Is illness important in professional soccer? An evaluation of incidence, risk factors and intervention.

Abstract

Background - Previous research has demonstrated that illness is not a major problem within professional soccer. However, this research did not record illness where performance is restricted or medical attention is given, instead focussing only on illness where time is lost from soccer activities. Therefore, the aim of the present thesis was to establish the importance of illness in professional soccer by evaluating illness incidence, proposed risk factors and an illness prevention intervention. Methods - Illness incidence was recorded from 1 professional soccer team (59 different players) across 3 seasons (2016-17 - 2018-19), using a system that recorded all illness definitions and a questionnaire to quantify performance-restriction illness. Illnesses were confirmed via physician diagnosis. During the congested fixture period of the 2017-18 season, illness incidence was compared to a recreationally active comparator population from a university institution. Physical load data (via microelectromechanical system and heart rate monitoring) and subjective wellbeing data (via a 1-5 Likert scale assessing fatigue, sleep quality, general muscle soreness, stress, mood and sleep hours) was also collected across this time period. 7 and 28-day average values for physical load and subjective wellbeing variables, prior to illness events, were compared to averages (indicative of normality) across the same time periods, using a paired samples t-test. In the 2018-19 season an illness prevention intervention was developed and implemented across 4 months (November - February). Illness incidence in this season was compared to the 2 previous seasons using a repeated measures analysis of variance (RM-ANOVA). Outcome measures for intervention evaluation assessed the reasons behind intervention effectiveness. Results - Using 2 seasons worth of data, chapter 3 demonstrated that illness incidence was greater than training injury incidence (91 vs 17 incidences) and greater than values reported in previous research (91 vs 46 incidences). Illness incidence was also greater in the soccer team compared to the recreationally active comparator group (15 vs 10 incidences). Temporal patterns showed that peaks in illness incidence were distributed throughout the 2 seasons, not just in the winter months that coincide with congested fixture scheduling (10 incidences in July, 8 in September, 6 in October, 7 in November and 10 in January). Chapter 4 showed that, prior to illness events, there was an increase in 7-day average values for training impulse per minute (0.4±0.4 vs 0.6±0.5, p=<0.01) and time spent above 85% of maximum heart rate (2.3±1.8 vs 2.8±2.2, p=0.02) (markers of internal physical load), whilst maximum velocity was reduced (4.1±0.3 vs 3.7±1.0, p=0.03) (external load), compared to normality. In the 28 days preceding illness events there also appeared to be a reduction in sleep quality (3.8±0.3 vs 3.7±0.4, p=0.01) compared to normality. Chapter 5 indicates that the intervention did not reduce illness incidence in comparison to previous seasons. A RM-ANOVA determined that there were significant differences in 1 illness incidence variable between seasons (F (2, 11) = 17.581, p = 0.001). Post hoc comparisons showed an increased total illness incidence per 1000 hours in the 2017-18 season (20.2 ± 9.2) compared to the 2016-17 (7.1 ± 9.4, p = 0.004) and 2018-19 seasons (9.2 ± 7.5, p = 0.015). There were no other significant differences between seasons. Evaluation revealed that the intervention appeared to be successful in improving awareness of illness prevention, but did not alter aspects of behaviour. Conclusions - Illness does appear to be a problem within professional soccer. This has implications towards training and match availability, performance, team success and therefore club finances. Findings suggest that illness is related to physical load and other risk factors within this population. Further exploration of these factors within this environment is required. Changes in the identified markers physical load and subjective wellbeing may identify players who are at risk of illness and allow intervention where appropriate. The illness prevention intervention did not reduce illness in comparison to previous seasons. The limited impact may have been due to increased competition demands during the 2018-19 season, elevated illness reporting due to the intervention itself and a lack of focus on influencing behaviour. Illness surveillance and prevention should be a future focus within professional soccer.