Using transcranial direct current stimulation to boost the hypoalgesic effects of exercise:: A randomised controlled experimental study

Participants will attend two sessions in the brain stimulation labs at Western Sydney University (‘Day 0’ and ‘Day 2’), which will take 30 and 90 minutes, respectively. On Day 0, individuals will complete formal screening to determine their eligibility for the study. If included, demographic data will be collected (pre-injection measures: age, height, weight, current levels of physical activity and beliefs about pain) and A 5 µg (0.2 mL) bolus of recombinant human NGF will be administered into the right extensor carpi radialis brevis (ECRB) muscle belly. One experimental will administer all injections. Participants will return on Day 2 and will be randomly allocated using a computer-generated random number table into the intervention group (active tDCS and exercise) or the control group (sham tDCS and exercise). Allocation will be concealed using opaque envelopes (prepared by an independent researcher) and will be opened by the researcher responsible for tDCS settings. The other researcher, responsible for collecting outcome measures, will remain blinded to the group allocation. Participants will receive active or sham tDCS and will complete a subsequent isometric grip exercise. At each timepoint (pre-injection, pre-tDCS, immediately post-exercise and 15-minute post exercise) pressure pain thresholds (PPTs), pain ratings, muscle soreness ratings and conditioned pain modulation (CPM) measures will be taken. ACTIVE tDCS (INTERVENTION): Participants in the active tDCS group will receive 20 minutes of direct current stimulation (using a DC-Stimulator, Rogue Resolutions Inc., IImenau, Germany) via two 35cm2 surface sponge electrodes soaked with 0.9% saline. The active tDCS electrode (anode) will be positioned on the scalp over the motor cortical representation for the ECRB muscle (the 'hotspot', determined using transcranial magnetic stimulation). Prior to application, the scalp will be lightly abraded using Nuprep skin prep gel (Weaver and Company, Colorado, USA) then cleaned with alcohol to reduce current impedance. The stimulation parameters used for active tDCS will be the same as those used in previous studies that have shown to produce hypoalgesic effects in chronic musculoskeletal pain (Chang et al., 2017). The Active tDCS is delivered at 1 milliAmps and the current will take 10 seconds to build in intensity (fade in) and 10 seconds to decrease in intensity (fade out). Following active tDCS, the grip force task will commence. Participants will remain seated with their right elbow resting on a table in 90 degrees of flexion and forearm in neutral (between pronation and supination). Participants will be asked to squeeze a grip dynamometer as hard as possible (termed ‘maximal voluntary contraction [MVC]’) for 5 seconds and this will be performed 3 times with 1-minute rest between trials. The exercise protocol itself will involve squeezing the grip dynamometer at 25% of the participant's MVC for 3 minutes. Visual feedback will be provided to ensure that the participant maintains this contraction. Researcher notes will be audited and reviewed by the supervisors of the student conducting the data collection. All participants will be blinded to group allocation. At the conclusion of each session, participants will be asked the group to which they believe they have been allocated. This will be done to assess the fidelity of participant blinding.