Currently, the Federation International of Football Association (FIFA) is assessing the safety and performance of Electronic Performance and Tracking Systems (EPTS). Regarding the accuracy of EPTS, the Performance Standard Test (FIFA Quality®) only evaluates the position and the velocity of tracking systems through optical- based systems, GNSS or LPS. For monitoring external workload, the ABELIs are becoming a widely used method because their reliability, precision and sensitivity are greater compared to other automatic and semiautomatic time- motion analysis (TMA) technologies such as optical-based systems, GNSS or LPS (Dalen et al., 2016; Fox et al., 2017). Instead, only four companies (Catapult Sports, GPSports, Mediatronic and RealTrack Systems) have published scientific papers about the validity and reliability of their accelerometers. From these publications, a proposal to evaluate the reliability and validity of accelerometers objectively is presented in Table 5.2.
Table 5.2 Proposal to evaluate the reliability and validity of accelerometry-based data
Evaluation Test Conditions Specific requirements Protocol Assessment
Laboratory Static Without stress Static device Place the device
static and in a flat zone for 3 minutes recording 1g in the vectorial sum of 3-axis accelerometers.
During 1 minute, each axis must be affected by gravity.
Reliability Intra-unit:
Repeat the test 3 times with the same device.
Inter-unit:
Repeat the test 3 times with two or more devices simultaneously.
Validity:
Assess if the device detects 1g in static trials
or Xg in
dynamic trials.
With stress A stress period of 5 minutes is needed (e.g. hand-shaking movements).
Dynamic Very low
impact
Generate a 0.5-g impact Place the device in a calibrated hydraulic shaker or criterion- reference accelerometer Common
impact
Generate a 3-g impact High impact Generate a 5-g impact Severe impact Generate a 10-g impact
Field Linear
running Incremental treadmill running
Place the device in the
anatomical body location that is to be measured
Realize an incremental protocol from walking speed 5 km/h to maximal sprinting (>25 km/h)
Reliability Inter-unit:
Repeat the test 3 times with two or more devices simultaneously in the same athlete at the same anatomical location.
Validity:
Concurrent:
Compare the accelerations respect with a gold standard device (e.g.
VICON).
Convergent:
Correlate the accelerations respect with to internal workload indexes as maximal oxygen consumption, heart rate, or muscle oxygen saturation among others.
Specific sport movements
Field test Place the device in the
anatomical body location that is to be measured
Design or use a previously validated protocol that includes linear running, accelerations, decelerations, change of direction and jumps (e.g.
SAFT90).
Small-sided game
Design a specific reduced situation considering the following aspects: 50% of players in the real situation, 25% of total match duration, use the same related area per player than in competition, and maintain goal-scoring (e.g. soccer: 22’ 5vs5 in half- pitch).
To evaluate the reliability and validity of accelerometry-based data, the reliability and concurrent validity should be calculated by the coefficient of variation (CV), percentage of differences (%diff), intraclass correlation
coefficient (ICC), standard error of measurement (SEM) or the standard estimate of error (SEE), and convergent validity by Pearson or Spearman correlation coefficients. To interpret reliability results, previous recommendations showed the following rating: (a) good (>5%), (b) moderate (5–10%) or (c) poor (>10%) (Hopkins et al., 2009); but no recommendations have been realized for validity in sport science, so previous research adapted the recommendation for reliability on validity analysis for consistency and congruency (Scott et al., 2016).
Lessons Learned and Concluding Remarks
From the information provided in this chapter, different recommendations are given for the use of microsensors for load monitoring in sport:
Accelerometers, gyroscopes and magnetometers are the most used microsensors in sport science. However, accelerometers have had most development for the quantification of workload in team sports. Its precision, validity and reliability are greater than other tracking technologies and it provides the total load of the athlete’s actions with and without movement.
The combination between the signal of microsensors allows improving of the accuracy of each of them and the calculation of new variables for external workload monitoring in team sports such as changes of direction, centripetal force or body orientation during displacements.
Different technical features such as attachment, body location, sampling frequency, number of accelerometers, measurement range, working temperature, measurement quality, previous calibration and data processing should be considered to obtain the highest data quality during registers.
The analysis of the reliability and validity of microsensors that composed the IMUs is fundamental to be able to compare data between subjects or sessions as well as provide the correct workload to achieve the correct adaptations according to the training plan.
Since there is no consensus in the scientific literature regarding tests for the evaluation of the validity and reliability of accelerometers, a proposal is realized for their evaluation in laboratory and field tests.
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