Given that most activities of daily living require the use of both hands, it is essential that physical therapy interventions target impairments in bimanual coordination after stroke. In healthy individuals, corticospinal tract excitability of a resting arm is enhanced by contraction of the contralateral limb muscles. This mechanism is known as "crossed facilitation" and has been shown to underlie interlimb coordination during bimanual movement. Although the loss or reduction in crossed facilitation has been linked with disruption in bimanual coordination after stroke, the functional relevance of this loss has received little attention. Therefore, to maximize functional recovery of bimanual tasks in a rehabilitation setting, it is essential to understand first, whether the extent of crossed facilitation is associated with affected upper extremity function, and second, which patients may benefit from interventions that promote crossed facilitation.

In this issue of Journal of Neurologic Physical Therapy, Yang et al1 sought to understand whether excitability of the corticospinal tract can be enhanced through unaffected limb contraction, and whether the degree of crossed facilitation relates to motor function of the affected hand in individuals with stroke. Using transcranial magnetic stimulation (TMS), the authors attempted to elicit motor-evoked potentials (MEPs) to assess corticospinal tract excitability from the affected hand under 3 conditions: rest, contraction of the unaffected abductor pollicis brevis, and contraction of the unaffected tibialis anterior (TA). Motor-evoked potentials were assessed by applying TMS to the affected primary motor cortex and by using electromyography to measure associated muscle activation. Results suggested that the presence of crossed facilitation was associated with improved motor status compared with those without evidence of crossed facilitation, as measured by the Fugl-Meyer assessment of the upper extremity (FMA-UE) function. Moreover, crossed facilitation was enhanced by tonic contraction of muscles of the unaffected arm and leg. Interestingly, crossed facilitation was observed even in some individuals who did not demonstrate an MEP with the affected limb at rest.

These findings have an important clinical value in supporting bimanual therapy, especially in lower functioning individuals. Typically, individuals who do not demonstrate MEPs with the limb at rest have been predicted to recover to a lesser extent than those in whom it was not possible to evoke MEPs.2 However, findings of MEPs in the affected hand during contraction of the unaffected limb shed light on the importance of bimanual task practice. Such movements can be categorized as in-phase (ie, mirrored), anti-phase (ie, opposite), or complementary movements.3 The latter of these movement patterns, in which one hand is used as a stabilizer and the other as a manipulator, is particularly relevant here. Given that excitability of the lesioned hemisphere is increased during a tonic, high-strength contraction of the unaffected hand, bimanual training can be designed as a functional extension to this neurophysiological occurrence. For example, use of the hands in their prestroke role (ie, stabilizer or manipulator) could help optimize motor output of the affected hand. As the stabilizing hand most often performs an isometric contraction, manipulative skills of the affected hand may be enhanced through crossed facilitation to the motor-dominant hemisphere. Moreover, evidence of crossed facilitation during strong tonic contraction of the unaffected TA may suggest the importance of whole body activities to facilitate functional improvements of the upper extremities during either bimanual or unimanual affected arm task practice.

The findings presented in this study show a possible extension of the clinical prediction model by Stinear et al.2 In this previously described model, Stinear et al2 report that an absence of ipsilesional hemisphere MEPs is related to reduced corticospinal integrity leading to increased synergistic movements. The present findings add to this predictive model by further characterizing individuals without an MEP at rest who demonstrate an MEP during contraction of the unaffected limb. The latter group can thus be considered to be higher functioning with greater potential for bimanual and whole body training, compared with the individuals in whom it is not possible to obtain an MEP even with contraction of the unaffected limb.

Although interest in the use of TMS for the stroke population continues to this technology is currently not clinically accessible, therefore the link between crossed facilitation and the FMA-UE provides a clinically accessible decision-making tool. The authors provide FMA-UE cutoff scores to identify the conditions under which facilitation of the affected corticospinal tract can be evoked. Scores above 28/66 were found in individuals demonstrating affected hand contraction both at rest and during contraction of the unaffected limb. These findings may suggest that rehabilitation interventions focused on unimanual and/or complementary bimanual training are appropriate. FMA-UE scores between 25 and 28 were associated with affected hand contraction, but only during contraction of the unaffected limb, suggesting that clinical focus be on bimanual or whole body activities that incorporate either the unaffected hand or postural tasks involving use of TA. Scores below 25 were observed in individuals without evidence of crossed facilitation. Therefore, it may be possible that this latter group could benefit from in-phase bimanual training with contractions of homologous muscles, which has been shown to enhance corticospinal excitability to a larger extent than the use of differing muscles.4

Understanding the role of crossed facilitation in bimanual functioning and the extent to which this interhemispheric relationship may be influencing function after stroke allows clinicians to decide upon an optimal and individualized treatment plan. Although TMS is primarily restricted to research laboratories at this point, the association of crossed facilitation with FMA-UE scores offers some guidance and predictive value in a clinical setting. For patients within a range of FMA-UE scores, interventions focused on promoting crossed facilitation through bimanual training may be most appropriate to remediate function and independence.

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