Research conducted over the last 30 years has demonstrated that a variety of factors can influence motor learning and the strength of motor memories. For example, the amount and structure of practice (ie, constant or variable) as well as feedback schedule have been shown to influence motor skill learning in healthy populations.1 In an effort to maximize patient outcomes, well-established principles of motor learning are commonly incorporated into therapeutic interventions in neurologic clinical practice. Yet, minimal work has been done to determine (1) how well various principles of motor learning translate into neurologic populations, and (2) whether they apply broadly across different learning mechanisms.

In this issue of the Journal of Neurologic Physical Therapy (JNPT), Helm and colleagues2 begin to address this knowledge gap by investigating how practice structure (in this case, variable vs constant practice) affects retention of a novel gait pattern learned through split-belt treadmill adaptation in persons after stroke. Individuals with chronic stroke learned to walk on a split-belt treadmill in either constant (2:1 speed ratio for 15 minutes) or variable (switching between 2:1, 1.5:1, and 2.5:1 speed ratios for 15 minutes in total) belt speed conditions. On a second day, participants again walked on the split-belt treadmill while the belts moved at a 2:1 speed ratio. Based on existing literature, the authors hypothesized that variable versus constant practice conditions would lead to better retention of the newly learned walking pattern on the second day. Instead, they observed a surprising result: both groups exhibited similar retention. These results reveal that the benefit in retention typically achieved from variable practice does not appear to apply to locomotor adaptation, a specific motor learning mechanism that has gained considerable attention in recent years for its clinical utility to address gait deviations after stroke.3

These new findings pose several questions regarding the role of variable practice in motor learning-specifically, locomotor adaptation. Although these results demonstrate little benefit of a specific variable practice paradigm, as the authors suggest, it is possible that variable practice delivered differently may still have some benefit. For example, the magnitude of the belt speed ratio variation between practice conditions may need to be larger (eg, switching between tied- and split-belt walking conditions4) than was provided here to augment retention of a novel walking pattern learned through adaptation. It is also possible that varying the spilt-belt walking parameters may facilitate retention of locomotor adaption. However, recent work in individuals who are neurologically intact demonstrated that the belt speed ratio is the key split-belt parameter that elicits transfer of a motor memory to new, unpracticed walking conditions.5 Therefore, focusing on changes of the belt speed ratio to vary practice, as done by Helm and colleagues,2 as opposed to another split-belt parameter (ie, absolute belt speed difference) is likely to be the most successful approach. More research is necessary to delineate the parameters of variable practice that will optimize retention of locomotor adaptation after stroke before we can determine potential benefits of incorporating it into clinical practice.

It is also important to consider that variable practice may be beneficial in facilitating other phenomena of motor learning beyond retention. For example, variable adaptive practice has been shown to lead to improvements in balance control6 and better generalization to overground walking.7 An important caveat is that these findings were observed in individuals who are neurologically intact; further research is needed to understand how these principles generalize to neurologic populations.

The authors also speculate that, in neurologic populations, patient-specific factors or characteristics may influence retention following variable practice. For instance, lesion location may have an impact, as the neural substrates engaged during learning depend on the practice structure.8,9 Furthermore, individuals with cognitive impairments have long been known to benefit more from constant practice.10 This suggests that cognitive function may be important to consider when determining whether it is appropriate to incorporate variable practice into an intervention for individuals after stroke.

Therapists currently work within a climate characterized by diminishing levels of payer coverage for rehabilitation services and increasing emphasis on value-based health care. Accordingly, we must focus on providing interventions that evidence suggests will confer the greatest benefit to patients. This has led to the implementation of well-established principles of motor learning in therapeutic interventions within neurologic clinical practice. However, the results presented by Helm and colleagues2 suggest that certain motor learning principles may not directly translate to neurologic populations or apply broadly to different learning mechanisms. From this work, it is clear that this line of inquiry deserves further investigation, so that we can broaden our understanding of how to leverage these concepts/principles in clinical practice in an effective, efficient manner.

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