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.

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.

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.

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.

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.

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.