Our ability to acquire and retain essential motor skills for activities of daily living, sports and rehabilitation settings is based on motor learning(Schmidt et al., 2018). Motor learning begins with practice, which is fundamental to encoding sensorimotor information used to build new motor memories(Robertson et al., 2004). This learning process does not end with practice but continues “offline” during consolidation(Dudai, 2012). Consolidation is a complex, multistage process that leads to either a stabilization or even an enhancement in skill performance without additional practice, which is usually measured through retention tests(Robertson et al., 2004). Strategies to improve the consolidation of motor memories are of great interest for sports and rehabilitation.

Cardiovascular exercise (CE) is a simple but effective strategy that has been shown to enhance the consolidation of procedural motor memories(Roig et al., 2013, Wanner et al., 2020). In particular, motor skill retention can be maximized when a single bout of CE is performed immediately after skill practice(Wanner et al., 2020, Roig et al., 2016, Thomas et al., 2016, Roig et al., 2012) and at vigorous intensity(Roig et al., 2013, Thomas et al., 2016). This favourable effect on motor memory consolidation has been associated with exercise-induced increases in brain plasticity(Roig et al., 2016, Dal Maso et al., 2018, Ostadan et al., 2016, Skriver et al., 2014). Despite these positive findings, recent meta-analyses have shown that CE is not under all circumstances effective at enhancing the long-term retention of motor skills, and the magnitude of its effect is variable(Wanner et al., 2020, Roig et al., 2016). Besides exercise parameters(Roig et al., 2016), differences in participants and motor task characteristics could explain this variability(Wanner et al., 2020, Roig et al., 2016). A task-related factor that has not been investigated is the task’s declarative load(Robertson et al., 2004, Willingham et al., 2002), which determines whether motor learning takes place intentionally and can thus be verbalized (i.e., explicit) or unintentionally and without or with minimal awareness (i.e., implicit)(Robertson et al., 2004, Willingham et al., 2002, Robertson, 2007).

The Reticular-Activating Hypofrontality (RAH) model(Audiffren, 2016, Dietrich and Audiffren, 2011) provides a theoretical framework that could explain why the task’s declarative load may modulate the effects of CE on memory processes (i.e., consolidation). This model, which has exclusively been validated for cognitive functions tested concomitantly during exercise, predicts that implicit and explicit cognitive processes can be influenced by acute exercise differently because different brain regions support these two systems(Poldrack et al., 2001). The explicit system and its functions rely on prefrontal cortical regions such as the dorsolateral prefrontal cortex (DLPFC). In comparison, the implicit system and its functions are supported by subcortical structures and sensory-motor-related cortical regions such as the primary motor cortex (M1). The RAH model assumes that exercise activates various arousal systems, leading to a transient enhancement in the activity of the implicit system. Conversely, given that movement is computationally demanding and the brain has limited neural resources, the RAH model proposes that exercise can transiently impair the function of the explicit system via downregulation of prefrontal cortical areas (i.e., hypofrontality). Given that the consolidation of explicitly encoded information relies on (pre)frontal brain areas(Sami et al., 2014, Simons and Spiers, 2003); exercise, under certain circumstances, might disrupt the consolidation of explicit (motor) memories.

Hypofrontality has been shown to surface during exercise involving larger muscle groups and performed at vigorous-to-maximal intensities, particularly among unfit individuals(Audiffren, 2016). The RAH model could therefore explain some of the variability observed in the effects of CE and motor memory processes(Roig et al., 2013, Wanner et al., 2020). Exercise performed at vigorous-to-maximal intensities while facilitating implicit motor learning may be detrimental to explicit motor memory processes, especially in persons with lower cardiorespiratory fitness(Dietrich and Audiffren, 2011) who tend to experience more severe hypofrontality(Rooks et al., 2010). Nonetheless, motor learning studies testing the assumptions and predictions of the RAH model to explain the variability of CE on motor memory consolidation are still lacking.

A task that offers the possibility to investigate the effects of CE on both explicit and implicit motor skills under comparable experimental conditions is the serial reaction time task (SRTT)(Robertson, 2007). The SRTT can be learned either with (explicit) or without (implicit) awareness that the visual stimuli follow a repeating sequence. While gains in offline skill changes are observed after practicing with both the explicit and implicit variants of the SRTT, the dissociation tied to the level of awareness activates different neural circuitries(Robertson et al., 2004, Sami et al., 2014, Breton and Robertson, 2017). During the initial consolidation phase, the explicit variant of the task activates preferentially prefrontal cortical regions (e.g., DLPFC), while the implicit version engages sensory-motor areas (e.g., M1)(Ostadan et al., 2016, Sami et al., 2014, Tunovic et al., 2014). The SRTT is, therefore, well suited to test the hypothesis that CE may have divergent effects on the consolidation of explicit and implicit motor memories.

However, while different variants of the SRTT can limit explicit and implicit knowledge, there remain differences between individuals. For example, participants practicing with the implicit variant of the SRTT may become aware of the hidden motor sequence and thereby gain explicit knowledge(Tunovic et al., 2014, Cohen et al., 2005, Brown and Robertson, 2007). Similarly, individuals practising with the explicit variant may fail to develop conscious knowledge of the repeating motor sequence and learn the task implicitly(Grafton et al., 1995, Willingham and Goedert-Eschmann, 1999). Critically, while previous studies excluded participants who did or did not show conscious knowledge of the motor sequence to investigate implicit and explicit memory processes specifically, Graffon et al.(Grafton et al., 1995) utilized a different approach. The authors demonstrated that the level of awareness developed during practice, rather than the type of task, determined whether the newly acquired motor sequence engaged prefrontal cortical networks or not (i.e., explicit vs implicit learning). Together, these lines of evidence suggest that the level of awareness developed during practice, rather than the type of task per se, can determine how motor memories are consolidated(Robertson et al., 2004, Sami et al., 2014, Grafton et al., 1995).

We designed a study that used different variants of the SRTT(Ostadan et al., 2016) to test whether an acute bout of CE performed immediately after practice has differential effects on the consolidation of motor memories depending on whether they were encoded via the implicit or explicit system. Therefore, we conducted our primary analysis to investigate the effects of CE on the consolidation of the implicit and explicit variants of the SRTT. We further explored whether the level of awareness developed during sequence acquisition (i.e., implicit vs explicit learning) independently of the task’s variant used during practice influenced the effects of CE on consolidation processes. Finally, we explored whether participants’ fitness levels altered these effects in our primary and explorative analyses(Dietrich and Audiffren, 2011, Loprinzi et al., 2022). We hypothesized that CE, while facilitating implicit memory processes(Ostadan et al., 2016), would have a negative effect on the consolidation of explicit motor memory and that this effect would be accentuated in unfit individuals(Audiffren, 2016, Rooks et al., 2010, Dietrich, 2006).