The effects of tDCS on the maximal PF in the leg motor tasks were evaluated by performing a two-way repeated measure of ANOVA with factors of intervention (anodal, cathodal and sham) and TIME (during, 30 and 60 min after stimulation) (Fig.
2a, left upper). The main effects of TIME [
F
(2,18) = 0.44, n.s.] and the intervention× time interaction [
F
(4,36) = 2.36,
P = 0.71] were not significant, whereas the main effect of the intervention was significant [
F
(2,18) = 6.87,
P < 0.01]. The significant main effect of the intervention indicated that the three types of tDCS influenced maximal leg PF differently. Post hoc analyses revealed that the maximal PF during anodal tDCS was significantly higher compared to sham (
P < 0.05 with Bonferroni correction) and cathodal tDCS (
P < 0.001). Thirty minutes after the offset of current flow, the maximal PF was still significantly higher for anodal than for cathodal tDCS (
P < 0.01) while there was no longer significant difference between anodal and sham stimulation. There were no significant differences in the maximal PF 60 min after stimulation. Individual maximal leg PF values before and during tDCS application are shown in Fig.
2b, where one sees that with anodal tDCS the maximal PF increased in all subjects (pre vs. during comparison, paired
t test,
t
(9) = 5.61,
P < 0.001) and with cathodal and sham stimulation there was no consistent change across subjects [cathodal (
t
(9) = 0.75, n.s.), sham (
t
(9) = 0.25, n.s.)]. These results indicated that anodal tDCS transiently enhanced maximal leg PF during and 30 min after its application but that neither cathodal tDCS nor sham stimulation affected maximal leg PF significantly. These findings are consistent with a previous report that anodal tDCS enhanced the excitability of the leg motor cortex (Jeffery et al.
2007). On the other hand, none of the interventions had a significant effect on leg RT [Fig.
2a left lower, intervention (
F
(2,16) = 0.20, n.s.) and time (
F
(3,24) = 0.63, n.s.), interaction (
F
(6,48) = 0.56, n.s.)].