The Acute Effects of Individually Timed Sodium Citrate Ingestion on Repeated Sprint Performance in Professional Female Soccer Players: A Randomized, Double-Blind, Placebo-Controlled Crossover Study

This study is a randomized, double-blind, placebo-controlled crossover trial designed to investigate the effects of acute sodium citrate (SC) supplementation on repeated sprint performance and acid-base responses in female soccer players. Based on an a priori sample size calculation using G*Power (version 3.1), a minimum of 20 participants is required and will be recruited. Eligible participants will be professional female soccer players competing in the Turkish Women's Third League.

Each participant will complete two experimental trials in a randomized order, separated by a washout period of at least three days. During the SC trial, participants will ingest SC at a dose of 0.5 g·kg-¹ body mass dissolved in a flavored beverage. During the placebo (PL) trial, participants will consume a taste- and appearance-matched sodium chloride solution. Both supplements will be prepared by a researcher not involved in data collection to ensure allocation concealment and maintenance of double blinding.

On each experimental visit, participants will arrive at the laboratory following an overnight fast and will consume a standardized breakfast before supplementation. Capillary blood samples will be collected before supplementation and at regular intervals thereafter to determine blood bicarbonate concentration. During the SC condition, exercise testing will commence once blood bicarbonate concentration has increased by at least 5.0 mmol·L-¹ above baseline (individualized peak bicarbonate response; Tpeak). During the placebo condition, exercise testing will begin 120-150 minutes after ingestion, corresponding to the expected time window for peak bicarbonate responses observed during SC supplementation. To minimize the influence of hormonal fluctuations on exercise performance and physiological responses, both experimental trials will be conducted during the same phase of the menstrual cycle, outside the menstruation period. Participants using oral contraceptives will be excluded.

Repeated sprint performance will be assessed using the Running-based Anaerobic Sprint Test (RAST), which consists of six maximal 35-m sprints separated by 10 seconds of passive recovery. Sprint performance outcomes will include sprint times, peak power, minimum power, mean power, and fatigue index. Capillary blood samples will be collected at baseline, immediately before exercise, and immediately after exercise to determine blood bicarbonate concentration, blood pH, and lactate concentration. Ratings of perceived exertion (RPE) will be recorded after each sprint using the Borg 6-20 scale. The success of participant blinding will be evaluated using a blinding questionnaire administered at three predefined time points during the study.