Acute Effects of Accelerated Transcranial Direct Current Stimulation on Leg Muscle Strength in Physically Inactive College Students

Background: Muscle strength comprises maximal strength, explosive strength and muscular endurance. Anodal tDCS over the primary motor cortex (M1) can increase motor cortical excitability, and several studies have reported improvements in muscle strength after single sessions. Neurophysiological studies show that a second session of anodal tDCS delivered within the after-effect period of the first (for example after 3 or 20 minutes) prolongs the increase in cortical excitability, whereas longer intervals do not. Protocols that deliver two or more sessions within one day are termed accelerated tDCS. Accelerated protocols have been applied in cognitive and clinical research, but their effects on muscle strength have not been examined.

Objective: To compare the acute effects of accelerated and single-session anodal tDCS over M1 on maximal strength, explosive strength and muscular endurance of the lower limbs in physically inactive college students.

Design: Single-center, randomized, single-blind, sham-controlled, five-arm parallel-group trial conducted at Beijing Sport University, Beijing, China. The protocol was approved by the Sports Science Experiment Ethics Committee of Beijing Sport University (No. 2025391H), and all participants provide written informed consent.

Participants: Seventy-five physically inactive college students aged 18 to 25 years (15 per group). Physical inactivity is defined as fewer than three sessions per week of moderate-intensity exercise lasting at least 30 minutes during the previous 3 months.

Randomization and blinding: Randomization is stratified by sex. A researcher not involved in testing or stimulation generates separate random number sequences for men and women in SPSS 27.0 and allocates participants of each sex to the five groups. Participants are blinded to the stimulation condition, and the researchers who collect and analyze the data are not involved in group allocation.

Interventions: tDCS is delivered with a dual-channel stimulator (TCS-E2, Shenzhen Yingchi Technology Co., Ltd., Shenzhen, China) through saline-soaked sponge electrodes (35 cm2, 5 x 7 cm). The anode is placed over Cz (leg area of M1), located according to the international 10-20 system and confirmed with a positioning cap, and the cathode over Fpz. Active stimulation is delivered at 1 mA. The five groups are: (1) two 10-minute sessions separated by 3 minutes; (2) two 10-minute sessions separated by 20 minutes; (3) one 10-minute session; (4) one 20-minute session; (5) sham stimulation, in which current is ramped up over 15 seconds and down over 15 seconds and the electrodes remain in place for 10 minutes (n = 8) or 20 minutes (n = 7). Participants remain seated at rest during stimulation.

Procedures: At a single laboratory visit, participants complete a 3-minute warm-up on the isokinetic dynamometer and the baseline test (T0). After a 10-minute rest, the assigned stimulation protocol is administered, and the same tests are repeated immediately after stimulation (T1), followed by a cool-down. Tests are performed on a Biodex System 4 Pro dynamometer in reciprocal concentric mode for the knee flexors and extensors of both legs, with the right leg tested first, in the order maximal strength, explosive strength and muscular endurance. Participants are asked to avoid strenuous exercise, sleep deprivation, alcohol, caffeine and related medications for 24 hours before testing. Sensations and adverse effects are recorded after the intervention with a transcranial electrical stimulation sensation questionnaire.

Outcomes: Peak torque at 60 degrees per second (maximal strength), average power at 180 degrees per second (explosive strength) and work fatigue during 20 repetitions at 180 degrees per second (muscular endurance) of the right and left knee flexors and extensors.

Sample size: An a priori calculation in G*Power 3.1 for a repeated-measures analysis of variance (effect size f = 0.35, alpha = 0.05, power = 0.80) indicated a minimum of 66 participants. Seventy-five participants (15 per group) are recruited to allow for dropout.

Statistical analysis: Change scores (T1 minus T0) are compared among the five groups with one-way analysis of variance, followed by least significant difference post hoc tests when the omnibus test is significant. Partial eta-squared and Cohen's d are reported as effect sizes. Welch's analysis of variance is used when variances are unequal, and a sensitivity analysis excluding extreme values (beyond three times the interquartile range) is performed. All randomized participants are included in the analysis, and the significance level is set at 0.05.