This study compared acute performance, neuromuscular, and thermal responses to a maximal isometric conditioning activity (ICA) versus a standardized volleyball pre-training activation (VPA) in highly trained male volleyball players, and to explore putative mechanisms underpinning post-activation performance enhancement (PAPE) considering training load. In a randomized crossover, 14 men (27 ± 3 y) completed two sessions: VPA (mobility/plyometric drills; 9 min), and ICA comprising 3×5×3-s maximal isometric back-squat contractions (knee angle 120°; 3-min inter-set rest). Countermovement jump (CMJ) height (primary), relative peak power (PP), RSImod, and contraction time (CT) were recorded pre and 3, 6, 9, and 12 min post. Rectus femoris muscle viscoelastic properties and skin surface temperature (SST) were assessed at matched time points. Analyses used repeated-measures ANOVA, responder analysis, correlations, and regression. Period×Sequence interactions favored ICA for CMJ and PP when delivered in Period 2 (lower training volume): ∆CMJ was higher under ICA versus VPA (p = 0.005, d = 1.20); PP likewise greater (p = 0.026, ηp² = 0.35). ICA yielded more beneficial responders than VPA (9/14 vs 1/14; McNemar exact p = 0.021). In Period 2, reductions in rectus femoris muscle stiffness independently predicted CMJ gains (β_std = -0.495, p = 0.005), whereas SST changes did not (p = 0.974). RSImod and CT showed no differences. A brief, high-effort isometric squat protocol was associated with superior acute improvements in jump performance compared with a standard volleyball warm-up, but this advantage emerged specifically when the preceding training volume was reduced (Period 2), suggesting that residual fatigue from training may reduce the effects of PAPE. Gains align with neuro-mechanical rather than thermal mechanisms, supporting ICA as a practical, equipment-minimal priming strategy for elite volleyball.

• The study used a randomized crossover design to compare a maximal isometric conditioning activity (ICA; 3 × 5 × 3 s maximal isometric squats) with standardized volleyball pre-training activation (VPA) in 14 highly trained male volleyball players.

• ICA produced greater improvements in countermovement jump height and relative peak power than VPA only in Period 2, which followed a lower-volume training microcycle.

• Individual response patterns favored ICA, with nine ICA-only responders compared with one VPA-only responder; however, these findings should be interpreted cautiously because of the small sample and measurement variability.

• In Period 2, greater reductions in rectus femoris muscle stiffness were associated with larger improvements in jump height, whereas changes in skin surface temperature were unrelated to performance; these relationships are associative rather than mechanistic.

• Maximal isometric squats may provide a practical, time-efficient, and equipment-minimal priming strategy when athletes are sufficiently recovered, highlighting the need to individualize conditioning activities according to preceding training load and current readiness.