Cerebral palsy (CP) refers to permanent, mutable motor development disorders stemming from a primary brain lesion, causing secondary musculoskeletal problems and limitations in activities of daily living . The prevalence of CP ranges from 1.5 to 2.5 per 1000 live births, with little or no differences among Western nations . Motor impairment is the main manifestation of this disease, with repercussions regarding the biomechanics of the body [3, 4].
The concept of functional mobility regards how an individual moves through his/her environment for successful daily interactions with family and society  and is an important goal in the rehabilitation of children with CP. Walking with or without assistance allows such children greater participation in activities of daily living as well as better physical development .
Ninety percent of children with CP have impaired gait due to excessive muscle weakness, altered joint kinematics and diminished postural reactions . Thus, such children have a diminished capacity for participating in games and sport activities at a sufficient intensity to develop an adequate degree of cardiopulmonary fitness [8, 9].
Exercise programs that include aerobic and muscle strengthening components have often been contraindicated for children with CP due to the belief that greater effort during exercise would result in an increase in muscle tone, along with a reduction in the gamut of movements and global function [10, 11]. However, a systematic review published in 2008  reports evidence of the physiological benefits of aerobic exercise in children with CP, but the effect of these benefits on function remains unknown.
A number of approaches have been used to favor selective muscle control, coordinated muscle action during gait [7, 12] and physical fitness [10, 11]. Two such approaches are treadmill training and mobility and balance training with the aid of virtual reality techniques.
The development of new therapeutic resources for use in combination with physical rehabilitation methods is of fundamental importance to the optimization of the functional outcome . Noninvasive cerebral stimulation has generated considerable interest in this context, as significant functional improvement has been demonstrated following short periods of cerebral stimulation in individuals with brain lesions [13, 14]. Transcranial Direct Current Stimulation (tDCS) is a promising method involving low-cost equipment that is easy to administer and offers good patient tolerance with minimum adverse effects . tDCS has been used in combination with physical therapy to potentiate neuroplastic changes .
In the last ten years, gait training on a treadmill has been used in the treatment of children with CP to optimize standing posture as well as improve gait speed and endurance. Although research in this field is still in the incipient stage, encouraging results have been demonstrated in children of different ages and different degrees of gross motor skill [14–22]. A treadmill can be used with or without partial weight support (PWS) to provide the training of a specific task with multiple repetitions of the steps of the gait cycle . This method activates central pattern generators (CPGs) in lumbar region of the spinal cord . CPGs are neural activations capable of forming motor patterns for the establishment of rhythmic, automatic steps, allowing the training of the biomechanical components of gait, postural control and balance [24, 25]. MacKay-Lyons  raised the hypothesis that children with CP have impaired CPGs. The activation of these generators and mechanisms of automatic reciprocation is an important aspect of the stimulation of gait through treadmill training .
Systematic reviews of the literature [27–32] published in the last four years stress the small number of randomized, controlled, clinical trials of adequate methodological quality on this subject. The studies analyzed generally evaluate the effects of treadmill training with PWS involving heterogeneous samples in terms of functional level, as determined by the Gross Motor Function Classification System (GMFCS, levels I to IV) [33, 34], and a relatively small number of participants. These reviews are categorical in stating that this form of intervention is safe, but the studies offer divergent results and further investigations are needed to determine the benefits of treadmill training on children with CP.
The equipment available for PWS is costly and a specific infrastructure is often needed for the installation of this equipment, which limits its use in the home environment and even in the therapeutic setting. Thus, a large number of physical therapists opt for treadmill training without PWS in children with mild (GMFCS I-II) [33, 34] to moderate (GMFCS III) [33, 34] motor impairment. However, a discerning evaluation of the effects of this form of intervention on functional aspects is needed. Moreover, few studies have addressed ways of determining effective therapeutic parameters (duration, frequency and intensity of training). In a recent study carried out by Grecco et al., treadmill training without PWS at a speed determined by the results of an effort test (at the aerobic threshold) demonstrated better results in comparison to overground gait training with regard to functional mobility (Six-Minute Walk Test and Timed Up-and-Go Test), gross motor function (walking, running and jumping), functional balance, static balance and cardiopulmonary fitness.
Training with virtual reality
Virtual reality is defined as an immersive, interactive, three-dimensional experience that occurs in real time [36, 37], allowing the user to have a multidimensional, multisensory experience in a virtual environment [37–39]. The use of video games with a virtual reality device has been gaining ground in the rehabilitation process, especially in physical therapy. Researchers and clinicians have explored the use of Nintendo Wii™ games as a rehabilitation tool for individuals with different forms of motor impairment involving deficits in balance and functional mobility .
Exergames is a relatively new term used to describe interactive electronic games that characterize the movements of the player as would occur in real life during the practice of a given exercise . The Nintendo Wii program is a new style of virtual reality using either a remote control or wireless platform that allows the individual to interact with the representation on the video screen through the use of a motion detection system. A sensor positioned on the television captures and reproduces the movement on the screen. The feedback provided by the image generates positive reinforcement, thereby facilitating the practice and perfection of the exercises. The games involve exercises of balance, functional mobility and aerobics .
It is believed that improvements can be achieved in the response to treatment through the play stimulus, adding a motivational factor to conventional treatment with the adoption of a specific game that motivates the patient to perform the desired movements [42, 43]. The practical advantages of the use of virtual reality through Nintendo Wii™ regard the possibility of providing feedback in real time on the performance and progression of the exercise  and the ability to train at home with or without supervision . Moreover, the system is a pleasant resource that can be used with family and friends .
