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TDCS is one of the most commonly used methods among studies with transcranial electrical stimulation and motor skills learning. Differences between study results suggest that the effect of tDCS on motor learning is dependent on the motor task performed or on the tDCS assembly specification used in the learning process. This systematic review aimed to analyze the tDCS effect on motor learning and verify whether this effect is dependent on the task or tDCS assembly specifications. Searches were performed in PubMed, SciELO, LILACS, Web of Science, CINAHL, Scopus, SPORTDiscus, Cochrane Central Register of Controlled Trials (CENTRAL), Embase, and PsycINFO. Articles were included that analyzed the effect of tDCS on motor learning through pre-practice, post-practice, retention, and/or transfer tests (period ≥24 h). The tDCS was most frequently applied to the primary motor cortex (M1) or the cerebellar cortex (CC) and the majority of studies found significant stimulation effects. Studies that analyzed identical or similar motor tasks show divergent results for the tDCS effect, even when the assembly specifications are the same. The tDCS effect is not dependent on motor task characteristics or tDCS assembly specifications alone but is dependent on the interaction between these factors. This interaction occurs between uni and bimanual tasks with anodal uni and bihemispheric (bilateral) stimulations at M1 or with anodal unihemispheric stimulations (unilateral and centrally) at CC, and between tasks of greater or lesser difficulty with single or multiple tDCS sessions. Movement time seems to be more sensitive than errors to indicate the effects of tDCS on motor learning, and a sufficient amount of motor practice to reach the "learning plateau" also seems to determine the effect of tDCS on motor learning.

To test Bernstein’s degrees of freedom (DF) hypothesis, the authors analyzed the effect of practice on the DF control and interjoint coordination of a Taekwondo kick. Thirteen inexperienced and 11 expert Taekwondo practitioners were evaluated. Contrary to Bernstein’s hypothesis, the inexperienced group froze the DF at the end of learning, reducing the joint range of motion of the knee. Moderate and strong cross-correlations between joints did not change, demonstrating that the interjoint coordination was maintained. The inexperienced group’s movement pattern was similar to that of the group of experts, from the beginning of the learning process. Thus, even after years of practice, experts continue to explore the strategy of freezing DF. The DF freeing/freezing sequence strategy was explored during the learning process, suggesting that DF-freezing/freeing strategies are task dependent.

According to Bernstein, the central nervous system solution to the human body’s enormous variation in movement choice and control when directing movement—the problem of degrees of freedom (DF)—is to freeze the number of possibilities at the beginning of motor learning. However, different strategies of freezing DF are observed in literature, and the means of selection of the control strategy during learning is not totally clear. This review investigated the possible effects of the class and objectives of the skill practiced on DF control strategies. The results of this review suggest that freezing or releasing the DF at the beginning of learning does not depend on the class (e.g., discrete skill class: football kick, dart throwing; continuous skill class: athletic march, handwriting) or objective of the skill (e.g., balance, velocity, and accuracy), in isolation. However, an interaction between these two skill elements seems to exist and influences the selection of the DF control strategy.

Para Bernstein (1967), a solução do problema dos graus de liberdade (GL) seria congelar e reduzir o número de GL a serem controlados. Com o efeito da prática, os GL seriam gradativamente descongelados e mais explorados. Estudos têm demostrado que tanto a estratégia de congelamento/descongelamento quanto a de descongelamento/congelamento dos GL podem ser utilizadas no início da aprendizagem e que a estratégia a ser explorada é dependente da habilidade e de seu objetivo. Entretanto, não há evidências que demonstrem em quais classes de habilidades e objetivos estas estratégias seriam aproveitadas. O objetivo deste estudo foi analisar o efeito da prática na coordenação e na reorganização dos GL durante o aprendizado de uma habilidade discreta contendo objetivo de velocidade e acurácia do movimento (chute Dwit Chagi do Taekwondo). Treze participantes com idade média de 20 anos formaram o grupo de inexperientes (GI). Estes realizaram 18 sessões práticas, totalizando 720 repetições do chute, e foram avaliados por meio de cinemetria (pré-teste, pós-teste e retenção). Onze atletas de Taekwondo com idade média de 21 anos formaram o grupo de experientes (GE) que foi utilizado como parâmetro para avaliação das alterações no chute do GI. Contrariamente à hipótese de Bernstein, o GI congelou os GL durante toda a aprendizagem, reduzindo a amplitude de movimento articular do joelho. Correlações moderadas e fortes entre as articulações adjacentes demonstraram que a coordenação interarticular em fase não se alterou durante a aprendizagem do chute. A análise do chute do GE demonstrou que o padrão de movimento do GI foi similar ao dos experientes desde o início da prática e que após anos de prática os experientes também exploram a estratégia do congelamento dos GL. Assim, a prática não alterou a coordenação interarticular do chute Dwit Chagi e a estratégia de reorganização dos GL durante o processo de aprendizagem de habilidades discretas com objetivo de velocidade e acurácia do movimento parece ser o descongelamento/congelamento dos GL. For Bernstein (1967), the solution to the problem of degrees of freedom (DF) would be to freeze and reduce the number of DF to be controlled. With the practice effect, DFs would be gradually thawed and more exploited. Studies have shown that both strategy the freezing/thawing and the thawing/freezing of DF can be used at the beginning of the learning process and that the strategy to be explored depends on the skill and its purpose. However, there is no evidence that demonstrates in which classes of skill and its purpose these strategies would be use. The objective of this study was to analyze the effect of the practice in the coordination and the reorganization of the DF during the learning of a discrete skill with speed and accuracy of the movement as its purpose (Taekwondo back kick). Thirteen participants with a mean age of 20 years formed the group of inexperienced (GI). They performed 18 practice sessions, totaling 720 repetitions of the kick, and were evaluated by means of cinemetry (pre-test, post-test and retention). Eleven Taekwondo athletes with a mean age of 21 years formed the group of experienced (GE) that was used as a parameter to evaluate the changes in the GI?s kick. Contrary to Bernstein's hypothesis, GI froze DFs throughout learning, reducing knee joint range of motion. Moderate and strong correlations between the adjacent joints demonstrated that the in phase inter joints coordination did not change during the learning of the kick. The GE?s kick analysis demonstrated that the movement pattern of the GI?s kick was similar to the experienced ones since the beginning of the practice and that after years of practice the experts explore the freeing DF strategy too. Thus, the practice did not alter the back kick inter joint coordination and the strategy of DF reorganization during the process of learning discrete skills with speed and accuracy of movement as its purpose seems to be the thawing/freezing of DF.