Plyometric-jump training with versus without unstable load to improve physical fitness in trained young volleyball players: a randomized controlled trial

Examination and comparison of the effects of plyometric jump training with unstable load versus plyometric jump training without unstable load on physical fitness indices in trained young volleyball players

A controlled clinical trial with parallel groups, single-blind, randomized, phase 2, conducted on 37 subjects. Randomization was performed using the RAND function in Excel software.

A single-blind, parallel randomized controlled trial was conducted at Shahid Beheshti University's Sports Science Laboratory and a volleyball gym. Participants were randomly assigned to three groups for a 4-week intervention (three sessions/week). Participants were unaware of their group allocation, and data analysis was performed by an independent, blinded analyst.

Trained male adolescent volleyball players (average age 17 years) with ≥2 years of experience in school or Tehran league competitions and ≥6 hours of weekly volleyball training were included. Inclusion criteria: good health, no training restrictions, and written consent from players and parents. Exclusion criteria: history of acute or chronic musculoskeletal injuries.

Intervention Group 1 (UL-PJT) Participants underwent 4 weeks (3 sessions/week) of plyometric jump training with an unstable load (10% body weight, using barbell + water-filled gym balls). Training included consecutive jumps over 35–50 cm obstacles, progressing from 48 to 104 jumps per session. Work-to-rest ratio was 1:7, with 2-minute rest intervals. Sessions lasted 15–25 minutes. Intervention Group 2 (S-PJT) Same protocol as UL-PJT but without unstable load. Control Group (CON) Participants continued regular volleyball training for 4 weeks without additional plyometric exercises.

Jump performance، Muscle strength، Balance performance

In this study, the allocation of participants to groups was performed using a combined approach of pair-matching and simple randomization in order to both reduce baseline heterogeneity between groups and ensure true randomization. First, all participants were matched in pairs based on their baseline performance in the Jump-and-Reach test, so that individuals with similar jump ability were grouped together. This pair-matching procedure was applied to achieve a more balanced distribution of baseline jumping ability across groups. Within each pair, participants were then assigned to the study groups using simple randomization. The random sequence was generated with Microsoft Excel using the RAND() function. Each participant received a random number, and after sorting these numbers in ascending order, participants were allocated to one of the three study groups: 1.Unstable Load Plyometric Jump Training (UL-PJT), 2.Stable Plyometric Jump Training without additional load (S-PJT), and 3. Active Control (CON). The unit of randomization was individual-based. The randomization sequence was generated and kept by a researcher who was not involved in data collection or analysis. To maintain allocation concealment and prevent any potential selection bias, the randomization codes were placed in opaque, sealed, and sequentially numbered envelopes, which were opened only at the time of participant assignment. Thus, the randomization procedure in this trial ensured both a reduction in baseline heterogeneity across groups and the preservation of a truly random allocation process.

This trial was conducted with a single-blind design. Participants were not informed about their group allocation or the specific differences between interventions. In addition, data analysis was performed by an independent statistical consultant who was blinded to group assignments. However, due to the nature of the intervention and the need for close supervision during exercise sessions and testing, the investigator responsible for conducting the interventions and outcome assessments was aware of group allocation. Therefore, blinding was ensured at the level of participants and data analysis, while assessor blinding was not feasible.

To adhere to ethical principles and protect participants' personal and confidential information, the raw data (individual participant data) from this study will not be shared for public use or secondary research. However, if required, and solely to assure journal reviewers or editors of the accuracy of statistical analyses, the data will be provided confidentially.

The Study Protocol, designed based on previous studies, along with the consent form, will be shared as a proprietary document. This protocol includes a comprehensive description of the study design, intervention methods, measured variables, and assessment timelines. The document will be provided as a PDF file upon formal request from reviewers or qualified researchers, subject to approval by the research committee. No individual participant raw data or identifiable information will be shared. Additionally, no analytical files or data dictionaries will be provided for public use or secondary research. Other documents will not be shared for public use or secondary research to comply with ethical principles and protect participants' personal and confidential information. However, if required, and solely to assure journal reviewers or editors of the accuracy of statistical analyses, the data will be provided confidentially.

