The purpose of this randomized controlled trial is to investigate the effects of core training and inspiratory muscle training (IMT) on muscle oxygenation and athletic performance in youth male soccer players aged 15-18. A total of 54 healthy soccer players will be included and randomly assigned into three equal groups: the Control Group, the Core Training Group, and the Core Training + IMT Group.

All interventions will be implemented for 8 weeks in addition to routine soccer training sessions. Assessments will be performed at baseline (Week 0), mid-intervention (Week 4), and post-intervention (Week 8). The primary outcome measures of the study are aerobic capacity assessed by the Yo-Yo Intermittent Recovery Test Level 1 and muscle oxygenation parameters monitored via near-infrared spectroscopy (NIRS) using a MOXY monitor. Secondary outcome measures include sprint performance, agility, dynamic balance, core muscle strength and endurance, inspiratory muscle strength, lower extremity explosive power, and soccer-specific technical performance tests.

Overall, this study aims to reveal the combined effects of core training and inspiratory muscle training on muscle oxygenation, respiratory functions, aerobic capacity, and soccer performance, while explaining the underlying physiological mechanisms in youth athletes.

Background:

Soccer performance is a multidimensional structure resulting from the interaction of technical, tactical, psychological, and physical components. In modern soccer, players frequently perform high-intensity running, sprinting, directional changes, and repeated efforts. Therefore, aerobic capacity, recovery ability, and the efficiency of muscle oxygen utilization are of critical importance.

Respiratory muscle fatigue during high-intensity exercise can limit performance by triggering the respiratory muscle metaboreflex, which reduces blood flow to peripheral muscles. Inspiratory muscle training (IMT) aims to attenuate these adverse effects by enhancing the strength and endurance of the diaphragm and auxiliary respiratory muscles. Similarly, core training improves trunk stability, enhances movement economy, optimizes force transfer along the kinetic chain, and contributes to athletic performance measures. Near-infrared spectroscopy (NIRS) is a non-invasive technology used to evaluate muscle oxygenation and hemodynamic responses during exercise. The MOXY monitor utilized in this study will allow real-time evaluation of muscle oxygen saturation (SmO2) and total hemoglobin (THb) levels.

Objectives:

The primary objective is to investigate the effects of core training and inspiratory muscle training on muscle oxygenation and aerobic performance in youth male soccer players. Secondary objectives are to examine changes in maximal inspiratory pressure, sprint performance, agility, dynamic balance, core strength/endurance, lower extremity explosive power, and soccer-specific technical skills.

Study Design & Interventions:

This study is designed as a three-arm, parallel-group, randomized controlled trial involving 54 healthy male soccer players. Participants will be randomly allocated into one of three groups:

1. Control Group: Continues routine soccer training only.
2. Core Training Group: Receives a structured core training program in addition to routine soccer training.
3. Core Training + IMT Group: Receives both the core training program and inspiratory muscle training in addition to routine soccer training.

The core training program will be implemented 3 days a week, lasting approximately 20-25 minutes prior to routine soccer practices. The program is structured on progressive overload principles, gradually increasing duration and difficulty. IMT will be performed 3 days a week using a threshold device. Participants will complete 2 sets of 30 breaths per session. Training intensity will start at 50% of maximal inspiratory pressure (MIP) and progress weekly up to approximately 70% MIP.

Assessments & Statistical Analysis:

All assessments will be conducted at baseline (Week 0), mid-intervention (Week 4), and post-intervention (Week 8). Primary outcomes include aerobic capacity (Yo-Yo IR1 distance) and muscle oxygenation parameters (SmO2, desaturation/reoxygenation rates, and THb). Secondary outcomes include 20-meter sprint (manual stopwatch), Illinois agility test, Y-Balance test, core strength/endurance tests, Standing Long Jump, MIP (cmH2O), Modified 10-Meter Dribbling Test, and the Wall Passing Accuracy Test. Linear mixed-effects models will be used to analyze group and time effects. Statistical significance will be set at p < 0.05.

Inclusion Criteria:

• Being between 15 and 18 years of age.
• Being a male soccer player.
• Actively and regularly participating in trainings at Alagöz Soccer Club.
• Having participated in regular soccer training for at least the last 6 months.
• Being able to continue routine trainings throughout the study period.
• Being fully informed about the research.
• Having obtained voluntary informed consent, and parental/guardian consent for participants under 18 years of age.
• Having the cognitive and physical capacity to understand and fulfill the specific test and exercise instructions.

Exclusion Criteria:

• Having a history of lower extremity, trunk-core, or athletic performance-limiting injury within the last 6 months.
• Having undergone orthopedic lower extremity or spine surgery within the last 1 year.
• Having active musculoskeletal pain or any orthopedic/neurological condition that prevents exercise.
• Having any health condition that prevents the safe execution of balance, sprint, agility, jump, or core endurance tests.
• Having asthma, chronic respiratory system diseases, exercise-induced bronchospasm, cardiovascular disease, or a physician-imposed restriction on high-intensity exercise that could affect respiratory muscle or exercise testing.
• Having open wounds, infections, or severe skin sensitivity that prevents the application of sensors placed on the skin.
• Currently participating in another structured respiratory muscle training or structured core exercise program that could affect the study results.
• Having a body mass index (BMI) of 30 kg/m² or above.
• Being deemed unsuitable for the study by the researcher due to safety concerns.
• Withdrawal from the study at the request of the participant or their parent/guardian.
• Missing two consecutive exercise sessions or having an insufficient overall attendance rate.
• Developing an exercise-preventing injury or illness during the study period.
• Occurrence of any safety risk during testing or exercise sessions.
• Non-compliance with the study protocol.
• Starting another additional exercise program during the study.
• Technical inability to obtain reliable measurement data.

Individual participant data will not be shared with third parties to ensure the confidentiality and privacy of the participants, in accordance with the informed consent limitations and ethical committee guidelines.