Acute Effects of Bilateral Versus Unilateral Isometric Conditioning Activity on Jump Performance in Volleyball Players (BI-UNI ICA)

Acute Effects of Bilateral Versus Unilateral Isometric Conditioning Activity on Bilateral and Unilateral Jump Performance in Highly Trained Male Volleyball Players: A Randomized Crossover Trial

This randomized crossover trial investigates the acute effects of bilateral and unilateral isometric conditioning activity on bilateral and unilateral jump performance in highly trained male volleyball players. Fourteen athletes will complete three experimental conditions in randomized order: bilateral isometric squat conditioning activity (BI), unilateral isometric squat conditioning activity (UNI), and a control condition consisting of low-intensity treadmill walking (CTRL). In both isometric conditions, participants will perform three sets of three maximal 3-second contractions, with 3-minute recovery intervals between sets. Jump performance will be assessed before each experimental condition and at 4, 8, and 12 minutes afterward using bilateral countermovement jumps and single-leg jumps performed separately with each lower limb. The primary aim of the study is to determine whether the mode of isometric conditioning activity, unilateral versus bilateral, influences the magnitude and time course of acute changes in jump performance. An additional objective is to assess whether the effects of the conditioning activity differ between bilateral and unilateral jumping tasks.

Study Overview

• Status: Enrolling by invitation
• Conditions: Athletic Performance; Post Activation Potentiation; Volleyball; Isometric Strength Training
• Intervention / Treatment: Behavioral: Bilateral Isometric Conditioning Activity; Behavioral: Unilateral Isometric Conditioning Activity; Behavioral: Low-Intensity Treadmill Walking

Detailed Description

This study is a randomized, counterbalanced crossover trial designed to examine the acute effects of unilateral and bilateral isometric conditioning activity on jump performance in highly trained male volleyball players. Fourteen participants will complete three experimental conditions on separate occasions in randomized order: bilateral isometric conditioning activity (BI), unilateral isometric conditioning activity (UNI), and a control condition (CTRL). Because each participant will complete all three conditions, every athlete will serve as his own control.

Each experimental session will begin with a standardized warm-up consisting of 5 minutes of cycling followed by bodyweight squats, walking lunges, thoracic spine rotations, and submaximal countermovement jumps. Baseline jump performance will then be assessed using two bilateral countermovement jumps and two single-leg jumps performed separately with each lower limb. The single-leg jump will involve take-off from one limb and landing on both limbs.

In the BI condition, participants will perform three sets of three maximal 3-second bilateral isometric squat contractions against an immovable resistance, with 3-minute recovery intervals between sets. In the UNI condition, participants will perform three sets of three maximal 3-second unilateral isometric squat contractions with each lower limb. The limbs will be alternated to reduce systematic differences in recovery time between sides. In the CTRL condition, participants will complete 9 minutes of low-intensity treadmill walking at 6 km/h.

Following each experimental condition, bilateral countermovement jump and single-leg jump performance will be reassessed at 4, 8, and 12 minutes. All jumps will be performed on force platforms. The primary outcome will be bilateral countermovement jump height. Secondary outcomes will include single-leg jump height and selected force-time variables derived from the bilateral and unilateral jump tests.

The primary objective of the study is to compare the acute effects of unilateral and bilateral isometric conditioning activity on bilateral countermovement jump performance. A secondary objective is to determine whether the type of conditioning activity produces task-specific effects on unilateral jump performance and whether the time course of post-activation performance enhancement differs between the BI and UNI conditions.

• Study Type: Interventional
• Enrollment (Estimated): 14
• Phase: Not Applicable

Contacts and Locations

Study Locations

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

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Child; Adult; Older Adult
• Accepts Healthy Volunteers: Yes

Description

Inclusion Criteria:

• minimum of 7 years of systematic volleyball training,
• at least 3 resistance training sessions per week during the preceding 5 years,
• no musculoskeletal injury requiring training cessation longer than 4 weeks within 6 months prior to enrollment,
• absence of tattoos over the rectus femoris muscle that could interfere with thermographic assessment.

