Unilateral vs Bilateral Isometric Training in Youth Soccer Players

April 24, 2026 updated by: The Jerzy Kukuczka Academy of Physical Education in Katowice

Effects of Unilateral and Bilateral Isometric Strength Training Integrated Into an 8-Week Complex Training Program on Neuromuscular Performance in Youth Soccer Players

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.

Study Overview

Status

Completed

Conditions

Intervention / Treatment

Detailed Description

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.

Study Type

Interventional

Enrollment (Actual)

41

Phase

  • Not Applicable

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Locations

  • Poland
    • Silesian Voivodeship
      • Katowice, Silesian Voivodeship, Poland, 40-065
        • Academy of Physical Education in Katowice

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Ages Eligible for Study

  • Child
  • Adult

Accepts Healthy Volunteers

Yes

Description

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.

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Basic Science
  • Allocation: Randomized
  • Interventional Model: Parallel Assignment
  • Masking: None (Open Label)

Arms and Interventions

Participant Group / Arm
Intervention / Treatment
Experimental: Unilateral Isometric Training
Participants perform unilateral isometric strength training integrated into an 8-week complex strength training program.
Behavioral: Unilateral Isometric Training
Isometric strength exercises performed unilaterally including plantar flexion, squat, and hip extension against an immovable barbell.
Experimental: Bilateral Isometric Training
Participants perform bilateral isometric strength training integrated into an 8-week complex strength training program.
Behavioral: Bilateral Isometric Training
Isometric strength exercises performed bilaterally including plantar flexion, squat, and hip extension against an immovable barbell.
Active Comparator: Complex Strength Training
Participants perform the same complex strength training program without additional isometric exercises.
Behavioral: Complex Strength Training
A structured complex training program consisting of paired resistance and ballistic exercises performed once per week for eight weeks.

What is the study measuring?

Primary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Countermovement Jump Height
Time Frame: Baseline (Week 0) and Week 9
Change in countermovement jump (CMJ) height measured using a dual force plate system. Jump height will be calculated using the impulse-momentum method and expressed in centimeters (cm).
Baseline (Week 0) and Week 9
30-Meter Sprint Time
Time Frame: Baseline (Week 0) and Week 9
Change in linear sprint performance measured during a 30-meter sprint test using electronic timing gates. Sprint time will be recorded in seconds (s) using electronic timing gates positioned at positioned at 0 m, 5 m, and 30 m.
Baseline (Week 0) and Week 9
Maximal Isometric Squat Peak Force
Time Frame: Baseline (Week 0) and Week 9
Change in maximal isometric squat peak force measured during an isometric squat test performed on a dual force plate system. Peak force will be expressed in Newtons (N).
Baseline (Week 0) and Week 9

Secondary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Reactive Strength Index Modified (RSI-mod)
Time Frame: Baseline (Week 0) and Week 9
Change in reactive strength index modified (RSI-mod) obtained during the countermovement jump test and calculated as jump height divided by contraction time.
Baseline (Week 0) and Week 9
Standing Broad Jump Distance
Time Frame: Baseline (Week 0) and Week 9
Change in horizontal jump performance measured using the standing broad jump test. Jump distance will be recorded in centimeters (cm).
Baseline (Week 0) and Week 9
Inter-Limb Asymmetry Index
Time Frame: Baseline (Week 0) and Week 9
Inter-limb asymmetry calculated for unilateral performance tests including countermovement jump, standing broad jump, and isometric squat peak force. The asymmetry index (AI) will be calculated using the formula: (DL - NDL) / ((DL + NDL) / 2) × 100, where DL represents the dominant limb and NDL represents the non-dominant limb.
Baseline (Week 0) and Week 9

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

The Jerzy Kukuczka Academy of Physical Education in Katowice

Publications and helpful links

The person responsible for entering information about the study voluntarily provides these publications. These may be about anything related to the study.

General Publications

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Actual)

September 30, 2025

Primary Completion (Actual)

December 11, 2025

Study Completion (Actual)

December 11, 2025

Study Registration Dates

First Submitted

April 10, 2026

First Submitted That Met QC Criteria

April 24, 2026

First Posted (Actual)

April 29, 2026

Study Record Updates

Last Update Posted (Actual)

April 29, 2026

Last Update Submitted That Met QC Criteria

April 24, 2026

Last Verified

April 1, 2026

More Information

Terms related to this study

Keywords

Other Study ID Numbers

  • 2-X/2025/001

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

UNDECIDED

IPD Plan Description

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

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

No

Studies a U.S. FDA-regulated device product

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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