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This study will examine whether caffeine (CAF) and Rhodiola rosea (RHO) supplementation, taken alone or in combination, can improve explosive performance and soccer-specific aerial duel performance in trained male soccer players. In a randomized, double-blind, placebo-controlled, parallel design, participants will be assigned to placebo, RHO, CAF, or RHO+CAF for 4 weeks. Performance outcomes will be assessed before and after the intervention using countermovement jump measures (including early take-off impulse) and standardized heading tests (heading reach height, ball exit velocity, and heading duel success rate), with additional measures of neck isometric strength and perceived exertion.
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| Label | Type | Description | Intervention Names |
|---|---|---|---|
| Placebo Control Group (CTR) | Placebo Comparator | Participants receive empty capsules (containing starch or rice flour) identical to the active supplements for 30 days. |
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| Rhodiola Rosea Group (RHO) | Experimental | Daily supplementation of 2.4g Rhodiola rosea extract (approx. 12mg salidroside) for 30 days, taken in two doses before breakfast and lunch. |
|
| Caffeine Group (CAF) | Experimental | A single dose of 3 mg/kg body mass of anhydrous caffeine administered 30 minutes before exercise testing. |
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| Combined Rhodiola and Caffeine Group (RHO + CAF) | Experimental | Combined daily supplementation of 2.4g Rhodiola rosea for 30 days and a single 3 mg/kg dose of caffeine before testing. |
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| Name | Type | Description | Arm Group Labels | Other Names |
|---|---|---|---|---|
| Caffeine | Dietary Supplement | Anhydrous caffeine capsules administered at a dose of 3 mg/kg body weight. Taken as a single dose 30 minutes prior to the physical performance tests. |
| Measure | Description | Time Frame |
|---|---|---|
| Early takeoff impulse (Impulse₀-₂₀₀) | Calculated as the integration of vertical force over the first 200 ms of the countermovement jump (CMJ) takeoff phase, relative to body mass (N·s·kg-¹). This measures the capacity for rapid force production. | Baseline (Day 0) and Post-intervention (Day 30), measured under both non-fatigued and fatigued conditions. |
| Measure | Description | Time Frame |
|---|---|---|
| Heading contact height | Participants perform a header and touch a football suspended on a vertical jump tester with their forehead. The highest touch height from three trials is recorded to reflect aerial reach capacity. | Baseline (Pre-test) and 4 weeks (Post-test), including measurements under both non-fatigued and fatigued conditions at each time point. |
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Inclusion Criteria:
Systematic soccer-specific training background and stable competitive level.
Suitable for high-intensity physical testing based on the Physical Activity Readiness Questionnaire (PAR-Q).
Exclusion Criteria:
Intake of caffeine (coffee/tea), other supplements, ergogenic aids, or alcohol within the last 3 months.
Current use of antidepressants or central nervous system stimulants.
Smokers.
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| Facility | Status | City | State | ZIP | Country | Contacts |
|---|---|---|---|---|---|---|
| China Football Institute, Beijing Sport University | Beijing | Beijing Municipality | 100084 | China | ||
| Beijing Sport University |
Individual participant data will not be shared due to participant confidentiality and institutional data protectionpolicies.
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| ID | Term |
|---|---|
| D002110 | Caffeine |
| ID | Term |
|---|---|
| D014970 | Xanthines |
| D000470 | Alkaloids |
| D006571 | Heterocyclic Compounds |
| D011688 | Purinones |
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This is a 4-week, randomized, double-blind, placebo-controlled, parallel-group trial with four arms (placebo, Rhodiola rosea, caffeine, and Rhodiola rosea plus caffeine; n=24 per arm). Participants complete identical pre- and post-intervention testing sessions. Rhodiola rosea is administered daily throughout the intervention period, while caffeine is administered acutely prior to the test sessions; matching placebo capsules are used to maintain blinding across all groups.
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Participants, outcome assessors, and data analysts are blinded to group allocation. Supplement distribution and group coding are managed by an independent researcher not involved in data collection or statistical analysis. All capsules are identical in appearance to maintain blinding throughout the study period.
| Rhodiola rosea extract | Dietary Supplement | Standardized Rhodiola rosea extract capsules. Daily total dosage of 2.4 g (containing approx. 12 mg salidroside), taken twice daily (before breakfast and lunch) for 30 days. |
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| Placebo | Dietary Supplement | Appearance-matched capsules containing inert ingredients (starch or rice flour). |
|
| Heading Ball Velocity | The maximum velocity of the ball after heading is measured using a speed device. A ball machine is used to ensure consistent delivery direction and speed. This measures momentum transfer efficiency and technical execution. | Baseline (Pre-test) and 4 weeks (Post-test), including measurements under both non-fatigued and fatigued conditions at each time point. |
| Aerial Duel Success Rate | Calculated as the percentage of successful duels out of a fixed number of trials in a standardized head-to-head setting. Success is defined as effectively contacting the ball first and directing it toward a target area. | Baseline (Pre-test) and 4 weeks (Post-test), including measurements under both non-fatigued and fatigued conditions at each time point. |
| Neck Isometric Strength | Maximum isometric force measured in neck flexion and extension using a handheld dynamometer. Values are normalized to body mass ($N \cdot kg^{-1}$) to assess the stability of the head-trunk system. | Baseline (Pre-test) and 4 weeks (Post-test). |
| Rate of Perceived Exertion (RPE) | Subjective effort level reported by participants on a scale of 1 to 10 (1 = nearly no effort, 10 = extreme effort) immediately following the fatigue protocol and post-fatigue tests. | Immediately after the fatigue-induction protocol and post-fatigue testing at Baseline and 4 weeks. |
| Beijing |
| China |
| D011687 |
| Purines |
| D006574 | Heterocyclic Compounds, 2-Ring |
| D000072471 | Heterocyclic Compounds, Fused-Ring |