Acute strength gain and corticospinal modulation following a single session of a-tDCS combined with strength training of the contralateral homologous muscle

Introduction: Unilateral strength training (ST) produces an increase in strength of the contralateral homologous muscle, known as cross education. This strength transfer is believed to be modulated by increased neural excitability within the inactive primary motor cortex (M1) due to an overflow of neural activity, termed ‘motor irradiation’. The application of anodal tDCS (a-tDCS) increases corticospinal excitability, with corresponding effects on motor function, including acute strength gain. The underlying mechanisms associated with a-tDCS may have the potential to increase the magnitude of motor irradiation, enhancing strength gain and cortical plasticity. Greater magnitudes of strength transfer may enhance rehabilitation outcomes following unilateral injury or disuse. Objectives: To determine the effect of a single a-tDCS session combined with ST of the contralateral homologous muscle on the acute changes in strength and neural activation of the non-dominant extensor carpi radialis (ECR). Materials and methods: In a double blinded cross-over design, each participant was exposed to 3 different interventions in random order with a 1 week washout between sessions. The interventions involved; a-tDCS alone, ST + sham tDCS, and ST + a-tDCS. 20 min of a-tDCS was applied at 2 mA over the M1 in the location corresponding with the non-dominant ECR. Training of the dominant ECR involved 4 6 wrist extensions with a dumbbell weighing 80% of the participants 1 repetition maximum (1RM). Maximal voluntary strength, cortical excitability, and short-interval intracortical inhibition (SICI) were assessed prior to and following each condition for the non-dominant ECR. In addition, the effect of contralateral muscle activity on MEP amplitude was assessed during maximal isometric voluntary contractions (MVICs) of the dominant ECR to evaluate motor overflow. Results: Preliminary data (7 participants) has shown that ST + atDCS increased maximal voluntary strength of the non-dominant ECR (6.88%, p < 0.05), however ST + sham tDCS and a-tDCS alone had no effect (1% and 2.39% respectively). MEP amplitude during contralateral muscle activity increased for the ST + a-tDCS group (18.17%, p < 0.05) but not ST + sham tDCS and a-tDCS alone ( 3.79% and 5.63% respectively). The ST + a-tDCS group also displayed increases in MEP amplitude (12.19%, compared to 3.32% and 2.86%) and decreases in SICI (14.07%, compared to 9.79% and 2.38%) however these measures have not yet reached statistical significance. Conclusion: These preliminary findings suggests that a single session of a-tDCS combined with strength training of the dominant ECR induces acute increases in maximal strength in the non-dominant ECR and motor cortical overflow by a greater magnitude than a-tDCS or strength training alone.