Background This study investigated delayed effects (6 h and 30 h) of a maximal isometric conditioning activity (ICA) versus a volleyball-specific warm-up (CON) on countermovement jump (CMJ) performance, viscoelastic properties (VP) of the rectus femoris (VP), and rectus femoris skin surface temperature (SST) in highly trained male volleyball players. Methods Fourteen male volleyball players (age: 27 ± 3 yrs; height: 197 ± 7 cm; body mass: 93 ± 8 kg) participated in a randomized, counterbalanced, two-condition crossover design. On two separate mornings (~ 10:00), 7 days apart, they completed either ICA comprising 3 × 5 × 3-s maximal isometric back-squat contractions (knee angle 120°; 3-min inter-set rest) or CON (mobility/plyometric drills; ~9 min). Participants then engaged in volleyball matches approximately 6 h later (~ 17:00) and again on the following day (~ 30 h post-intervention). CMJ height, peak power (PP), modified reactive strength index (RSImod), VP, and SST were assessed at baseline, 6 h, and 30 h post-intervention session. Data were analyzed using a repeated-measures analysis of variance (ANOVA). Results A significant main effect of time was detected for CMJ height (F(2,24) = 25.1; p < 0.001; np²=0.68), with increases at 6 h (mean difference [MD] = + 3.42 cm; g = 1.51; p < 0.001) and at 30 h in Microcycle 1 (MD = + 4.65 cm; g = 1.77; p < 0.001). CMJ PP also increased at both time points (F(2,24) = 36.8; p < 0.001; np²=0.75), as did RSImod (F(2,24) = 8.3; p = 0.002; np²=0.408). VP demonstrated small but significant time effects for stiffness (F(2,24) = 5.95; p = 0.008). SST showed a condition × time interaction, though without significant post-hoc differences (p = 0.078). Conclusions Both ICA and CON yielded comparable delayed improvements in CMJ performance after 6 h, with partial retention at 30 h. These increases in jump height appear to be primarily driven by increased PP, and RSImod rather than changes in VP or SST. The findings support the use of both strategies as practical means to optimize neuromuscular readiness within competitive volleyball schedules, providing practitioners with alternatives. Trial registration ClinicalTrials.gov ID: NCT07483931, Release Date: March 12, 2026. The trial is retrospectively registered to the Clinical Trials.gov, Protocol Registration and Results System.
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