A Buyer’s Guide to IMU Sport Sensor Devices for Professionals

The microsensor market, specifically motion sensors found in many devices in sports tracking, exploded over the past few years. Unfortunately, due to the low cost of the sensors, the market has been overrun by gadgets instead of precision instruments. The velocity-based training market is now saturated with low-cost sensors and, due to the availability of the technology, the sensors are usually included in body tracking products such as GPS systems and similar wearable technologies.

Inertial motion units, or IMUs, are sensors that collect motion data that can be used in calculating estimates of work, speed, and even movement angles. IMUs are only as useful as the methodology of the users and the algorithms developed by the companies that provide sport systems. Simple accelerometers can be found in sleep trackers and even in the smartphones we use every day, and while they are acceptable tools for estimating athlete motion, they are far from perfect. In this guide, we provide a serious look at IMU technology and applications, and review example options that are excellent for sports training and rehabilitation providers.

What Is an Inertial Measurement Unit?

An IMU is typically a combination of sensors that assist in collecting the motion of a body or sporting implement, such as a ball. IMUs are integrated with sports training equipment because they are small and collect multiple types of data, including acceleration, orientation in time and space, and direction. IMUs are also inexpensive and don’t require much energy to collect and transmit data, so they are ideal solutions for sports training and competition.

IMUs collect many types of data, including acceleration, orientation in time & space, and direction. Share on X

An IMU system has three primary sensors: an accelerometer, a magnetometer, and gyroscope. All three have individual value, but together they form a superb measurement solution for general sporting action. The amount of accuracy and precision is dependent on the calculations of algorithms when engineered, so the development of a system of checks and balances is necessary.

Accelerometer: An accelerometer is exactly what it sounds like—it detects changes in speed and is invaluable for general body motions. Without other sensors though, it’s very limited and can’t give context to what is occurring.

Gyroscope: A gyroscope helps determine the orientation of the sensor, determining how it is moving in three dimensions. Multiple sensors connected together can model the motion of an entire body if carefully designed.

Magnetometer: Assisting in direction orientation, a magnetometer is useful in calibration and directing the data to the right position. Magnetometers are not as important as the other two sensors, but they do add another layer of data that is appropriate for outdoor sports.

The expectations beyond the sensor side of an IMU is that the system has battery and wireless components, as well as the necessary parts that allow for functionalities such as charging the device and possible indicator lights. More and more sports technology equipment includes other components unrelated to movement because they add context, but they are not IMU sensors.

What Applications Are Appropriate for IMU Sensors?

The versatility of IMUs make them prime candidates for use as sports technology solutions, especially because of their size and ability to collect data continuously. While inexpensive, producing a full turnkey solution requires a lot of development, especially on the algorithm and software side. Most of the IMU solutions solve small motion needs, but some companies use IMUs for very demanding purposes. For example, a cable television remote may have an IMU to simply detect if it’s picked up or placed down to illuminate the buttons. More advanced applications use IMUs, such as monitoring jet engines, while many simple devices in sports training help coaches detect jumping.

Their size and ability to collect data continuously make IMUs versatile sports technology solutions. Share on X

Some barbell velocity and performance products use IMUs to detect changes in speed or orientation of path of the stroke. While IMUs are great for shifts in acceleration, they are not ideal for slow motions such as isometric activities. Although calculations can improve, one issue of note is that IMUs favor simple actions. Drift, or when the data loses its accuracy over time, is a problem that requires refinement in algorithm and firmware modification. Most wearable products on the market now use IMUs to collect simple activity data, like walking or running, but they are more powerful than pedometers. A modern IMU can collect more sensitive data and, if used creatively, can acquire information such as hand gestures and even small deviations in body posture.

There is a growing interest in IMU use in collision and combat sports, as well as military activities. High-speed collisions create accelerations to the head and torso, thus making IMUs appropriate for impact detection. Based on the research, most of the calculations and algorithms available are valid and reliable for detecting tackles, scrums, and head impacts with helmet-based sensors. While calculations from IMUs are refined, the individual state of brain, health, and neck strength can determine the severity of a blow, so other factors outside the forces delivered to the body should be considered.

How IMUs Calculate from Different Locations of the Body

The location of an IMU sensor is of high importance for several reasons, and the first is comfort. For example, some sports collisions are common and a normal part of the game, and a sensor can be damaged or cause pain during use if it impacts the body during a hit or fall. IMU placement is also important for validity and accuracy requirements. Placing a sensor near the center of mass of an athlete does approximate a more true measure, but comfort and ergonomics have made the upper back a prime location for the athlete tracking market. Other sites, such as the foot and wrist, do help coaches and athletes see relationships between the ground and throwing or lifting, but those are still indirect estimates of what is happening in sporting actions.