However, few studies have investigated the use of exergames in the realm of child neuromotor rehabilitation. Most studies involve adults and analyze the effects of balance in patients with sequelae stemming from a stroke  or sedentary, obese individuals . By stimulating the displacement from the center of body mass and alterations in the support base, the games facilitate improvements in both static and dynamic balance during functional tasks . Another benefit demonstrated in the literature regards the possibility of using the games as an alternative for the performance of aerobic exercises, promoting an improvement in physical fitness, according to the guidelines of the American College of Sports Medicine [49, 50].
Practical guidelines for the use of virtual reality in the treatment of children with CP were published in February 2012 . According to the manuscript, there is evidence to affirm that virtual reality is a promising tool in the treatment of such children. Despite the small number of studies carried out on this population, the findings demonstrate improvements in postural control, balance, upper limb function, selective motor control and gait .
Transcranial direct current stimulation
Transcranial direct current stimulation (tDCS) is a noninvasive method that stimulates the cerebral cortex by means of a direct, low-intensity, monophasic electric current (1 to 2mA) through surface electrodes. This method has advantages over other transcranial stimulation techniques, such as its ease of application, lower cost and more prolonged modulating effect on the cerebral cortex. Moreover, this type of intervention is better suited for comparison with placebo stimulation, thereby offering greater specificity in the results of a study [52–54].
The effects of tDCS are achieved through the movement of electrons due to the electrical charges between them. The anode pole of the electrode is positive and the cathode pole is negative. The electric current (electron flow) moves from the positive to the negative pole and has different effects on biological tissues. During the administration of tDCS, the electric current flows from the electrodes and penetrates the skull, reaching the cortex. Although a large portion of the current is dissipated among the overlying tissues, a sufficient amount reaches the structures of the cerebral cortex, altering the membrane potential of the surrounding cells [55, 56].
tDCS has demonstrated effects on the excitability of the cerebral cortex. Short-term application has had short-lasting effects, whereas long-term application has generated long-lasting effects related to plastic mechanisms . A large number of studies conducted on animal models report the polar effects of tDCS on the cerebral cortex, demonstrating that polarized currents applied to the brain surface may enhance spontaneous firing  and initiate paroxystic activity  when the anodal pole is used, whereas the cathode pole generally depresses these events. Based on these findings, studies involving humans have evaluated the effects of each pole on cortex excitability through the stimulation of the primary motor cortex, demonstrating that anodal and cathodal stimulation respectively enhances and diminishes cortex excitability .
tDCS is a neuromodulation technique that has piqued the interest of a large number of researchers in recent years. The results of clinical studies demonstrate the potential of this method in the treatment of neurological conditions and investigations into the modulation of cerebral cortex excitability .
In the rehabilitation process, the aim of neuromodulation techniques is to enhance local synaptic efficacy by altering the maladaptive plasticity pattern that emerges following a cerebral cortex injury. The largest benefit of tDCS may come from its use in combination with different forms of physical therapy, as this method is a way to modulate the activity of the cerebral cortex, opening a path for the enhancement and prolongation of the functional gains provided by physical therapy. It can therefore be said that stimulation evokes a change in the dysfunctional excitability pattern so that physical therapy can model the functional pattern of cortex activity with the activation of specific neural networks .
Studies involving the use of tDCS on the primary motor cortex in stroke victims report improvements in upper limb function (active movements of wrists and fingers), movement velocity, active movements of the ankle and motor function. However, very few studies have analyzed the effects of tDCS on children with CP. Findings reported in the literature refer to Transcranial Magnetic Stimulation (TMS) as a way to analyze the evoked potential [60–62] and as a resource to reduce spasticity in children with CP . A recent study investigating TMS found significant changes in the motor cortex maps of children with hemiparetic and diparetic CP (lateralization of upper limbs and motor representation of lower limbs), demonstrating the occurrence of reorganization following affections in one or both hemispheres of the brain .
The present project proposes three innovative intervention techniques (treadmill training, mobility training with virtual reality and transcranial direct current stimulation [tDCS]) that can be safely administered to children with cerebral palsy (CP). As CP results from a primary injury of the developing brain, with limitations in functional mobility in 90% of cases, it is reasonable to assume that the motor impairment found in patients stems from the combination of the brain lesion and maladaptive plasticity pattern that emerges following a cerebral cortex injury. Forms of physical therapy seek to promote motor learning through the administration of functional training and multiple sensory stimuli. However, motor learning depends on a change in the excitability of the cerebral cortex, with a reduction in cortex inhibition following an injury. In this context, stimulation appears to be a way to modulate cortex activity, enhancing and extending the functional gains achieved with physical therapy .
Children with CP enter the rehabilitation process early and spend their entire lives performing a significant number of physical therapy sessions, which can have a negative effect on motivation. Thus, treadmill training and mobility training with the use of virtual reality offer such children a new therapy environment, which can pique their interest and enhance their motivation. Moreover, the combination of tDCS and physical therapy resources will provide the training of a specific task with multiple repetitions of the phases of the gait cycle, promoting rich sensory (proprioceptive and visual) stimuli with a modified threshold of excitability of the primary motor cortex (enhanced local synaptic efficacy), thereby potentiating motor learning.