Exclusion Criteria:

• less than 7 years of systematic volleyball training,
• less than 3 resistance training sessions per week during the previous 5 years,
• any musculoskeletal injury within the 6 months prior to enrollment that required cessation of training for longer than 4 weeks,
• current musculoskeletal pain or injury that could affect jumping performance or participation in maximal isometric exercise,
• tattoos, scars, or skin conditions over the rectus femoris muscle that could interfere with thermographic measurements,
• use of medications or substances that may affect neuromuscular performance or muscle function,
• failure to comply with study procedures or testing requirements.

Study Plan

How is the study designed?

Design Details

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

Number of Arms

3

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Bilateral Isometric Conditioning Activity; Participants will perform a bilateral isometric conditioning activity consisting of three sets of three maximal 3-second isometric squat contractions against an immovable resistance. Each set will be separated by 3 minutes of recovery.	Behavioral: Bilateral Isometric Conditioning Activity; Three sets of three maximal 3-second bilateral isometric squat contractions performed against an immovable resistance, with 3-minute recovery intervals between sets. Participants will be instructed to push as hard and as fast as possible during each contraction.; Other Names: BI
Experimental: Unilateral Isometric Conditioning Activity; Participants will perform a unilateral isometric conditioning activity consisting of three sets of three maximal 3-second isometric squat contractions with each lower limb against an immovable resistance. The lower limbs will be alternated, and each set will be separated by 3 minutes of recovery.	Behavioral: Unilateral Isometric Conditioning Activity; Three sets of three maximal 3-second unilateral isometric squat contractions performed with each lower limb against an immovable resistance. The lower limbs will be alternated, and 3-minute recovery intervals will be provided between sets. Participants will be instructed to push as hard and as fast as possible during each contraction.; Other Names: UNI
Experimental: Control Condition; Participants will complete 9 minutes of low-intensity treadmill walking at a speed of 6 km/h. This condition will serve as the control condition for comparison with the bilateral and unilateral isometric conditioning activities.	Behavioral: Low-Intensity Treadmill Walking; All participants will complete all three experimental conditions in a randomized and counterbalanced order, with a standardized recovery interval between sessions. Each participant will therefore serve as his own control.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Changes in Bilateral Countermovement Jump Height	Bilateral countermovement jump height will be assessed using dual force platforms operating at a sampling frequency of 1000 Hz. Jump height will be calculated from center-of-mass take-off velocity using the impulse-momentum method and expressed in centimeters. Participants will perform two valid trials at each time point, and the mean value will be used for analysis. Changes from the condition-specific baseline will be compared between the bilateral isometric, unilateral isometric, and control conditions.	Baseline and 4, 8, and 12 minutes after each experimental condition
Changes in Single-Leg Jump Height	Single-leg jump height will be assessed separately for the right and left lower limbs using force platforms. Each jump will be performed with take-off from one limb and landing on both limbs. Jump height will be calculated from center-of-mass take-off velocity using the impulse-momentum method and expressed in centimeters. Two valid trials will be performed with each limb at each time point, and the mean value for each limb will be used for analysis.	Baseline and 4, 8, and 12 minutes after each experimental condition

