Objective To investigate the independent and combined effects of caffeine and vitamin C on maximal strength and strength endurance in female adolescents with mild mental fatigue.
Methods A randomised, double-blind, crossover experimental design was adopted. Forty female college students presenting mild mental fatigue were recruited as participants. Each participant completed five laboratory sessions in total. The first session determined their one-repetition maximum (1RM) for bench press and back squat. The remaining four experimental sessions involved four supplement interventions administered in random order: placebo control, vitamin C (VC) alone, caffeine (CAF) alone, and combined caffeine plus vitamin C (CAF+VC). A one-week washout interval was arranged between every two supplementation trials.
In each test session, participants completed a 45-min Stroop task to induce mental fatigue prior to exercise assessments. Measurements included the Visual Analogue Scale for Mental Fatigue (VAS-MF), 1RM bench press and squat performance, as well as the maximal number of repetitions at 65% 1RM for both movements. Immediately after the strength endurance test, perceived exertion and muscle pain were rated. Blood glucose and blood lactate concentrations were sampled before and after all exercise protocols.
Supplementation was administered 1 hour prior to testing: a standard caffeine dosage of 3.0 mg per kg body mass and a single 500 mg dose of vitamin C were provided for the corresponding intervention groups.
Aged between 18and 19years (adolescent)
Able to complete standardized 1RM maximal strength test and muscular endurance protocol for back squat and bench press
Meet criteria for mild mental fatigue verified by validated scale (PVT + VAS Mental Fatigue ≥4/10 or Brief Fatigue Inventory) prior to experimental session
Habitual daily caffeine intake ≤ 150 mg/day (low-to-moderate habitual consumption)
History of cardiovascular disease, arrhythmia, hypertension, asthma requiring regular medication
Known hypersensitivity or adverse reaction to caffeine, vitamin C
Chronic neurological disorders: epilepsy, migraine disorder, ADHD with stimulant medication
Current psychiatric diagnosis (anxiety, depression) receiving psychotropic medication
Sequential
1RM squats and bench presses; repetitions at 65% of 1RM for squats and bench presses
BehavioralA randomized, double-blind, crossover design was employed. Professional coaches assessed the subjects' bench press and back squat performances to examine the effects of caffeine combined with vitamin C on maximum strength (1RM) and strength endurance (repetitions to failure at 65% of 1RM) during bench press and back squat training. 65% of 1RM is a widely recognized load for strength endurance testing \[28\]. The formal experiment consisted of five test sessions: one for familiarization with the entire experimental procedure and a preliminary 1RM test to reconfirm the 1RM measured three weeks prior to the study. The remaining four sessions were conducted sequentially for the control group, vitamin C (VC), caffeine (CAF), and caffeine plus vitamin C (CAF+VC) groups. Each test was spaced 7 days apart to eliminate carryover effects \[30\]. All tests were scheduled between 12:00 PM and 4:00 PM to control for the effects of circadian rhythms \[30\].
1RM Squats and Bench Presses；Number of repetitions for 65% 1RM squats and bench presses
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Yunliang Zhang
Lead sponsor
Tianjin University of Sport
Sponsor institution
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