Background: Lower limb stiffness plays a crucial role in volleyball, a sport requiring explosive actions like jumping, sprinting, and quick directional changes. While both ballistic training, which emphasizes speed and reactive force, and heavy resistance training, which builds muscular strength, are used to improve athletic performance, their comparative effects on lower limb stiffness have not been thoroughly examined in university-level volleyball players. Furthermore, research comparing their short-term and long-term outcomes remains limited. This study aims to address these gaps and provide evidence-based guidance for training protocols.

Research Question: The study investigates whether ballistic training or heavy resistance training more effectively improves lower limb stiffness in university-level volleyball players.

Aim: The primary aim is to evaluate the effectiveness of ballistic and heavy resistance training on lower limb stiffness and related performance measures in this athletic population through a randomized controlled trial.

Objectives: The specific objectives are: (1) to evaluate the long-term effect of ballistic training on lower limb stiffness, (2) to evaluate the long-term effect of heavy resistance training on lower limb stiffness, and (3) to compare both short-term and long-term effects of these training methods on lower limb stiffness and performance outcomes, such as agility, jump height, and power.

Study Design: This study follows a randomized controlled trial design.

Subjects and Sampling: The study will include male university-level volleyball players aged 18–25 years. Participants will be selected through random sampling. A priori sample size estimation using G*Power software determined that 38 participants are required to detect a medium effect size (f = 0.50) with a power of 0.95 and a significance level of 0.05. This calculation assumes a correlation of 0.5 among repeated measures across three time points.

Inclusion and Exclusion Criteria: Inclusion criteria include male players aged 18–25 who regularly participate in volleyball and have no recent lower limb injuries. Exclusion criteria include any significant lower limb injury within the past six months, chronic neuromuscular conditions, current involvement in structured strength or plyometric training, physical limitations preventing assessment completion, or use of medications that affect musculoskeletal function.

Study Procedure: After baseline assessments, participants will be randomly assigned to either a ballistic training group or a heavy resistance training group. The ballistic group will engage in plyometric exercises (e.g., squat jumps, drop jumps, bounding) performed 3–4 times per week, focusing on enhancing neuromuscular reactivity and explosive power. The heavy resistance group will perform compound strength training exercises (e.g., back squats, deadlifts, power cleans) at 70–90% of one-repetition maximum (1RM), 2–3 times per week. The total intervention period will be 8–16 weeks, with outcome measures assessed at baseline (T0), after 8 weeks (T1), and after 16 weeks (T2).

Outcome Measures: The primary outcome measures include the Double Leg Hop Test and the Dynamic Strength Index to assess stiffness and strength. Secondary performance measures include the T-Test for Agility, Lateral Medicine Ball Throw Test, and Vertical Jump Test.

Statistical Analysis: Data will be analyzed using repeated measures ANOVA to detect within- and between-group differences over time. Tests of normality and sphericity will be conducted, and the Greenhouse-Geisser correction will be applied where necessary. Post hoc comparisons with Bonferroni adjustments will be used to identify specific time-point differences. Effect sizes will be reported using Cohen’s d and partial eta squared, with statistical significance set at p < 0.05. All analyses will be conducted using SPSS software.