Improved isometric force endurance after transcranial direct current stimulation over the human motor cortical areas

Neuromuscular fatigue is the exercise-dependent decrease in the ability of muscle fibres to generate force. To investigate whether manipulation of brain excitability by transcranial direct current stimulation (tDCS; 1.5 mA, 10 min, 0.026 C/cm2) modulates neuromuscular fatigue, we evaluated the effect of brain polarization over the right motor areas of the cerebral cortex of healthy subjects on the endurance time for a submaximal isometric contraction of left elbow flexors. In 24 healthy volunteers the study protocol comprised an assessment of the maximum voluntary contraction (MVC) for the left elbow flexors and a fatiguing isometric contraction (35% of MVC), before and immediately after brain polarization. One hour elapsed between baseline (T0) and postconditioning (T1) evaluation. After tDCS, MVC remained unchanged from baseline (mean ± SEM; anodal tDCS: T0, 154.4 ± 18.07; T1, 142.8 ± 16.62 N; cathodal tDCS: T0, 156 ± 18.75; T1, 141.86 ± 17.53 N; controls: T0, 148.8 ± 6.64; T1, 137.6 ± 7.36 N; P > 0.1). Conversely, endurance time decreased significantly less after anodal than after cathodal tDCS or no stimulation (-21.1 ± 5.5%, -35.7 ± 3.3% and -39.3 ± 3.3%, respectively; P < 0.05). None of the evaluated electromyographic variables changed after tDCS. Anodal tDCS could improve endurance time by directly modulating motor cortical excitability, modulating premotor areas, decreasing fatigue-related muscle pain, increasing motivation and improving synergist muscle coupling. Our findings, showing that anodal tDCS over the motor areas of the cerebral cortex improves muscle endurance, open the way to increasing muscle endurance and decreasing muscle fatigue in normal (i.e. sports medicine) and pathological conditions.