This study aims to examine the acute effects of an isometric conditioning activity on sprinting, jumping, and change-of-direction performance in elite youth soccer players. Twelve male soccer players aged 15-19 years from a professional academy will participate in a randomized crossover study. Each participant will complete two experimental sessions separated by one week.

During each session, participants will perform a standardized warm-up followed by baseline performance tests including countermovement jumps, drop jumps, a 30-m sprint test, and a 505 change-of-direction test. After baseline testing, participants will complete one of two conditions in randomized order: (1) an isometric conditioning activity consisting of unilateral standing isometric calf raises, or (2) a control condition consisting of low-intensity treadmill walking. Performance tests will be repeated seven minutes after the intervention to assess acute changes in neuromuscular performance.

Jump performance will be assessed using a dual force plate system sampling at 1000 Hz, and sprint performance will be measured using electronic timing gates. The study will be conducted on an indoor athletics runway located in a gymnasium. The results will help determine whether isometric conditioning activities can acutely enhance explosive performance in youth soccer players.

This study investigates the acute effects of an isometric conditioning activity on neuromuscular performance in elite youth soccer players. The study uses a randomized crossover design in which participants complete two experimental conditions in a randomized order.

Twelve male soccer players aged 15-19 years from a professional soccer academy competing in the Polish league system will be recruited. Eligible participants must have at least one year of resistance training experience, regularly participate in organized soccer and strength training and be free from neuromuscular or musculoskeletal disorders.

Each participant will complete two testing sessions separated by seven days. Each session will begin with a standardized warm-up consisting of cycling, dynamic mobility exercises and submaximal jumps. Baseline testing will include two countermovement jumps (CMJ), two drop jumps (DJ) from a 45-cm box, one maximal 30-m sprint and two 505 change-of-direction tests performed using the dominant and non-dominant leg.

Following baseline testing, participants will complete one of two conditions:

i) Isometric conditioning activity (experimental condition): unilateral standing isometric calf raises performed as 3 sets of 3 repetitions with a 3-second contraction for each repetition with a 3-minute rest period between sets.

ii) Control condition: low-intensity treadmill walking at 6 km·h-¹ for 9 minutes.

Seven minutes after completing the assigned condition, participants will repeat the performance tests to assess acute changes in neuromuscular performance.

Jump performance will be measured using a dual force plate system (Dual Force Plate System, CC Athletics, Denmark) sampling at 1000 Hz. Force-time data will be recorded and processed using the manufacturer's software according to standardized procedures. Linear sprint performance will be assessed using a 30-m sprint test performed from a two-point staggered start. Sprint time will be recorded using electronic timing gates (Witty Timing System, Microgate, Bolzano, Italy) positioned at 0, 5 and 30 m.

All testing will be conducted on a short indoor athletics runway located inside a gymnasium. The study aims to determine whether an isometric conditioning activity can acutely enhance explosive performance in youth soccer players and provide practical recommendations for warm-up strategies in soccer.

Inclusion Criteria:

• Male soccer players aged 14-19 years
• Currently participating in organized soccer training within a competitive soccer academy
• Minimum of one year of experience in resistance training
• Regular participation in both soccer training and resistance training for at least one year prior to the study
• Free from neuromuscular and musculoskeletal disorders that could affect performance

Exclusion Criteria:

• Presence of any current musculoskeletal injury affecting the lower limbs.
• History of neuromuscular disorders or medical conditions that could influence physical performance
• Any injury or health condition preventing maximal sprinting or jumping efforts during testing.

De-identified individual participant data will be available upon reasonable request after publication of the study results.

This study is conducted as an open-label trial. Participants and investigators are aware of the assigned condition. Performance outcomes are measured using objective instrumentation including force plates and electronic timing gates. The order of experimental conditions was randomized using a computer generated randomization sequence "random.org".