GPS Units (i.e. Data Loggers)
As previously discussed, the sampling frequency refers to the speed of which the unit gathers data (32). The current body of research indicates that units with a higher sampling frequency (i.e. 10 and 15Hz) provide greater reliability for measurement of distance. However, the sampling rate of a 15Hz unit is in fact calculated by supplementing a 10Hz GPS sampling rate with accelerometer data (43, 44). As a 15Hz sampling rate GPS is actually indirectly produced using calculus, there is a great need for future research in order to verify the validity and reliability of this method.
In fact, a 2014 study demonstrated that a 10Hz GPS unit had greater validity and inter-unit reliability than a 15Hz GPS unit when measuring total distance (44) – perhaps suggesting that a 10Hz unit is more accurate and reliable than a 15Hz GPS. Regardless of the variations between 10 and 15Hz units, these have both been repeatedly proven to be more reliable at monitoring performance than 1 and 5Hz models (44).
- Speed of Movement – Consistent amounts of research have shown that the slower the movement velocity of the athlete, the more accurate the GPS technology becomes and vice versa – the faster the athlete’s velocity, the less reliable the device. For example, one study demonstrated that the GPS was capable of accurately measuring an athlete’s movements at <20km/h, but struggled once speeds reached >20km/h (45). This would make sense given that the athlete is travelling larger distances, but with the same sampling rate. This information suggests that caution should be taken with the reliability of the data if the athlete(s) was achieving velocities of >20km/h.
- Short-distance, high-speed movements – Most research on GPS technology has been conducted using linear running patterns consisting of moderate-long distances, with little research looking at intricate movements and random changes of direction typically seen in most sports (46). To our knowledge, only two articles have been published to date identifying the validity and reliability of intricate and sport-specific movements using GPS technology (46, 47). One study identified that 1 and 5Hz GPS units significantly underestimated total distance, and speed of sport-based movements when compared to a high-resolution camera system (47). The second also suggests low inter-unit reliability and therefore advises practitioners to take caution when using GPS to monitor distance and speed – particularly when the distance is short, and speed is high (46). Despite this, it is currently understood that GPS systems provide acceptable levels of accuracy and reliability when measuring moderate-long distances whilst running at slow-moderate speeds, but this is largely reduced when measuring higher speeds over shorter distances (47, 48). Put simply, practitioners should take caution when analysing data derived from short-distance, high-speed movements (e.g. small-sided games).
The United States Global Positioning System (GPS) is currently the Earth’s only fully functional Global Navigation Satellite System (GNSS) (49). At present, there are twenty-four GPS satellites currently orbiting Earth at approximately 11,000 nautical miles, each transmitting signals back and forth to GPS receivers – like those worn by the athletes. This transmission of information allows the satellites to determine (49):
Different GPS models possess different technology, meaning some are able to connect to more satellites and obtain stronger connections than others. Units which can connect to more satellites and with stronger connections can typically provide more accurate data. The strength of these signals can often also be affected by dense foliage and urban surroundings such as heavy tree cover, tall buildings, and stadiums (49).
For example, using GPS systems inside stadiums and indoor sport complexes can potentially reduce the number of available satellites, and therefore affect the transfer of data. This interference, therefore, affects the transmission of signals between the satellites and the GPS units, and thus results in ‘lost’ data. Though some researchers have developed software that identifies the cause for the signal loss, and provide calculations to manage it (50), to the best of our knowledge, we are unaware if:
- This calculation method is reliable in a sporting context.
- Whether any of the leading wearable technology manufactures in sport use any of this software whatsoever.
Consequently, using GPS systems in urban areas, in sport complexes, or even in areas surrounded by dense foliage, can potentially reduce the reliability of the data, suggesting significant caution should be taken if this is attempted.
Without going into too much detail, satellite geometry can also affect the accuracy of GPS positioning. This effect is called Geometric Dilution of Precision (GDOP). Typical GPS receivers usually report the quality of satellite geometry by reporting the “Position Dilution of Precision”. In certain devices, you can typically check the quality of the satellite configuration your receiver (i.e. GPS unit) is currently using by looking at the PDOP value. A low Dilution of Precision indicates better accuracy, and a high value indicates lower accuracy (34). However, to our knowledge, the commercially available GPS units used for monitoring athletic performance, do not allow coaches to determine this quality of precision – suggesting the need for this feature.
As wearables are notorious for lacking reliability during short-distance, high-speed movements (e.g. accelerations) (51), the accelerometer data is often combined with the GPS data in an attempt to improve its reliability. Yet, to our knowledge, no research has proven the validity or reliability of such a method. In fact, one study reported that the 10Hz unit was more reliable at predicting movement demands than the 15Hz models (44). Interestingly, though the authors were aware of the issue when measuring short-distance, high-intensity movements, they did not measure accelerations or decelerations. As a result, perhaps combining accelerometer data with GPS data to improve the accuracy of short-distance, high-speed movements actually has an opposite effect – meaning the reliability is reduced, and maybe it is better to just use 10Hz units.
A growing amount of research has attempted to use the tri-axial accelerometer to assess sport-specific activities (52-55). Both interestingly and importantly, one study identified that this micro-sensor is a reliable and useful tool for measuring low- and high-intensity locomotive movements in Australian Rules Football, and that this can be used to monitor loads over time (e.g. a season) (55). However, it was reported that this sensor, and variable (Player Load), fails to account for skill- and contact-based activities such as: jumping, kicking, tackling, passing, blocking and so on and therefore may underestimate actual workloads. Therefore, the “Player/Body Load” variable which appears to be derived purely from the accelerometer data (see formula below), and is often monitored by sports scientists, remains questionable as it may underestimate total workloads (33, 56).