How to Use GPS Thresholds
Welcome to the latest issue of The Applied Sport Scientist.
In this week's issue, I will be discussing the use of thresholds when using GPS devices to monitor athletes.
I hope you enjoy!
GPS Thresholds: A Brief History
The use of GPS devices is commonplace across professional team sports to monitor the external load of athletes.
GPS technology was first utilized by the Australian Institute of Sport (AIS) in a collaborative project with a start up company (who would later become Catapult Sports) in the early 2000's.
Despite 20+ years worth of use, practitioners are still confused as to how exactly to utilize the technology to monitor their athletes.
One key question that I'm often asked is "what GPS thresholds should I use to monitor my athletes?".
It is only in the past decade that we have seen a switch from the use of arbitrary GPS thresholds to a more individualized approach across various team sports.
But where did the original arbitrary thresholds come from?
In soccer, a paper published in 2009 by Paul Bradley and authors was the first to use an optical tracking system (Prozone Sports) to quantify the high speed running demands of Premier League players.
The arbitrary thresholds used in this study (19.8 km/h for high speed running and 25.1 km/h for sprinting) were then adopted as industry standard and became the default thresholds for GPS companies.
These thresholds from soccer also spread across to other sports such as rugby.
As researchers began to question the scientific rationale behind these thresholds, a new wave of individualized thresholds linked to physical measures were born.
Common physical measures include maximal sprint speed (MSS), maximal aerobic speed (MAS) and anaerobic speed reserve (ASR).
But the questions remain - are individualized thresholds 'better' to monitor team sport athletes? And if so, what threshold should you use?
Individualized GPS Thresholds: Friend or Foe?
From a scientific perspective, it makes absolute sense to base a threshold upon some form of physiological rationale.
In endurance sports, this has been done for decades, with the use of training zones based on tests such a lactate profiling and critical power/speed, being the norm.
In these sports, practitioners are concerned with the transition between zones, which typically relates to the moderate, heavy and severe exercise domains.
Whilst in endurance sports, there are specific sessions that include fluctuations between these domains (e.g. interval based), they occur less frequently compared to the intermittent nature of team sports.
For years, academics and practitioners have been trying to identify the best way to quantify this individualization and determine how such data can be useful to enhance the training practices of athletes.
Original work from Grant Abt, Ric Lovell and others in this area applied the use of laboratory-based testing through continuous, incremental treadmill testing to derive key physiological thresholds such as the 2nd ventilatory thresholds.
Whilst the validity of such approaches is very high, the practicality of testing 20+ athletes on a regular basis to derive such thresholds makes it practically improbable.
Thus, practitioners have adopted field tests to create individualized thresholds for their athletes.
Arguably, the most common method adopted in recent times is the use of MSS derived through a simple 30-40m linear sprint test to base thresholds upon. This is typically expressed as a % of an athletes individual MSS, such as 80% or 90%.
The issue with this approach is that it is based upon a singular velocity based point, which doesn't take into account other physiological factors such as aerobic capacity. There is also no physiological rationale for arbitrary % of thresholds derived from MSS.
Back in 2013, Alberto Mendez-Villanueva and the group from Aspire Academy in Qatar published a series of papers (see link for main paper) utilizing the combination of MAS and MSS testing to calculate the ASR of youth soccer players.
The authors found this approach provided an improved representation of the players external dose to soccer matches which is more fitting to physical programming.
This was further emphasized in the PhD work of John Fitzpatrick (see link to main paper), who investigated the relationship between individualized thresholds and changes in physical capacities.
The research found that individualized thresholds based upon players MAS (both time and meters covered above the thresholds) outperformed traditional arbitrary thresholds.
Practitioners can also programme HIIT type sessions based upon athletes MAS values, which gives it an advantage over other field-based aerobic tests such as the Yo-Yo.
Our recent paper as part of Tom Bennett's PhD (see link) discusses the pros and cons of the beforementioned approaches in a lot more detail and I would highly recommend giving the paper a read. Below is a figure that we created for practitioners to help understand the complexities when selecting which test/threshold to use:
Metabolic Power: The Future?
Back in 2018, I read a paper by Ted Polglaze (see link) and some of the authors of the original metabolic power methodological paper.
In this study, the authors utilized concepts taken from endurance sports through the critical speed approach to determine an individualized threshold for metabolic power.
The authors created a field-based 3 minute test and attempted to validate it against gold standard methods (i.e. time trials).
Whilst this approach has both a strong physiological rationale and is practical in nature, the authors failed to use appropriate time trials to compare and validate the 3 minute test with (they were too short in distance/duration compared to established literature in this area).
In our own lab, we tried to replicate this study using established time trial distances and replicated the same 3 minute test, finding that the test underestimated metabolic power compared to the gold standard approach (unpublished findings).
However, we do believe this approach could be a long term solution to individualization of thresholds for team sports.
For all it's negatives, metabolic power is a useful metric in that it combines both velocity and acceleration based movements within the calculation.
The 3 minute test approach has been found to be valid to determine critical power and speed in endurance sports (e.g. work of Anni Vanhatalo, link) without the need for athletes to perform multiple time trials across multiple days.
So if we can create and prove a valid 3 minute test using the metabolic power approach, this can be used in multiple ways.
It can be used to monitor an athletes load over time, with further research to establish the dose-response relationship of such an approach.
It could also potentially be used to calculate W' prime, which is essentially your 'battery' of energy above critical power.
This could be incorporated into live GPS monitoring, similar to how this metric is currently used in cycling in which W' prime is built into a cycle computer live for the cyclist to monitor their exertion.
Summary Points
Throughout this article, I've provided some critique and practical advice around using individualized GPS thresholds.
At the time of writing, we still don't have a clear gold standard approach across team sports.
My advice is as follows:
1) Choose an approach and stick with it. Changing between thresholds and methods will make longitudinal monitoring impossible.
2) Use an approach that relates to your sport/scenario. For example, if you are interested in improving your athletes aerobic capacity, it makes sense to monitor individualized MAS for this.
3) Although lacking a physiological rationale, arbitrary thresholds allows direct comparison across all players. This can be useful for benchmarking - e.g. the profile for a senior player of a certain position from match play vs. an academy player trying to break through.
4) Most GPS software's now allow for export of both arbitrary and individualized thresholds. This is easy to include in a database, even if you only use a certain approach in a specific scenario.
Thank you for reading, see you next week.
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