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.
Study Type
INTERVENTIONAL
Allocation
RANDOMIZED
Purpose
OTHER
Masking
TRIPLE
Enrollment
96
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.
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.
Appearance-matched capsules containing inert ingredients (starch or rice flour).
China Football Institute, Beijing Sport University
Beijing, Beijing Municipality, China
Beijing Sport University
Beijing, China
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.
Time frame: Baseline (Day 0) and Post-intervention (Day 30), measured under both non-fatigued and fatigued conditions.
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.
Time frame: Baseline (Pre-test) and 4 weeks (Post-test), including measurements under both non-fatigued and fatigued conditions at each time point.
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.
Time frame: 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.
Time frame: 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.
Time frame: 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.
Time frame: Immediately after the fatigue-induction protocol and post-fatigue testing at Baseline and 4 weeks.
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