Acute cardiovascular exercise does not enhance locomotor learning in people with stroke

Converging evidence indicates that the positive effects that cardiovascular exercise has shown to have on memory may be regulated in a time-dependent manner (Roig et al. 2016). These positive effects have been demonstrated in different types of memory, including motor memories. For example, a single bout of intense exercise performed in close temporal proximity to the practice of a novel motor task enhances the long-term retention of the motor skill (Roig et al. 2012). The transient changes in brain plasticity triggered by exercise in cortico-motor brain networks are thought to explain, at least in part, the enhancing effects that this intervention has on the neural mechanisms underlying motor memory formation processes (Ostadan et al. 2016). The majority of studies supporting the positive effects of a single bout of cardiovascular exercise on motor memory, however, have used upper extremity visuo-motor tracking tasks requiring isolated joint movements and the application of low isometric forces (Roig et al. 2016). The potential benefits of this type of exercise on other, more complex motor tasks, involving multi-joint movements of lower limbs, such as those required for locomotion, are less well established (Helm et al. 2017). Furthermore, most previous studies demonstrating that acute exercise can improve motor memory and skill learning have involved young, healthy individuals. Therefore, the transferability of these findings into clinical practice is still to be established. Using an upper-limb visuo-motor task that required the accurate modulation of isometric grip force to maintain a moving cursor on different targets, Nepveu et al. (2017) showed, for the first time, that 15 min of high intensity interval training performed immediately after motor practice improved skill retention also in people with chronic stroke. Compared to a non-exercise group, participants who performed the bout of exercise displayed a better retention of the motor skill 24 h after motor practice. This seminal finding opened the exciting possibility that the time-dependent effects of memory shown in previous investigations involving non-disabled young individuals could be used as a rehabilitation tool to improve motor learning also in people with neurological impairments. The findings of a study in this issue of The Journal of Physiology, however, appear to temper this possibility. Charalambous et al. (2018) investigated whether a single bout of exercise could improve the retention of a novel locomotor task in people with chronic stroke. The potential mediating effects of exercise intensity and timing were also investigated. Participants were allocated into one of a treadmill walking, a total body recumbent cycling or an active control group and performed 5 min of cardiovascular exercise performed at low or high intensity and also before or after practicing a locomotor task. The locomotor task consisted of walking on a split-treadmill at a 2:1 speed ratio (100% and 50% fast-comfortable walking speed) for 15 min. A retention test of the task was performed 24 h after motor practice. In contrast to what the study by Nepveu et al. (2017) had shown previously, in this study, acute cardiovascular exercise did not improve the retention of the locomotor task. Furthermore, neither exercise intensity nor timing appeared to affect the response to exercise. In light of these results, Charalambous et al. (2018) concluded that the effects that acute exercise has been shown to have on motor memory might be task-specific. When complex locomotor tasks are involved, acute exercise does not appear to enhance retention. This new finding, which provides important novel insights into the role of acute exercise to enhance motor learning in people post-stroke, illustrates the complexity of translating research evidence into clinical practice. Given the differences in the intensity and duration of the exercise protocols, the specific reasons for the discrepancies between the study by Charalambous et al. (2018) and Nepveu et al. (2017) are difficult to ascertain. It is possible, however, that the positive priming effects of acute exercise on the brain could be circumscribed only within a specific group of networks (Dal Maso et al. 2018) involved in the consolidation of motor memories developed during the practice of some very specific types of tasks (e.g. visuo-motor tracking) and not others (e.g. sensorimotor adaptation). This would explain the divergent results between the two studies and why one specific type of motor learning appeared to be more responsive to the effects of acute cardiovascular exercise on motor memory and skill retention. The study by Charalambous et al. (2018) also explored the potential role of brain-derived neurotrophic factor (BDNF) genotype in mediating the response to acute exercise on the capacity to retain the locomotor task. BDNF is a neurotrophin that regulates rehabilitation-induced recovery after stroke (Ploughman et al. 2009). Carrying the Val66Met polymorphism of the BDNF gene tends to reduce brain plasticity and hampers post-stroke recovery (Di Lazzaro et al. 2015). Their results indicated that individuals carrying the Val66Met polymorphism of BDNF showed a lower capacity to relearn the locomotor task during the retention test but that cardiovascular exercise ameliorated this effect. Larger studies will be needed to confirm whether the Val66Met, or other polymorphisms (Mang et al. 2017), explain the variability of the response to acute cardiovascular exercise in relation to motor learning. None declared. Both authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

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