To investigate the effects of different intensity of cathodal tDCS on muscle strength in healthy adults.

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that can modulate cortical excitability. In 2000, Nitsche and Paulus reported the polarity-dependent effect of tDCS in humans, i.e., anodal tDCS facilitates cortical excitability while cathodal tDCS decreases it. (Nitsche & Paulus, 2000). TDCS studies have been replicated by many researchers and reported similar effects of tDCS in humans when using anodal and cathodal tDCS with an intensity of 1 mA. The linear effect of tDCS also has been reported by various studies, i.e., when increase intensity, the polarity-dependent effect of tDCS also enhances. However, conflicting results of cathodal tDCS efficacy have been reported.

Cortical excitability has been used to represent the efficacy of tDCS on neuroplasticity. The linear effect of cathodal tDCS on cortical excitability have been reported by researchers when using intensities at 1, 2, and 3 mA stimulated at the primary motor cortex in healthy participants (Batsikadze et al., 2013; Jamil et al., 2017; Kuo et al., 2013; Mosayebi Samani et al., 2019; Nitsche et al., 2003). Nevertheless, some studies showed the non-linear effects of cathodal tDCS (i.e., when increases intensity, cathodal tDCS enhances the cortical excitability) when using intensities at 1.5 and 2 mA in healthy participants (Batsikadze et al., 2013; Jamil et al., 2017; Mosayebi Samani et al., 2019). Hence, cathodal tDCS efficacy on cortical excitability is still controversy.

Muscle performance is an outcome that represents clinical change induced by tDCS. Previous studies reported the tendency of cathodal tDCS efficacy in decreasing muscle performance of both upper and lower extremities in healthy participants (Cogiamanian et al., 2007; Tanaka et al., 2009). However, previous studies used a single intensity of tDCS in each study. Hence, there was no direct comparison between different intensities of cathodal tDCS on muscle performance in healthy participants.

In our study, we aim to investigate the effect of different intensities of cathodal tDCS on muscle strength in healthy participants.

Inclusion Criteria:

1. Male and female healthy adults
2. Age range between 18 - 60 years old
3. Right-handed dominant identified by the Edinburg Handedness Inventory
4. No recent muscle injuries of both limbs for a past 6-months

Exclusion Criteria:

1. Consumed caffeine within 24 hours prior the experiment
2. History of neurological symptoms i.e. seizures, weakness, loss of sensation or unclear history of pass illness
3. Presence of metal implantation, intracranial shunt, cochlear implantation or cardiac pacemakers
4. Presence of opened wound or infectious wound around scalp
5. Presence of pain in evaluating muscle groups
6. History of surgery in evaluating limbs