The Effect of Low Dose Caffeine on Serve Speed, Spike Speed, and Speed-Endurance in Elite Sitting Volleyball Players: A Randomized Double-Blind Crossover Study
Phase: NA
Conditions: Caffeine, Physical Endurance, Muscle Strength, Paralympic Athletes
Sponsor: Ulaç Can YILDIRIM

 
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Cover Page 
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Official Study Title: 
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Effect of Low-Dose Caffeine on Performance in Elite Sitting Volleyball Players 
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NCT Number: 
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NCT number pending 
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Date of Document: 
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June 28, 2025 
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Materials and Methods 
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Participants 
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Thirteen elite male sitting volleyball players (29.84 ± 11.50 years) from the 2024 Paravolley European Champion Turkish 
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Sitting Volleyball Men's National Team participated in this study. as the inclusion criteria were: (1) having a minimum level of 
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disability according to the World ParaVolley classification, (2) not having any neuromuscular or musculoskeletal disorders that 
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would prevent participation in the measurements, (3) not smoking or consuming alcohol, and (4) not using any ergogenic support 
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to increase performance in the last 3 months. Exclusion criteria were: (1) failure to complete all test sessions, (2) occurrence of 
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injury or illness during the study period that could affect performance and (3) changes in habitual caffeine consumption during the 
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study. 
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All athletes were given detailed information about the study, their voluntary participation was ensured, and an informed 
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consent form was obtained before any measurement. The study was approved by the Sinop University Human Research Ethics 
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Committee (approval no: 2025/91) and was conducted in accordance with the ethical standards of the Declaration of Helsinki [19]. 
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This study adheres to the Consolidated Standards of Reporting Trials (CONSORT) guidelines to ensure transparency and compre-
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hensive reporting of the randomized controlled trial design and methodology (Figure 1). 
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Figure 1. CONSORT flowchart of the study 
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Study Design 
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This study had a randomized, double-blind, counterbalanced, and cross-over design. Athletes participated in a total of three 
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sessions (one familiarization session and two test sessions) (Figure 2). Participants were randomly divided into two groups (n=7 and 
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n=6) using a computer-based random number generator (simple randomization, 1:1 ratio), one group receiving low-dose caffeine 
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(L-CAF) and the other group receiving placebo (PLA) As required by the design, each participant received both supplements, but 
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the order of supplementation was randomized. A 48-hour washout period was given between each test sessions. All measurements 
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were performed at the same time interval (e.g., 09:00 - 10:00) to minimize the possible effects of circadian rhythm differences. 
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Participants were asked not to change their eating habits, to avoid intense exercise, and to consume alcohol during the study. They 
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were also advised to maintain their individual daily caffeine consumption habits. Moreover it taken all participants daily caffeine 
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consumption survey before sessions. 
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Assessed for 
eligibility      
n=13
Enrolled and 
randomized 
n=13
Caffeine 
intervention n=7
Crossover to 
placebo n=7 
Placebo 
intervention n=6
Crossover to 
caffeine n=6 
Completed both 
arms n=13
Included in final 
analysis n=13

 
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Figure 2. Test Protocol 
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In each test session, participants consumed a randomly assigned supplement containing caffeine (LCAF) or placebo (PLA) 
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with 250 ml of water 45 minutes before the tests. All participants completed a standard warm-up protocol which consisted of a 5-
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minute low-intensity general warm-up, followed by 10 minutes of dynamic stretching and technical work, 15 minutes before the 
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test. After the warm-up period, athletes performed the serve speed, spike speed, and speed-endurance performance tests, respec-
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tively. In the serve and spike speed tests, each athlete performed three trials, and the highest value was recorded. Participants were 
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given a 30-second rest period between trials and a 2-minute rest period between tests (Figure 3.). Rating of Perceived Exertion 
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(RPE) during the speed-endurance course were recorded using the Borg 6-20 scale immediately at the end of the test. Caffeine and 
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placebo doses were prepared using an analytical laboratory balance (Shimadzu, Tokyo, Japan) capable of measuring to within pre-
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cision of 1 mg, and Oxford brand pure caffeine powder (ISO 14001; The Oxford Vitality Health Company ltd; London, UK) and 
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polydextrose was used throughout the study. 
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Figure 3. Timeline of Daily Experimental Procedures 
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Data Collection Tools 
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Serve Speed 
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The serve speed of sitting volleyball players was evaluated with a standardized measurement protocol within the framework 
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of World ParaVolley game rules and court dimensions. Participants positioned themselves on the service line with their hips touch-
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ing the court surface and used a serve. During the test, participants were instructed to hit the ball within the target area at the highest 
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possible speed appropriate to their individual techniques. Each participant was given three attempts, and the best score was recorded. 
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Serve speed measurements were performed with a high-precision handheld radar (Bushnell Velocity Speed Gun, USA). 
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Spike Speed 
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The spike performance was performed by positioning the participants in front of the net in accordance with the real condi-
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tions in the game. The participants were instructed to hit the ball at the highest speed possible into the 3×3 m target area. The spike 
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speed measurements were performed with a high-precision handheld radar (Bushnell Velocity Speed Gun, USA). The data were 
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recorded in miles. 
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Speed-Endurance Performance 
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The speed and endurance test developed by Marszalek et al. (Figure 4) was applied with a single-trial protocol to evaluate 
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the anaerobic endurance capacity of the athletes [7]. The test begins with athletes in a seated position behind the starting point at 
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cone A. The participants completed the specified route by moving back and forth between cones A, B, C, D, E and G respectively 
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in the shortest time possible in accordance with the requirements of the test. All athletes were required to maintain physical contact 
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with the base of each cone throughout the test. The results were measured precisely by the Optojump photocell system (Microgate, 
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Bolzano, Italy) mounted on a tripod. 
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Figure 4. Speed-Endurance Test Parkour 
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Data Analysis 
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All data are reported as mean ± standard deviation (Mean ± SD). Normality of distribution of data was assessed by Shapiro-
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Wilk normality test. Paired t-test was applied to examine the differences between LCAF and PLA conditions in serve speed, spike 
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speed and speed-endurance performance. Power analysis was conducted using G*Power for a paired-samples t-test (two-tailed, α = 
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0.05, n = 13, power = 0.80). Cohen's d coefficient was calculated for effect sizes and interpreted according to the following classi-
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fication: insignificant (<0.2), small (0.2-0.5), medium (0.5-0.8) and large (>0.8) [20]. Statistical significance was accepted as p < 
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0.05. All analyses were performed using IBM SPSS Statistics 25.0 (IBM Corp., Armonk, NY, USA) software. 
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