Access to the study protocol will begin six months after the publication of the final research results and will remain available for five years. The document will be provided only upon formal request from journal reviewers or qualified researchers, subject to approval by the research committee.

The study data and documentation will be provided only to individuals requesting them for scientific and research purposes, with a commitment to maintaining confidentiality and ethical use of the information. Eligible individuals may include: - Researchers employed at universities or research institutions - Graduate students working on topics related to the study - Independent researchers with credible and relevant projects - Individuals in related industries requiring data for scientific and practical development Access to the data is subject to a written request and approval by the ethics committee or study authorities. Recipients are also required to comply with data protection and participant confidentiality regulations.

**Conditions for the Use of Anonymized Data and Documentation:** Anonymized data and documentation will be provided to ensure the privacy and confidentiality of participants. Use of these data is permitted solely for scientific and research purposes, including statistical analyses, meta-analyses, trend evaluations, and scientific comparisons. Commercial use, unauthorized copying, or dissemination of data without written permission is strictly prohibited. **Governing Mechanisms for Use:** - Recipients must adhere to research ethics principles and maintain confidentiality. - Data may only be used for the purposes specified in the request. - Any publication of results must acknowledge the source and obtain approval from the principal investigator. - Any use beyond the stated purposes requires renewed approval from the ethics committee. **Conditions and Criteria for Submitting a Request:** To obtain data and documentation, applicants must: - Submit a written request specifying the exact research or application purpose of the data. - Provide a research CV or relevant credentials to verify scientific qualification. - Sign a confidentiality and research ethics commitment. - Agree to use the data solely for the stated purposes and not transfer it to unauthorized individuals or entities. - Obtain approval from the ethics committee or study authorities if required.

**Guidelines for Requesting Study Data and Documentation:** To request anonymized study data and documentation, please follow these steps in order of priority: 1. **Contact the Principal Investigator:** Initiate contact with the principal investigator of the study. Their contact information is typically provided in the introduction or abstract of the study. 2. **Submit a Formal Written Request:** Send a written request (via email or official letter) to the principal investigator, including the precise research purpose, a research CV, and an ethical commitment letter. 3. **Contact the Institutional Ethics Committee:** If ethical approval is required, reach out to the ethics committee of the relevant university or institution. 4. **Receive Data After Approval:** Upon approval of the request and signing of the confidentiality agreement, the requested data and documentation will be provided. **Contact Information:** - **Principal Investigator:** Dr. Mohammad Fashi - **Email:** fashi84.u@gmail.com - **Phone:** +98 912 863 4907 - **Office Address:** Shahid Beheshti University, Faculty of Sport Sciences, Second Floor, Head of Exercise Physiology Department

**Process for Obtaining Documentation and Data and Related Details:** 1. **Request Submission and Review:** Upon submission of a written request including the research purpose, supporting documents, and an ethical commitment letter, the request will be reviewed by the responsible researcher and, if necessary, the ethics committee. This stage typically takes 7 to 14 business days. 2. **Request Approval and Ethical Agreement:** After verification of qualifications and ethical considerations, the applicant must sign a confidentiality and research ethics commitment letter. This may be done electronically or in person and usually takes 1 to 3 business days. 3. **Data Preparation and Analysis:** Anonymized data and documentation will be prepared based on the request. If specific data processing or extraction is required, this stage may take 3 to 7 business days. 4. **Data Delivery:** Once prepared, the data will be delivered as electronic files (e.g., Excel, SPSS, PDF) via secure email or a secure data-sharing platform. 5. **Support and Clarifications:** After data delivery, the responsible researcher will be available to provide guidance or address any questions. **Estimated Total Time:** The entire process, from request submission to data delivery, typically takes 2 to 4 weeks, depending on the data volume and processing complexity.