Wireless force-sensing insoles
Figure 1. Wireless force-sensing insoles send data via Bluetooth to an app on a tablet or smartphone (Loadsol, Novel, St. Paul, Minn).

Bringing the Lab to the Field

By Chase Straw, Ph.D., and Athol Thomson, Ph.D.

Several devices are commercially available to quantify surface properties of a sports field (e.g. surface hardness, turfgrass shear strength, infill depth, etc.). These data are useful in creating thresholds for data-driven management decisions, rather than basing them solely on intuition. Once certain surface property thresholds are met, the field is oftentimes deemed as safe and playable; however, safety and playability have another important component: the players themselves. 

Players interact with the surface through the shoes on their feet to perform movements on the field. Ensuing player loads during these movements trigger mechanobiological responses in the musculoskeletal system that make those tissues (e.g. bone, tendons, ligaments, etc.) stronger or weaker. Understanding surface properties alongside player biomechanics is important to minimize injury risk, maximize performance, and better grasp what exactly field safety and playability is.

One issue is that the collection of high-quality biomechanical data during athletic movements has historically been limited to laboratory settings with expensive force plates embedded in the floor and 3D motion capture systems. Nevertheless, rapid improvements in the cost, size, weight, and data collection sampling frequency of wearable technologies may be a game-changer. Evaluation of player biomechanics during practices or games across different climatic conditions, surfaces, footwear, and movement strategies opens exciting possibilities to improve understanding of complex, on-field interactions between player and playing surface. Of course, context is king, and there are some pitfalls. Here we present a brief introduction to a couple wearable technologies, and then consider their use in conjunction with surface property data to advance our knowledge of player-surface interactions and adjust field management strategies accordingly.

What is wearable technology? 

Wearable technologies (WT) are devices (usually electronic sensors) worn or attached to a person. Sensor specifications depend on the measurement needs; for example, a force sensor to measure ground reaction forces and an accelerometer to measure leg accelerations. For force or acceleration data it is important to collect a measurement many times a second (high sampling frequency), as the rate of loading in certain player movements occur across a short timeframe, such as when sprinting or landing from a jump. WT sensor data may be viewed in real-time via WiFi with a computer, phone or tablet. They can also be stored onboard the sensor and downloaded or sent via Bluetooth to an online dashboard after practices and games. Most companies even offer dashboard solutions to retrospectively analyze and view the data. 

Examples of wearable technologies 

Wireless insoles

Wireless insoles work essentially like mini force plates inside a shoe or cleat (see Figure 1, above). Metrics involving the foot-to-surface interaction can include ground reaction forces, accumulation of impact forces, ground contact time, loading rate, regional pressure within the foot, and others. Usually, wireless insoles measure the vertical component of force (not horizontal or mediolateral) and have a lower sampling frequency rate than an embedded force plate in a laboratory. Caution should be used when interpreting some metrics, such as loading rate, when insoles have low sampling frequency, as a few milliseconds can make a difference to the peak or maximum force measured. The collection of multiple foot impacts across a playing session on the field of play is still a major advantage compared to traditional force plates in a laboratory, since it is difficult to replicate outdoor environmental conditions and player movement during actual competition.

Figure 2. Example of an inertial measurement unit (IMU) placed on the lower leg (IMeasureU, Auckland, New Zealand). Data can be streamed in real time or downloaded after practices or games to a dashboard for further analysis. It is important to check the protocols for data security and Cloud storage of personal data, which may differ in each state or country.

Inertial measurement units (IMU)

An IMU is a collection of sensors – which includes a gyroscope, accelerometer and magnetometer – that collect data based on movement of the unit. In sports, they are often used to evaluate accelerations and decelerations of the body or segments of the body [e.g. the tibia (leg)], angular rates or movements, and body orientations (see Figure 2, above). Examples include integration into American football helmets to measure impact decelerations at the head or strapping them to shoes or cleats to measure the timing and magnitude of foot impacts (see Figure 3, below).

Figure 3. Example of an IMU fastened to back of soccer cleats with custom-made strap (PlayerMaker, Tel Aviv, Israel).

Furthermore, IMUs with global positioning system (GPS) technology allow for measuring speed and distance metrics, such as the time spent running above a speed level or distance ran while running within a certain speed range. Locational data (i.e. latitude and longitude) obtained from the GPS let you know where some of these measurements take place at exact locations within a field (see Figure 4, below). Good battery life, high sampling frequency for essential measurements (e.g. force, acceleration, GPS location), adequate range of measurement unit for the metric of interest (e.g. +/- 200g for accelerometers covers the majority of sporting movements and collisions), and waterproof housing (for sweat or water off the playing surface) are all important considerations.

Figure 4. Locations within a field where athletes were running ≥11 mph. Data were collected using Titan 2 GPS player trackers (Integrated Bionics, Houston, Texas). Speed values are on a color scale, where green areas are speeds ~11 mph and increase as the color transitions to red.

The future of wearable technologies in sports field management

So, what do these wearable technologies and their data mean for sports field management? As an increasing number of teams begin adopting and wearing these technologies, coaches and trainers may begin to express interest in the role of the field and its influence on their players. If you are already testing your field, then this may allow for more open dialogue with the team to compare datasets and adjust management strategies to better meet their expectations. If you are currently not capable of testing your fields, it may provide some justification for purchasing surface testing equipment so that these types of comparisons can be made. Platforms for simple integration of player and surface property data are coming regardless, especially at the collegiate and professional levels. In fact, a company in another country has already developed software to incorporate data from wearable technologies into their online dashboard to identify relationships with surface property data. This type of software is going to provide a platform that helps us better understand player-surface interactions under a variety of real-world scenarios, which will take data-driven sports field management to a whole new level.

Chase Straw, Ph.D.is an assistant professor in Turfgrass Management and Physiology at Texas A&M University in College Station, Texas.

Athol Thomson, Ph.D.is a podiatrist and research scientist at Aspetar Sports Medicine Hospital in Doha, Qatar.