Introduction

Small‐sided games (SSG) are game‐based training formats that simulate the physical, technical, and tactical demands of competitive soccer (Sarmento et al. 2018) by manipulating so called task constraints (Davids et al. 2013) possibly consisting of one or a combination of the following: player numbers (F. M. Clemente et al. 2025; Rumpf et al. 2026), pitch size (Rumpf, Jäger, Clemente, et al. 2025), and game rules, such as touch restrictions, that is, 2‐touch limitation per player possession (Coutinho et al. 2022; Rumpf, Jäger, Altmann, et al. 2025). Touch restrictions alter the physical and technical involvement of players with no clear indication on tactical variables (Giménez et al. 2018; Rumpf, Jäger, Altmann, et al. 2025). Nevertheless, effects were observed regarding decision making, player interactions with team mates (Coutinho et al. 2022), and ball circulation (Sarmento et al. 2018). Touch‐restriction constrains the range of immediate action options available to the player in possession (Jäger et al. 2026) and players may need to select subsequent actions earlier, often before or immediately upon receiving the ball (Coutinho et al. 2022; Vilar et al. 2013). Under these conditions, players must continuously monitor teammates, opponents, and ball movement increasing the perceptual‐cognitive demands such as attentional control, visual scanning, and spatial awareness (Birinci et al. 2025; González‐Fernández et al. 2026) requiring to process relevant game information and commit to decisions within limited ball‐contact opportunities (Jäger et al. 2026; Rosten et al. 2025). However, the mechanisms underlying these cognitive responses remain unclear.

Exercise‐related cognitive benefits are thought to involve multiple biological pathways, including molecular and cellular processes linked to neuroplasticity (Herold et al. 2018; Stillman et al. 2016). At the molecular level, acute exercise may upregulate neurotrophic factors (Piepmeier and Etnier 2015) and myokines (Kim et al. 2019), thereby promoting neuroplasticity and supporting cognitive functions (Basso and Suzuki 2016). Among the potential neurobiological candidates, brain‐derived neurotrophic factor (BDNF) is particularly relevant as an exercise‐responsive neurotrophin implicated in neurocognitive adaptation (Knaepen et al. 2010). BDNF supports neuronal survival and synaptic plasticity by promoting dendritic growth, synaptogenesis, and long‐term potentiation (Park and Poo 2013), thereby contributing to learning‐ and memory‐related processes across the lifespan (Erickson et al. 2011). Acute responses to exercise are generally associated with increases in circulating BDNF, although the magnitude of this response appears to vary according to exercise intensity (Birinci et al. 2026; Szuhany et al. 2015), duration (Dinoff et al. 2017), modality (Behrendt et al. 2021), and task demands (Hung et al. 2018). This task‐demand perspective is also important in soccer, where successful performance depends not only on physical and technical abilities but also on executive functions that support information processing, decision‐making, attentional control, response inhibition, and cognitive flexibility under rapidly changing game conditions (Vestberg et al. 2012).

In soccer contexts, BDNF has also been discussed as a candidate biomarker reflecting training‐related stress and potentially sport‐relevant cognitive functioning (Andrzejewski et al. 2022; Gutiérrez ‐Vargas et al. 2021). However, evidence specific to SSG remains limited, with available studies reporting no significant acute changes in BDNF after 4v4 free‐play SSG (Birinci et al. 2025) or after a soccer‐specific session combining technical drills with 5v5 SSG (Williams et al. 2020). Although the latter was not specified in greater details, the exercise protocol of Birinci et al. (2025) in the initial 4v4 free play consisted of four 4‐min bouts without any task‐constraints. Because broader evidence suggests that combining physical and cognitive stimulation may provide a stronger stimulus for cognitive adaptation than either component alone (Herold et al. 2018; Mao et al. 2024), touch restriction may represent an ecologically valid soccer‐specific strategy for embedding cognitive stimulation within physical training. Considering the well‐established involvement of BDNF in exercise‐related neuroplasticity and executive‐function processes, determining whether a rule‐induced cognitive constraint within SSG is accompanied by acute serum BDNF responses and executive‐function changes would address an important gap in understanding the neurochemical context of soccer‐specific cognitive demands. Therefore, this study aimed to clarify whether imposing a touch restriction during 4v4 SSG influences (i) serum BDNF responses, (ii) executive‐function performance, and (iii) technical‐action performance compared with free play. We hypothesized that, compared with free play, the touch‐restricted format would elicit greater perceived mental effort, a larger acute serum BDNF response, and greater improvement in executive‐function performance. Technical actions were additionally examined to determine whether the two formats produced distinct game‐behavior profiles.