IMU placement is important for athlete comfort, as well as validity and accuracy requirements. Share on X

Several rounds of revisions are necessary to create a robust algorithm, such as planned activities and exercises. Depending on the location on the body, the data can be useful and revealing, or simply inappropriate. Algorithms are only as smart as the engineering of where the sensor is placed and how well that location interacts with the body action. Placing several sensors on the body does increase the likelihood that the data will be valid, but more information doesn’t help if the integrity of the data isn’t solid with one location.

You should not view IMUs as interchangeable with motion capture, but they are often used as a proxy to camera systems because they are clinical grade and useful for basic activities. Most of the IMU systems that perform motion capture for very basic applications are considered clinical tools. Clinical-grade tools are accurate enough to perform very low level tasks, but they don’t provide the rich precision of research-grade options. Usually, companies tag each sensor with a specific location on the body, as inferred information enables the software to calculate more intelligently. Additionally, a calibration or orientation period is typically needed before using multiple IMU systems.

Features and Functions of Hardware and Software

As stated earlier, IMU hardware is normally wearable, and that means it must be water-resistant, small, wireless, and battery-powered, and store data if needed. What makes IMU sensors special is that they can be programmed and designed to solve many different problems with measuring athlete motion. Some IMU sensors attach to the athlete, while some attach to equipment. Hardware is the most important factor with IMU technology because if an athlete will not wear it or finds the user experience unsatisfactory, software features and high data accuracy are pointless benefits.

Wireless data from IMU sensors is now continuous and live, and the expectation is that the process will provide immediate feedback. Low-energy Bluetooth is the standard because the data volume is reasonable. One main difference between IMU sensors is that modern options can be used with smart devices. While a full IMU motion capture system still uses software on a computer, many small devices can connect wirelessly to a tablet or smartphone. Some systems do connect to traditional setups, such as laptop and desktop computers, but most of those are for research or enterprise systems.

A strong point with IMU sensors is the sheer volume of data they collect. Share on X

One of the strong points of IMU sensors is the sheer volume of data they collect, so an interest in machine learning and other forms of artificial intelligence is growing. Most software programs for sport IMUs are web applications that manage team data, and companies may provide an API that connects to other platforms, such as athlete management systems.

Example Solutions for Coaches and Medical Professionals

The market has over a hundred companies that provide IMU products to fitness enthusiasts and researchers, and the turnover of products is very rapid. We provide the list of companies and products below to give you insight into what is available. The guidelines above are timeless advice that will enable you to make an informed choice for years.

Motus

This U.S. company specializes in baseball tracking, but technically its product can be used for other sports and body motions. The Motus band tracks arm motion and has specific metrics that inform the user of fatigue, abnormal mechanics, and possible compensation. Due to the frequency of elbow and shoulder injuries in pitchers, the Motus has potential with tracking, but it’s still a long shot for injury reduction due to the constraints of measuring entire body motion and actual forces. An IMU doesn’t have enough sensitivity to account for small arthrokinetic changes to the elbow and shoulder that occur in live motion throwing. There is currently not enough research available to determine if the sensor can actually detect those responses and whether they are valid to use as tracking metrics anyway.

Runscribe

Most commercially available running sensors are designed for cadence only, but Runscribe uses their metrics to evaluate the entire foot strike. Currently, the product is more prosumer, meaning it caters to high-end users and some coaches in the endurance market, but it’s not a product that can support sprinting or other motions unless you export the files into a .csv reader. The wearable sensor is mounted on the rear of the sneaker or top of the laces, making it user-friendly and practical for different sports. Endurance runners, and even milers and other middle distance athletes, can benefit from the system. Countless systems are available on the waist for stride analysis, but actually tagging the right and left leg reduces error and calibration issues. 

MuscleLab

Ergotest provides an integrated and synchronized family of measurement tools that are designed for coaches, researchers, therapists, and sport scientists. The MuscleLab line of products is excellent for applied settings, but valid for performing scientific investigations. The IMU sensor is mainly a gait analysis tool, but coaches can integrate it with jump training and other modalities. The IMUs are location-agnostic, meaning the data is designed to be interpreted by an expert and is not a motion capture system. The IMUs can be integrated with force plates, load cells, electromyography, contact grids, and lasers. Ergotest’s founder is one of the pioneers in velocity-based training, and produced a system in the 1990s before the popularity we see today. The company is based in Norway, and has partnerships worldwide.

Jawku

This U.S. company offers an electronic timing system that is part smartphone camera-based and part accelerometer-assisted. Jawku recognizes first movement using the accelerometer set to detect real initiation, and the app detects movement of the finish of the run. Unfortunately, the product can’t get instantaneous speed or even splits, but due to the price point it’s likely serviceable by some coaches. As of mid 2018, no validation study is available, but the engineering and hardware side does have the potential to provide a rather reliable product. In addition to the clever use of the smartphone, the product is technically a wearable, as the system is worn on the wrist. Teams and large facilities should think about the limited use in group settings, as wearables are cumbersome when doing combines or testing large groups.

