Tjerk Zult

Role of the Mirror-Neuron System in Cross-Education

The present review proposes the untested hypothesis that cross-education performed with a mirror increases the transfer of motor function to the resting limb compared with standard cross-education interventions without a mirror. The hypothesis is based on neuroanatomical evidence suggesting an overlap in activated brain areas when a unilateral motor task is performed with and without a mirror in the context of cross-education of the upper extremities. The review shows that the mirror-neuron system (MNS), connecting sensory neurons responding to visual properties of an observed action and motor neurons that discharge action potentials during the execution of a similar action, has the potential to enhance cross-education. After a literature search we narrowed the review to studies that examined healthy young adults who performed unilateral strength training and unilateral motor tasks with or without a mirror and assessed outcome measures in relation to the changes in brain activity, motor cortical excitability, and corticospinal excitability. We identified six chronic studies that examined the effects of unilateral strength training on neural adaptations and 15 cross-sectional studies that examined acute changes in brain activation, motor cortical and corticospinal excitability using imaging, electroencephalographic, magnetoencephalographic, and magnetic brain stimulation. There were two chronic and nine cross-sectional studies in which participants performed unilateral motor tasks while viewing the image of the active hand superimposed on the resting hand’s image. Collectively, the data suggest that the MNS is involved in cross-education and the hypothesis is tenable. However, future studies are needed to elucidate the precise mechanism of how the use of a mirror in a cross-education study augments transfer to the non-exercised limb. Recent studies show a strength-sparing effect in the immobilized arm after strength training of the free arm in healthy individuals, and improved bilateral function after unilateral exercise therapy in stroke patients. It is thus conceptually justified to conduct randomized clinical trials that supplement cross-education protocols with a mirror. Such a treatment could reduce muscle weakness caused by limb fractures, anterior-cruciate ligament reconstruction surgery, stroke, and other unilateral motor dysfunctions.

Mirror training to augment cross-education during resistance training: a hypothesis

Resistance exercise has been shown to be a potent stimulus for neuromuscular adaptations. These adaptations are not confined to the exercising muscle and have been consistently shown to produce increases in strength and neural activity in the contralateral, homologous resting muscle; a phenomenon known as cross-education. This observation has important clinical applications for those with unilateral dysfunction given that cross-education increases strength and attenuates atrophy in immobilized limbs. Previous evidence has shown that these improvements in the transfer of strength are likely to reside in areas of the brain, some of which are common to the mirror neuron system (MNS). Here we examine the evidence for the, as yet, untested hypothesis that cross-education might benefit from observing our own motor action in a mirror during unimanual resistance training, thereby activating the MNS. The hypothesis is based on neuroanatomical evidence suggesting brain areas relating to the MNS are activated when a unilateral motor task is performed with a mirror. This theory is timely because of the growing body of evidence relating to the efficacy of cross-education. Hence, we consider the clinical applications of mirror training as an adjuvant intervention to cross-education in order to engage the MNS, which could further improve strength and reduce atrophy in dysfunctional limbs during rehabilitation.

Mirror Training Augments the Cross-education of Strength and Affects Inhibitory Paths

PURPOSE Unilateral strength training strengthens not only the muscles on the trained side but also the homologous muscles on the untrained side; however, the magnitude of this interlimb cross-education is modest. We tested the hypothesis that heightened sensory feedback by mirror viewing the exercising hand would augment cross education by modulating neuronal excitability. METHODS Healthy adults were randomized into a mirror training group (MG, N = 11) and a no-mirror training group (NMG, N = 12) and performed 640 shortening muscle contractions of the right wrist flexors at 80% maximum voluntary contraction (MVC) during 15 sessions for 3 wk. Maximal strength and specific transcranial magnetic stimulation metrics of neuronal excitability, measured in the mirror and no-mirror setup at rest and during unilateral contractions at 60% MVC, were assessed before and after the strength intervention. RESULTS Trained wrist flexor MVC increased 72% across groups, whereas cross-education was higher for the MG (61%) than NMG (34%, P = 0.047). The MG showed a reduction (15%-16%) in the contralateral silent period duration measured from the contracting left-untrained flexor carpi radialis, whereas the NMG showed an increase (12%, P ≤ 0.030). Interhemispheric inhibition, measured from the trained to the untrained primary motor cortex, increased in the MG (11%) but decreased in the NMG (15%) when measured in the mirror setup at rest (P = 0.048). Other transcranial magnetic stimulation measures did not change. CONCLUSION Viewing the exercising hand in a mirror can augment the cross-education effect. The use of a mirror in future studies can potentially accelerate functional recovery from unilateral impairment due to stroke or upper limb fracture.

Mirror illusion reduces motor cortical inhibition in the ipsilateral primary motor cortex during forceful unilateral muscle contractions.

Forceful, unilateral contractions modulate corticomotor paths targeting the resting, contralateral hand. However, it is unknown whether mirror-viewing of a slowly moving but forcefully contracting hand would additionally affect these paths. Here we examined corticospinal excitability and short-interval intracortical inhibition (SICI) of the right-ipsilateral primary motor cortex (M1) in healthy young adults under no-mirror and mirror conditions at rest and during right wrist flexion at 60% maximal voluntary contraction (MVC). During the no-mirror conditions neither hand was visible, whereas in the mirror conditions participants looked at the right hands reflection in the mirror. Corticospinal excitability increased during contractions in the left flexor carpi radialis (FCR) (contraction 0.41 mV vs. rest 0.21 mV) and extensor carpi radialis (ECR) (contraction 0.56 mV vs. rest 0.39 mV), but there was no mirror effect (FCR: P = 0.743, ηp (2) = 0.005; ECR: P = 0.712, ηp (2) = 0.005). However, mirror-viewing of the contracting and moving wrist attenuated SICI relative to test pulse in the left FCR by ∼9% compared with the other conditions (P < 0.05, d ≥ 0.62). Electromyographic activity in the resting left hand prior to stimulation was not affected by the mirror (FCR: P = 0.255, ηp (2) = 0.049; ECR: P = 0.343, ηp (2) = 0.035) but increased twofold during contractions. Thus viewing the moving hand in the mirror and not just the mirror image of the nonmoving hand seems to affect motor cortical inhibitory networks in the M1 associated with the mirror image. Future studies should determine whether the use of a mirror could increase interlimb transfer produced by cross-education, especially in patient groups with unilateral orthopedic and neurological conditions.