Discusión de los resultados 110

GPS devices have dramatically shrunk over their short lifespan and are now as small as a mobile phone and weigh less than 100 g. In built into these devices are not only GPS transmitters and memory hardware but accelerometers, gyroscopes and magnetometers. These pieces of technology provide a greater depth of information for sports practitioners to assess the physical movements of athletes during their training and in competition. The main benefit of microtechnology is the higher sampling rates in comparison to GPS transmitters – the highest available GPS sampling rate is 10 Hz compared to the basic sampling rate of accelerometers 100 Hz.

1.4.1 Validity, Specificity, Reproducibility and Reliability

The microtechnology available allows accelerations, decelerations and mechanical load to be quantified alongside distance and speed during competition and training. Catapult MinimaxX systems report total external load (Player Load) by using an algorithm which combines accelerations in all planes (further explained in Chapter 2.4.1.3). This parameter is suggested by the manufacturers to display similar trends to total locomotor distance from GPS between playing positions. Further to this the Player LoadSLOW parameter (all accelerations under 2 m·s-2) is suggested to be a

parameter for displaying information on small, low speed movements such as lunging or changing direction (Catapult Innovations Help Document, 2013).

The validity of accelerometer data has been reported in a small number of studies within the literature. Akenhead and colleagues (2013) assessed the accuracy of acceleration data in comparison to laser timing. The GPS unit was attached to a

93 | P a g e monorail and towed behind an athlete. As the unit was attached to a monorail acceleration was restricted to one plane. SEE of the accelerations were low although increased when above 4 m·s-2 (0.12 ± 0.02 to 0.32 ± 0.06, 0 - 1 m·s-2 and > 4m·s-2 respectively). Overall SEE was 0.19 ± 0.01 for all trials. The authors concluded that accelerometer accuracy is good up to 4 m.s-2 but anything over this and the accuracy of findings is reduced. In comparison to GPS accuracy the accuracy of accelerometer data is far superior. However similar to the findings in GPS the validity is very specific and not in a competitive environment.

However, Gabbett (2010c, 2011 and 2013) reported on the validity of microtechnology to measure the number and magnitude of impact during collision sports such as rugby league. One of these studies, in rugby league skills training, a positive relationship was shown between the number of collisions recorded by Catapult MinimaxX devices and by video analysis (Gabbett, 2010a). This was during drills which were focused on improving skills such as passing, catching, tackling, support play, defensive skills and ball control. However it is not reported if the outputs of MinimaxX units and video outputs were similar during small-sided games or competition. Both of these situations are more random than skill practice and may be more susceptible to inaccuracies such as opposition players pulling the shirt of the player making the unit experience an acceleration which would not be classed as a contact when viewed using video analysis.

In a review on the subject Gabbett (2013) discussed the ability of current systems to measure impact loads. It was reported that head mounted accelerometers in the NFL

94 | P a g e which measure at a greater frequency than those used in the GPS-accelerometer combined units (MinimaxX and GPSports) had been validated and shown to be acceptable for use in this environment. However he reported that only the MinimaxX unit had been validated in the other systems - this study also conducted by Gabbett (2010a) is discussed above. In the review it was concluded that only the Catapult MinimaxX devices, and not GPSports devices, are capable of giving a valid measurement of impact loads during contact sports.

Boyd and colleagues (2011) assessed the reliability of a GPS device with in built microtechnology – the Catapult MinimaxX 5 Hz device. The validity of the devices was assessed in a laboratory setting as well as during Australian rules football games. Lab based testing involved static and dynamic testing. Static testing – units secured in a vertical position using a specifically designed cradle on a level surface - was conducted on 10 units, each of which were recorded during 6 periods of 30 seconds with 2 minutes. Dynamic testing involved 8 units being taken through a protocol consisting of a range of accelerations on a hydraulic shaker. The results of the study showed both static and dynamic CV of between 0.91 to 1.10 %. The reliability during sport specific games was similar (1.94 %).

This study is one of the first to attempt to describe the reliability of accelerometers during competition. By attaching the two units together during games the between- unit reliability was assessed. This means that the inaccuracy of each unit compounds the reliability results. However in this case it was reported that the between-device CV was less than the within-device reliability (1.05 % versus 1.02 %, within-device

95 | P a g e and between-device respectively) during the dynamic laboratory test. This study is limited by the size of the sample used – 8 units tested during static testing, 10 during dynamic testing and on 10 players during competitive games. The authors allude to the need for test within specific populations when determining the validity and reliability of the accelerometer data (Boyd, Ball and Aughey, 2011). This concurs with the previous assertions made that GPS validity must be considered within the specificities of the situation with which it is used. The literature perhaps more strongly evidences the reliability of accelerometer data in comparison to GPS data however both technologies require more specificity and as such it may be more appropriate to utilise mean values rather than those of individuals.

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