise competitors, it requires no dedicated sideline devices, no large team charging units, and no mandatory monthly subscription to access the core data.
How ACT Head Impact Tracker head sensor Pro works in practice
ACT Head Impact Tracker head sensor Pro attaches to a tight-fitting headband worn on a head, ideally also when wearing a helmet below a helmet, or ACT Head Impact Tracker Dock on a tight-fitting head or hair band of athlete’s choosing. During training or competition, the sensor captures every significant force event – recording magnitude, rotation, timing, and load – and transmits the data to the ACT cloud.
The data is available immediately in the free ACT Head Impact Tracker mobile app and the browser-based Dashboard. In the app, the athlete or coach can review individual event recordings , and compare with previous events. In the Dashboard, coaches can create season and session records, view also the individual athletes’ and full squad’s cumulative load in sessions and seasons, flag events for further review, add notes to the events and get access to all the data collected.
The data can also be exported to Excel or CSV via the browser access for deeper analysis or record-keeping.
A note on responsible use
Head impact sensors can be powerful tools for awareness, understanding and prevention – but they require responsible use.
The data does not replace medical judgement. If an athlete reports symptoms of concussion, or a head injury is possible or suspected, the athlete must leave the activity immediately regardless of what the sensor readings show. Sensors capture forces; they do not diagnose injuries.
Thresholds in the research literature are statistical associations, not individual cutoffs. An impact or event above a certain g-force does not mean the athlete is definitely injured; an impact below it does not mean they are definitely fine.
Used correctly – as part of a broader athlete safety culture, alongside proper procedures, protocols, educated coaches, and clear return-to-play procedures – head impact sensors are among the most meaningful tools available for protecting athletes’ long-term brain health.
The bottom line
A head impact sensor makes the invisible destructive forces visible and objectively measured. It turns the silent, painless forces into data – data that can be tracked, compared, acted on, and used to make sport safer for every athlete who plays it.
The ACT Head Impact Tracker head sensor Pro represents the most complete, most accessible, independently validated version of this technology available today for grassroots and competitive sport alike. It measures both the linear and rotational forces that science tells us matter most, works across virtually any sport, and puts that data in the hands of coaches, physiotherapists, and athletes through a free app that works on any smartphone.
Because there is no medication. No cure. Prevention – informed, data-driven, consistent and validated prevention – is the key.
ACT Head Impact Tracker is a measurement device. ACT Head Impact Tracker is not a medical device. It does not provide medical advice, diagnostics, or treatment suggestions. In case of suspected head injury, immediately and safely remove the athlete from activity and seek assessment from a licensed medical professional.
For more information, or to buy your sensor, visit