This randomized controlled trial investigates the effects of unilateral and bilateral isometric strength training integrated into an 8-week complex training program on neuromuscular performance in elite youth soccer players. Forty-one male players competing at the national level will be randomly assigned to one of three groups: unilateral isometric training (UNI), bilateral isometric training (BI), or a control group performing the same training program without additional isometric exercises.

All participants will complete a structured complex strength training program once per week for eight weeks during the competitive season. Neuromuscular performance will be assessed before and after the intervention using countermovement jump height, broad jump distance, maximal isometric squat peak force and 30-m sprint time. The aim of the study is to determine whether the mode of isometric training (unilateral vs bilateral) influences adaptations in strength, power and sprint performance in youth soccer players.

An additional objective of the study is to determine whether the type of isometric training intervention influences inter-limb asymmetries in neuromuscular performance. Selected tests will therefore be performed both bilaterally and unilaterally to assess potential differences between dominant and non-dominant limbs.

This study is a randomized, parallel-group intervention designed to examine the effects of unilateral and bilateral isometric strength training integrated into an 8-week complex strength training program on neuromuscular performance in elite youth soccer players.

Participants will be recruited from youth teams competing at the national level and randomly assigned to one of three groups: unilateral isometric training (UNI), bilateral isometric training (BI), or a control group (CRL). All groups will perform the same structured complex strength training program once per week for eight weeks during the competitive season. The UNI and BI groups will perform additional isometric exercises at the beginning of each session, whereas the control group will complete the same training program without the isometric component.

The isometric protocol will include three multi-joint exercises: isometric plantar flexion, isometric squat, and isometric hip extension performed against an immovable barbell in a power rack. In the BI group exercises will be performed bilaterally, whereas in the UNI group each limb will be trained separately.

The complex training program will consist of paired resistance and ballistic exercises including squats, jumps, bench press, medicine ball throws, rows, and Romanian deadlifts. Training intensity and volume will progress across two phases during the eight-week intervention.

Neuromuscular performance will be assessed before and after the intervention using countermovement jump height measured with a force plate system, standing broad jump distance, maximal isometric squat peak force measured during an isometric squat test and 30-meter sprint time measured with electronic timing gates.

In addition, selected performance tests (countermovement jump, standing broad jump, and isometric squat test) will be performed both bilaterally and unilaterally in order to assess potential inter-limb asymmetries. Participants will first perform the tests bilaterally, followed by unilateral trials with the dominant limb and then the non-dominant limb. Sprint performance will be assessed only bilaterally.

The primary objective of the study is to evaluate the effects of unilateral versus bilateral isometric strength training on changes in jump performance, sprint performance and maximal isometric strength. A secondary objective of the study is to assess whether the type of training intervention influences inter-limb asymmetry in neuromuscular performance.

Inclusion Criteria:

• Male youth soccer players competing at the national level.
• Age between 15 and 19 years.
• Minimum of one year of resistance training experience.
• Regular participation in organized soccer training and strength training.

Exclusion Criteria:

• Current neuromuscular or musculoskeletal injury or disorder.
• Injury occurring during the intervention period preventing participation in training or testing.
• Failure to complete the prescribed training program or attend at least 80% of training sessions.
• Participation in additional resistance training programs outside the study protocol during the intervention period.

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