Acute Performance, Neuromuscular and Thermal Effects of Isometric Conditioning versus Volleyball Warm-Up in Highly Trained Male Players.

We tested whether a brief, high-effort isometric conditioning activity (ICA: three sets of five 3-s maximal isometric back-squats at a 120° knee angle, 3-min rests; total 45 s) produces greater post-activation performance enhancement (PAPE) than a volleyball warm-up of matched duration (VCA). We further hypothesized that any gains would align with neuromechanical changes—specifically reductions in rectus femoris passive stiffness—rather than with superficial thermal responses (skin surface temperature; SST).

14 highly trained male volleyball players completed a randomized two-period crossover (VCA→ICA or ICA→VCA; 7-day separation) during preseason. After a standardized general warm-up, we recorded baseline and +3, +6, +9, +12-min measures in each period: countermovement-jump (CMJ) kinetics on a force platform (1,000 Hz; jump height [JH], peak power [PP], RSImod, contraction time), rectus femoris viscoelastic properties via MyotonPRO (dynamic stiffness, natural frequency, log decrement), and anterior-thigh SST by infrared thermography under controlled conditions. Period 2 followed a microcycle with lower external training volume than Period 1, with relative intensities largely preserved.

ICA outperformed VCA in Period 2: CMJ height increased by ~2.7–3.0 cm (large effect) with concomitantly higher peak power. Responder analysis showed many more ICA-only than VCA-only responders (9/14 vs 1/14; paired OR ≈ 9, exact p=0.021). The practical potentiation window clustered between 6–12 minutes post-ICA; under the higher preceding load (Period 1), later timepoints tended to exceed earlier ones.
Mechanistic signal. In Period 2, changes in jump height were negatively associated with changes in rectus femoris stiffness (standardized β ≈ −0.50, p=0.005), indicating that greater reductions in passive stiffness co-occurred with larger performance gains. SST changes were not related to performance (p≈0.97). A simple main-effects model including treatment, Δstiffness, and ΔSST explained a large share of variance (R²≈0.85), supporting a neuromechanical—not thermal—interpretation of the PAPE observed here.
Interpretation. The ICA is effective, but its expression is context-dependent: when recent training volume is reduced, potentiation likely outpaces residual fatigue, revealing clear gains in CMJ height and power. The coupling of improved performance with reduced passive stiffness points to short-term alterations in muscle–tendon behavior (e.g., compliance–stiffness balance during the stretch–shortening cycle) as the proximate driver, whereas additional heating beyond a high-quality warm-up appears unnecessary.

The sample was modest (n=14) and limited to highly trained male volleyball players; strong period effects underscore sensitivity to background load; and direct neural/metabolic indices (e.g., EMG, NIRS) were not collected. Replication across sexes, sports, and with additional physiological instrumentation is warranted.