Team Solutions for GPS monitoring in sports first became available approximately ~15 years ago and have evolved rapidly in recent seasons. Nowadays, it has become almost inconceivable that a professional outfit does not possess “a set of units” to monitor distances travelled at various speeds, despite the technology’s hefty expense, and are deploying them on a daily basis…
Consequently, the commercial side of GPS solutions for Sports has grown rapidly (currently dominated by 2-4 companies worldwide) and the ‘elite team sports wearables’ industry has boomed as the impressive client lists and key testimonials have grown, with GPS companies competing fiercely for client signatures such as FC Barcelona, CF Real Madrid, AC Milan and Manchester United to show to the rest of the world that they have the most superior system. With this kind of marketing power, the Sports Scientist who might stop to consider and ask questions about the service (such as validity & reliability of the technology, actual impact on performance and return on investment) may just find themselves criticised by coaches and players for not even knowing their own field of science and if it’s good enough for Clubs X, Y & Z to be using GPS, then so must we…
However, with this challenge in mind, I’m going to take a closer review at a particular paper published back in July 2013 in the International Journal of Sports Physiology and Performance entitled “Monitoring Accelerations With GPS in Football: Time to Slow Down” by Martin Buchheit and colleagues. Firstly, it needs to be noted that there have only been a small handful of independent attempts to validate GPS measurements (from different, but not all, brands) as well as investigations on within-unit reliability (see Petersen et al., 2009; Coutts & Duffield, 2010; and Varley et al., 2012 as good examples). Overall, the studies have shown “acceptable levels of accuracy” (mind you, it depends on what variable you decide to choose to determine as being accurate and how long you decide to measure it for!), with a consistent acknowledgement that as player speed increases, error measurement increases along with it. In this regard, we’ve also seen large differences in direct measurement comparisons between brands.
The aim of Martin’s study was to 1) examine the magnitude of between-GPS model differences in commonly reported running-based measures in football, 2) examine between-unit variability and 3) assess the effect of software updates on these measures. Fifty same-brand GPS units (although 15 was of one model type and 35 of another model type) were attached to a custom-made plastic sled towed by a player performing simulated match running activities. GPS data collected during training sessions over 4 weeks from 4 professional football players (n = 53 files) were also analysed before and after 2 manufacturer-supplied software updates.
There were substantial differences between the different models (e.g., standardized difference for the number of acceleration >4 m.s-2 = 2.1; 90% confidence limits (1.4, 2.7), with 100% chance of a true difference). Between-unit variations ranged from 1% (maximal speed) to 56% (number of deceleration >4 m.s-2). Some GPS units measured 2 to 6 times more acceleration/deceleration occurrences than others. Software updates did not substantially affect the distance covered at different speeds or peak speed reached, but one of the updates led to large and small decreases in the occurrence of accelerations (-1.24;-1.32,-1.15) and decelerations (-0.45; -0.48,-0.41), respectively. Martin’s team concluded that practitioners should be advised to apply care when comparing data collected with different models or units, or when updating their software. The metrics of accelerations and decelerations show the most variability in GPS monitoring and must be interpreted cautiously.
Despite these impacting findings we’ve just been reading, there’s also many obvious issues associated with this study to consider as well. For example, we do not know what effects the unit proximity (50 units on one sled) had on the measurements and we must also remember that this is a one-time study in isolation, with a particular set of 50 units (2 different models and therefore ages) in a particular part of the world at a particular time of the year. In other words, although the findings are indeed very interesting and should be noted, they are far from being definitive right across the board. In addition, these studies quickly become outdated as the GPS Manufacturers seek get back ahead of the academic research by releasing “new & improved” models of their products, or a latest firmware update for their units, often around the same time as the scientific research may be exposing any of their weaknesses and the investigation process needs to start again.
Nick & I often discussed how nervous we got when the GPS providers were releasing an upgrade to firmware, software or hardware (particularly mid-season or mid-tournament) when we had a level of confidence and contentment in the particular set of units at our disposals. However, we still both appreciated that the product needed to be allowed to continue to evolve, but always fearing that a feature we were both using, or a measurement variable that we had become content with, would suddenly be altered because of requests made by other practitioners, in other sports, in other parts of the world.
Even with scientific studies such as the one I’ve highlighted today have provided seemingly damaging conclusions for GPS technology, the fact remains that nowadays a serious lack of attention is being provided to scientific rigour of this technology in favour of strong marketing campaigns, client lists & key testimonials. In fact, it has reached an extent whereby some of the main brands have seemingly made absolutely no attempt whatsoever to have their technology independently validated & openly published. Oddly though, despite this lack of transparency, it has not hindered their growing volume of sales to the Sports Science Community or the prevention of Sports Science Journal Referees from rejecting GPS based studies within the academic press.
To conclude, I’d like to recommend just two things to practitioners. 1) If you acquire a set of GPS units, it should be your own responsibility to conduct the validation & reliability experiments necessary on the particular set of equipment you’ve been provided with & to periodically repeat this experiment to continually examine the lifespan of the equipment.
In other words, we are scientists, so let’s continue to ask questions and keep seeking answers.
2) Think cautiously before updating software or firmware, or changing brands, particularly mid-season and demand a full technical report and explanation of any changes made to their programme before running any, and then apply all these changes to all of your historical data as well in order to update your entire database.
In my next Blog, I will discuss my views on how best to test the validity and reliability of your own GPS units…
1. Aughey RJ. Applications of GPS technologies to field sports. Int J Sports Physiol Perform. 2011;6:295-310.
2. Coutts AJ and Duffield R. Validity and reliability of GPS devices for measuring movement demands of team sports. J Sci Med Sport. 2010;13:133-135.
3. Varley MC, Fairweather IH, and Aughey RJ. Validity and reliability of GPS for measuring instantaneous velocity during acceleration, deceleration, and constant motion. J Sports Sci. 2012;30:121-127.
4. Petersen C, Pyne D, Portus M, and Dawson B. Validity and reliability of GPS units to monitor cricket-specific movement patterns. Int J Sports Physiol Perform. 2009;4:381-393.
5. Hopkins WG, Marshall SW, Batterham AM, and Hanin J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sports Exerc. 2009;41:3-13.
6. Mendez-Villanueva A, Buchheit M, Simpson B, Peltola E, and Bourdon P. Does on-field sprinting performance in young soccer players depend on how fast they can run or how fast they do run? J Strength Cond Res. 2011;25:2634-2638.