3 55 MACHINE LEARNING SUPPORTING SUBSTITUTIONS IN SOCCER 2018-2019 season. For our analysis, two matches with erroneous and missing data were excluded. Also, the extra time at the end of the first and second half and goalkeepers were excluded from the dataset. The effect of substitution on the match was controlled by identifying both entire match players and substitutes. Thus, entire match players played the full match where the substitutes entered the match at a later stage. 2.2 Subjects Four hundred and eighty players participated in the 300 matches. Four thousand nine hundred thirty-five times, entire match players were identified. In addition, 1533 substitutes were identified. The majority of substitutions happened at half-time (50-minute mark) and between the 60- and 90-minute marks (Figure 2). The number of substitutions in the first half and the 55-minute mark is significantly lower (P < 0.001) compared to the second half and between the 60- and 90-minute marks. The Ethics Committee CTc UMCG, of the University Medical Center Groningen, The Netherlands, approved the study, approval number: 201800430. 2.3 Data The sample includes tracking data of all players of 302 matches. The players’ time, position, speed, and acceleration were detected and recorded by SportsVU optical tracking system (SportsVU, STATS LLC, Chicago, IL, USA). Linke et al. (2018) tested the SportsVU optical tracking system and rated the system as being adequately reliable [18]. 2.4 Variables The type-1 variables distance covered, distance in speed category, and the type-2 variable energy expenditure in power category were applied to examine the decline in physical performance [14]. The variables were calculated as (i) distance covered per five minutes, 15 minutes, half and entire match [15], [19], [20] (ii) distance in speed category per five minutes, 15 minutes, half, and entire match: the speed categories were categorized as Very Low Intensity Running (VLIR; 0.7–7.2 km·h-1), Low Intensity Running (LIR; 7.2–14.4 km·h-1), Medium Intensity Running (MIR; 14.4–19.8 km·h-1), High Intensity Running (HIR; 19.8–25.1 km∙h-1), and Very High Intensity Running (VHIR; >25.2 km∙h-1 ) [15], [21] (iii) energy expenditure in power category per five minutes,15 minutes, half and entire match, calculated conform Osgnach et al. [22]. The power categories were categorized as Low Power (LP; from 0 to 10 W*kg-1), Intermediate Power (IP; from 10 to 20 W*kg-1), High Power (HP; from 20 to 35 W*kg-1), Elevated Power (EP; from 35 to 55 W*kg-1), and Maximal Power (MP; >55 W*kg-1) [22]. The descriptive statistics of the variables were calculated for entire match players and substitutes and reported as mean ± standard deviation for each variable. The difference between entire match players and substitutes was reported for all variables as well.
RkJQdWJsaXNoZXIy MjY0ODMw