Group findings of load and recovery in soccer are not generalizable to individual players

The process of load and recovery is a key concept to optimize soccer performance and well-being. To date, studies are typically conducted at the group level, which can pose limits on applying research findings to individual athletes (Davids et al., 2003; Glazier & Mehdizadeh, 2019; Hill et al., 2020). Indeed, researchers in psychology have demonstrated that group-level statistics are often not generalizable to processes at the individual level, which is called the “ergodicity problem” (Fisher et al., 2018; Mangalam & Kelty-Stephen, 2021; Molenaar & Campbell, 2009). The aim of the present research was to investigate this ergodicity problem in the study of load and recovery in soccer. We collected internal training load (RPE*training duration) and recovery (TQR) data on a daily basis across two seasons among 82 youth male players of a professional soccer club. Next, we calculated the univariate distributions and bivariate correlations on both the group and the individual level. Group- and individual-level analysis resulted in different statistical outcomes, particularly with regard to load. Standard deviations of individuals were up to 7.63 times larger than standard deviations of the group. In addition, at either level, we observed different correlations between load and recovery. The results thus suggest that the process of load and recovery in athletes is nonergodic. Implications of these results are that recommendations for training programs of individual athletes may be suboptimal, or even erroneous, when guided by group-level outcomes. We therefore propose that researchers consider shifting their focus to the individual level when studying load and recovery in the soccer field.