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Changes in Bilateral Countermovement Jump Modified Reactive Strength Index	The modified reactive strength index during the bilateral countermovement jump will be calculated as jump height in meters divided by time to take-off in seconds and expressed in meters per second. Participants will perform two valid trials at each time point, and the mean value will be used for analysis.	Baseline and 4, 8, and 12 minutes after each experimental condition
Changes in Inter-Limb Concentric Impulse Asymmetry During the Bilateral Countermovement Jump	Concentric impulse will be calculated separately for the right and left lower limbs from force-time data recorded using dual force platforms. Inter-limb asymmetry will be expressed as a percentage and calculated using the following formula: absolute difference between limbs divided by the higher limb value, multiplied by 100. A value of 0% represents perfect symmetry, while higher values indicate greater inter-limb asymmetry. The mean of two valid trials will be used for analysis.	Baseline and 4, 8, and 12 minutes after each experimental condition
Changes in Inter-Limb Single-Leg Jump Height Asymmetry	Inter-limb asymmetry in single-leg jump height will be calculated from the mean jump height obtained separately for the right and left lower limbs. Asymmetry will be expressed as a percentage and calculated as the absolute difference between limbs divided by the higher limb value, multiplied by 100. A value of 0% represents perfect symmetry, while higher values indicate greater inter-limb asymmetry.	Baseline and 4, 8, and 12 minutes after each experimental condition

Collaborators and Investigators

• Sponsor: The Jerzy Kukuczka Academy of Physical Education in Katowice

Publications and helpful links

General Publications

• Helbin J, Gawel D, Terbalyan A, Wilk M, Krzysztofik M, Lum D, Jarosz J. Acute Effects of Isometric Contraction Distribution on Jump Performance in Volleyball Players. J Funct Morphol Kinesiol. 2025 Sep 9;10(3):343. doi: 10.3390/jfmk10030343.
• Jarosz J, Szwarc A. Isometric Conditioning Activity and Jump Performance: Impact of Training Status in Male Participants. J Clin Med. 2025 Sep 3;14(17):6214. doi: 10.3390/jcm14176214.
• Krzysztofik M, Wilk M, Pisz A, Kolinger D, Tsoukos A, Aschenbrenner P, Stastny P, Bogdanis GC. Effects of Unilateral Conditioning Activity on Acute Performance Enhancement: A Systematic Review. J Sports Sci Med. 2022 Dec 1;21(4):625-639. doi: 10.52082/jssm.2022.625. eCollection 2022 Dec.
• Lum D, Barbosa TM. Brief Review: Effects of Isometric Strength Training on Strength and Dynamic Performance. Int J Sports Med. 2019 May;40(6):363-375. doi: 10.1055/a-0863-4539. Epub 2019 Apr 3.
• Jarosz J, Drozd M, Gawel D, Wilk M, Helbin J, Krzysztofik M. Acute effects of isometric conditioning activity with different distribution contraction on countermovement jump performance in resistance trained participants. Sci Rep. 2025 May 15;15(1):16960. doi: 10.1038/s41598-025-02126-4.

Study record dates

Study Major Dates

• Study Start (Estimated): July 23, 2026
• Primary Completion (Estimated): August 3, 2026
• Study Completion (Estimated): August 3, 2026

Study Registration Dates

• First Submitted: July 13, 2026
• First Submitted That Met QC Criteria: July 13, 2026
• First Posted (Actual): July 16, 2026

Study Record Updates

• Last Update Posted (Actual): July 16, 2026
• Last Update Submitted That Met QC Criteria: July 13, 2026
• Last Verified: July 1, 2026

More Information

Terms related to this study

• Keywords: Resistance training; Jump performance; Maximal voluntary isometric contraction; Post-Activation Performance Enhancement
• Other Study ID Numbers: 2-X/2025/002

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: YES
• IPD Plan Description: De-identified individual participant data will be shared upon reasonable request after publication of the study results. Requests should be directed to the corresponding author.
• IPD Sharing Time Frame: De-identified individual participant data and supporting documents will be available beginning 3 months after publication of the primary study results and will remain available for 5 years.
• IPD Sharing Access Criteria: De-identified individual participant data and supporting documents will be made available to qualified researchers who submit a methodologically sound research proposal. Requests should be directed to the corresponding author and will be reviewed by the study investigators. Access will be granted for analyses consistent with the approved proposal and may require a data-use agreement.
• IPD Sharing Supporting Information Type: STUDY_PROTOCOL; SAP

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No