Choosing The Right GPS Metrics
Welcome to the latest issue of The Football Scientist.
In this week's issue, I'm going to discuss key considerations when selecting which GPS metrics to monitor your athletes.
Enjoy!
Why are there so many GPS metrics?
When I first started using Global Positioning System (GPS) devices back in 2009, there were a small number of metrics that you could use with some sort of confidence. The hardware and software really wasn't at the level we take for granted in the current day.
In addition, there were only a handful of GPS companies providing devices in sport. Fast forward over 10 years and we now have a very competitive market in which each company is trying to outdo the other.
One of the main ways they compete is by releasing novel metrics each year to provide something new and interesting to the user. This has resulted in over 400+ GPS metrics (including derivatives) being available for the practitioner to choose from - an absolute nightmare!
What factors to consider when choosing the right metrics When choosing the right GPS metrics to use with your players, I would suggest asking the following 3 key questions: 1. Does the metric have acceptable levels of reliability/validity? 2. Is the metric clearly linked to the sporting demands? 3. Can the metric be easily interpreted by the key stakeholders? Here I will break down how to answer each of those questions in more detail:
Reliability and Validity
It is important not just with GPS metrics but also any measure we use in applied sport science that it has acceptable levels of reliability and validity. As we are not collecting data in a tightly controlled, lab-setting then there needs to be some flexibility to this.
Reliability is defined as how repeatable a measurement tool is. This should be assessed based on how you will use the measure. In our case, the majority of the time this will be day to day, so reliability needs to be assessed from, for example, day 1 to day 2.
Validity is defined as something that measures what it should measure. In athlete monitoring, we are often concerned about ‘criterion validity’ which is how your measure is compared to the gold standard. We are also concerned about ‘ecological validity’, which refers to how well the monitoring tool can be applied to real-life settings.
It's important that you know the reliability and validity for the specific metric and device you are using. For example, total distance derived from a 5-Hz device is not the same as from a 20-Hz device, even though technically it has the same name.
I would recommend firstly checking the published literature to see what has been done. If reliability/validity data is not available, then you may need to do some in-house testing yourself to assess.
Does it link to the sporting demands?
This point should be pretty self explanatory, but it's where some go wrong when selecting their GPS metrics.
When we monitor athletes, we want to understand the link between training load, adaptation and injury. We are always trying to make our players better (if we can) or at least maintain what they have.
This is where a lot of new GPS metrics fall down. They look and sound great in principle, but when you do a detailed analysis linking to performance and injury data, they come up short.
Below is an image created from a systematic review by Jaspers et al. (2017) that demonstrates just how few metrics are linked to changes in physical performance.
Can it be easily interpreted by the key stakeholders?
Again this sounds quite obvious, but it's something that some GPS metrics are lacking.
An example of this is when metabolic power metrics were first released to consumers. When I asked the typical practitioner to explain to me what met power was, they couldn't properly. So then how are they supposed to explain to coaches and players, the ones that actually matter?
Using metrics that can be easily understood makes sense on all levels. A coach understands total distance, they understand accelerations/decelerations and sprint distance covered. It's when newer metrics which have limited practical application cause confusion and potentially lead to reduced buy in from the key stakeholders.
Summary
Below is a figure from a paper we published back in 2020 in Science and Medicine in Football that provides practitioners with a decision making framework when selecting the most appropriate GPS metrics.
Thank you for reading, see you next week.
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