Cathodal transcranial direct current stimulation over the Cz increases joint flexibility

- We investigated whether tDCS over the Cz modifies joint flexibility.
- We observed that cathodal tDCS over Cz increase ankle joint flexibility.
- Joint flexibility is changed by modulating the excitability of sensorimotor cortex.

Joint flexibility depends on both mechanical and neural factors. However, the contribution of neural factors is not fully understood. To test the hypothesis that the sensorimotor cortex is involved in joint flexibility, we investigated whether transcranial direct current stimulation (tDCS) over the Cz modifies ankle and wrist flexibility in healthy human participants. In eight male participants, range of motion of the left ankle and wrist were measured during a passive-dorsiflexion test. We also assessed passive torque, which represents involuntary resistance to dorsiflexion at the ankle. Participants performed passive-dorsiflexion tests before and after anodal, cathodal, and sham tDCS over the Cz. The current was applied for 10 min with an intensity of 2.0 mA during anodal and cathodal tDCS. Cathodal tDCS resulted in a 10.5% increase in range of motion of the ankle, but no significant increase in range of motion of the wrist. Neither anodal nor sham tDCS had a significant effect. Cathodal tDCS over the Cz may have affected neural factors, such as perception of joint angle or pain, because the passive torque at 0°, 5°, 10°, and 15°, which indicates mechanical effects, did not change. These results suggest that the sensorimotor cortex is involved in joint flexibility.