Reduced isometric knee extensor force following anodal transcranial direct current stimulation of the ipsilateral motor cortex

Background: The goal of this study was to determine if 10-min of anodal
transcranial direct current stimulation (a-tDCS) to the motor cortex (M1)
is capable of modulating quadriceps isometric maximal voluntary
contraction (MVC) force or fatigue endurance contralateral or ipsilateral
to the stimulation site. Results: The main finding of this study was a
significant interaction effect for stimulation condition x leg tested x
time [F(1,60) =7.156, p = 0.010, ηp2 = 0.11] which revealed
 significant absolute KE MVC force impairments in the
contralateral leg following s-tDCS (p < 0.001, d =1.2) and in the
ipsilateral leg following a-tDCS (p < 0.001, d = 1.09). A
significant interaction effect for condition x leg tested [F(1,56) = 8.12,
p = 0.006, ηp2 = 0.13], showed a significantly lower left quadriceps
(ipsilateral to tDCS) relative MVC force with a-tDCS, versus s-tDCS [t(15)
= -3.07, p = 0.016, d = -0.77]. There was no significant difference
between the relative right quadriceps (contralateral to tDCS) MVC force
for a-tDCS and s-tDCS. Although, there was an overall significant [F(1,56)
= 8.36, p < 0.001] 12.1% force decrease between the first and
twelfth MVC repetitions, there were no significant main or interaction
effects for fatigue index force. Conclusion: Hence, a-tDCS may be
ineffective at increasing maximal force or endurance and instead may be
detrimental to quadriceps force production.