The Effects of Caffeine and Vitamin C on Maximum Strength and Strength Endurance in Adolescents With Mild Mental Fatigue

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.

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Key information

Conditions

Maximum Force Behavior Behavioral Symptoms Fatigue Mental Fatigue Pathological Conditions, Signs and Symptoms Signs and Symptoms Strength Endurance

Age range

18 year–19 year

Sex eligibility

Female

Study type

Interventional

Phase

Not applicable

Who can participate

Healthy volunteers accepted: No

Only the study team can determine whether someone qualifies for participation.

Inclusion criteria

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)
Non-smoker; no regular alcohol consumption
Consistent strength training status: No physical training for the past 3 months, or maintenance of a consistent daily activity routine throughout the study
Able to attend all scheduled testing visits, comply with pre-test restrictions (diet, sleep, exercise, supplements)
Participant provides written assent; parent/legal guardian provides written informed consent
Able to understand and follow verbal and written study instructions

Exclusion criteria

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
Musculoskeletal injury in the past 3 months limiting lower/upper body maximal strength testing
Regular intake of ergogenic supplements (creatine, beta-alanine, energy drinks) within 4 weeks before screening
Use of any stimulants, anti-fatigue drugs, corticosteroids within 30 days prior to enrollment
Chronic gastrointestinal disease, kidney stone history (vitamin C related risk)
Irregular sleep-wake cycle, diagnosed sleep disorder
Females: current pregnancy; regular oral contraceptive use
Participated in other human intervention trials within past 8 weeks
Investigator judges any condition that compromises participant safety or interferes with outcome measurements

Treatment and study plan

1RM squats and bench presses; repetitions at 65% of 1RM for squats and bench presses

Behavioral

A 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].

Primary outcomes

1RM Squats and Bench Presses；Number of repetitions for 65% 1RM squats and bench presses

Time frame: 5 week

Study contacts

Contact information is provided by the study sponsor or research team.

Zhizheng REN

CONTACT

[email protected]

+8619862079078

Sponsors and collaborators

Lead sponsor

Yunliang Zhang

Other

Registry information

Important dates

Study start: 2026
Primary completion: 2026
Study completion: 2026
First posted: Jul 24, 2026
Registry last updated: Jul 27, 2026

OpenTrials presents study information sourced from ClinicalTrials.gov. The official registry record should be consulted for the latest information.

View the official ClinicalTrials.gov record (opens in a new tab)

This listing is for discovery and informational purposes only. It is not medical advice, does not guarantee that a study is recruiting, and does not determine eligibility. Contact the study team and a qualified healthcare professional when considering participation.

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