VERT

VERT is more of a jump tracking product than a jump testing system, as it estimates work rate with jumping. The system is designed to be worn around the waist, and is popular with volleyball teams. Basketball, being a higher revenue sport in general, has used wearable GPS or tracking systems for jump detection, but the systems are limited for jump testing. Over the years, the system was marketing to the consumer side, but some teams have adopted it as a way to move towards a more automated approach to workload for jumping athletes. A lot of traction started in 2012 when the momentum of the product hit a commercial tipping point. As of today, some companies are attempting to follow suit with a similar product.

Assess2Perform

This U.S. company produces three products that address the ball, body, and barbell tracking markets. With more than 10 years of experience in the medical and performance spaces, the founder created a complete line of solutions that empower coaches, rehabilitation specialists, and even researchers. The ballistic ball was validated for explosive power recently in the scientific literature, and the company’s other products, which include Bar Sensei, continue to sell well internationally. The main goal of Assess2Perform is to provide a practical solution for all levels of sport and fitness, and support those in the wellness and rehabilitation market.

Xsens

Based in the Netherlands but with offices internationally, Xsens is an enterprise solution for large industries and sports. The Xsens is a complete turnkey solution, and is open for experimentation-type activities including research. The company is gaining traction in the sports market, and has tracking tools available for coordinate data. The company provides 3-D motion capture and is available for research, manufacturing, rehabilitation, and worker safety. While the teams using Xsens are currently unknown, the product is gaining momentum with awareness in the North American market.

TritonWear

The only aquatic product listed in this buyer’s guide is made in Canada, and is focused on the sport of swimming. Each TritonWear sensor is designed to detect surges or acceleration in the water, and those metrics are helpful for seeing overall patterns in distance per stroke and similar indices specific to swimming. TritonWear is an enterprise product, but individual purchases can be made for those wanting to test and monitor individual athletes. The data is sophisticated enough for deep analysis, as it can detect small motions such as breathing pattern and time underwater. In addition to the metrics it collects, the solution is useful for building workouts and race analysis.

IMeasureU

This New Zealand company has traction with a few teams and was acquired by Vicon in mid 2017. IMeasureU has its U.S. headquarters in Denver, Colorado, and touts the Philadelphia 76ers as one of its clients. In addition to the NBA, over the last two years several pro teams have tried the system, which requires the user to wear it around their ankle. The algorithm is a simple strain calculation that detects step impact, and the data is sent to a smart device using Bluetooth. The product can be used for other means, but so far most of the development is for impact load tracking for sports like basketball and running-based field sports.

dorsaVi

This international company offers two sensors, an EMG option and an IMU wearable. dorsaVi’s strategy is to hit all levels of business-to-business solutions, including teams and colleges. Most of the IMU systems are valid enough for simple work rate or activity detection, but they are limited because they are IMU-based. So, while they are marketed as motion capture, they are clinical grade for applied settings and not as effective as camera-based solutions. The company has a strong presence in Europe, Australia, Asia, and North America. Over the last few years, the company demonstrated strong management of an array of different applications, including sports performance.

Plantiga

Plantiga provides a movement analytics platform supported by sensor-equipped insoles. They quantify the biomechanics of sport-specific movements by measuring the accelerations of each foot strike during running, jumping, and changes of direction. The company’s insoles slot into existing athletic footwear and capture performance and asymmetry data in training and competition. Cloud-based algorithms then translate the data into metrics and insights which users interact with on Plantiga’s web and mobile apps. The solution helps trainers, physical therapists, and coaches screen athletes at intake and identify trainable deficits in order to prevent injuries, guide return-to-sport, and improve performance. Headquartered in Vancouver, Canada, Plantiga’s offering has garnered interest from the four major North American sports leagues and other pro sport organizations, such as the United States Tennis Association.

The market will likely see an increase in IMU options, and a decrease in their cost and size. Share on X

The future of IMU markets is unknown, but what is likely to happen is an increase in options and decrease in the size and cost of the technology. In addition to hardware companies, we will likely see the rise and fall of different software players and possibly new opportunities from innovation. While IMUs will always be seen as indirect measures of kinetic and kinematic output, their usefulness and reliability is at a standard where they are accepted as appropriate sport science tools for data collection.

Adopting IMU Solutions for Your Organization or Needs

Highly trained sports professionals can use IMU tools in conjunction with other systems to determine the context of the data they are collecting with sensors or know how to work around the limitations of the data. The usefulness of IMU sensors is the opportunity they provide to have specialized data assist with small needs, and extensive options like motion capture are possible with much larger budgets. IMU sensors can create a network, or add accuracy and validity to other sensors, but in isolation they can only estimate most measurements. Scientific validation from actual peer review research is recommended before trusting the data, and when necessary, new systems can be vetted internally with accepted gold standards.

Since you’re here…
…we have a small favor to ask. More people are reading SimpliFaster than ever, and each week we bring you compelling content from coaches, sport scientists, and physiotherapists who are devoted to building better athletes. Please take a moment to share the articles on social media, engage the authors with questions and comments below, and link to articles when appropriate if you have a blog or participate on forums of related topics. — SF