A brief maximal isometric squat protocol produced superior acute jump performance improvements compared to a standard volleyball warm-up, but only when preceded by lower training volume, with gains aligning with neuro-mechanical rather than thermal mechanisms.
ICA produced greater CMJ height improvements than VPA specifically when delivered in Period 2 (lower preceding training volume).
ICA produced greater relative peak power than VPA in Period 2.
ICA produced substantially more beneficial responders than VPA.
Reductions in rectus femoris muscle stiffness independently predicted CMJ gains following ICA in Period 2.
Skin surface temperature changes did not predict CMJ performance gains, suggesting thermal mechanisms did not underpin the PAPE effect.
RSImod and contraction time showed no significant differences between ICA and VPA conditions.
Preceding training volume moderated the PAPE response, with residual fatigue from higher training loads appearing to attenuate ICA benefits.
The ICA protocol consisted of 3 sets of 5 repetitions of 3-second maximal isometric back-squat contractions, while VPA consisted of mobility and plyometric drills lasting 9 minutes.
This research suggests that a short, intense isometric squat warm-up — where athletes push maximally against an immovable bar for brief intervals — can boost jump performance more effectively than a typical volleyball warm-up routine involving mobility and plyometric drills. The study tested 14 highly trained male volleyball players in both protocols and found that the isometric approach led to meaningful improvements in countermovement jump height and power output, with 9 out of 14 players responding positively compared to only 1 out of 14 with the standard warm-up. However, this advantage was not universal — it appeared specifically when athletes had lower training loads in the preceding session. When athletes came into the session more fatigued from prior training, the isometric protocol's benefits were diminished. This suggests that the overall fatigue state of an athlete plays an important role in determining whether a high-effort priming strategy will be effective, and coaches may need to consider training load when planning such warm-up protocols. The study also found that improvements in jump performance were linked to reductions in muscle stiffness in the quadriceps (rectus femoris), rather than increases in muscle temperature — a common assumption about how warm-ups work. This research suggests the isometric squat protocol works through mechanical and neural pathways rather than simply by heating the muscles, and that it could serve as a practical, equipment-minimal tool for priming elite volleyball players before training or competition when fatigue levels are appropriately managed.
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Upload Your LabsTerbalyan A, Gepfert M, Szymczyk P, Koźlenia D, Lum D, Jarosz J. (2026). Acute Performance, Mechanical and Thermal Effects of Isometric Conditioning Versus Standardized Volleyball Pre-Training Activation in Highly Trained Male Players.. Journal of sports science & medicine. https://doi.org/10.52082/jssm.2026.728