The Pressure System, Part I, Volume 2: Temporal Dynamics of Tube Internal Pressure in Sit-to-Stand and Vertical Jump: A Three-Stage Model

Part I, Volume 1 demonstrated that tendon elastic storage (35 J) accounts for only 12% of the energy required for vertical jumping (~300 J), and proposed muscle compartment pressure as a third support mechanism. However, the temporal dynamics of how this pressure system functions remained unspecified. This paper (Part I, Volume 2) defines when and how tube internal pressure (TIP) operates through a three-stage model: Stage 0 (filling via capillary pressure), Stage 1 (passive pressurization through geometry and tendon pull), and Stage 2 (active amplification via near-isometric contraction with proximal pressure escape). The model generates falsifiable predictions: - P1: EMG temporal sequence (tibialis anterior precedes seat-off → multi-muscle synchronization → triceps surae intensification) - P2: Ankle-dominant joint power during early lift-off with minimal knee contribution - P3: Near-isometric gastrocnemius fascicle behavior during Achilles tendon lengthening - P4: Fatigue-induced breakdown and recovery patterns Critical finding: knee extension emerges as a consequence of hydraulic push-up rather than as the causal driver. The fatigue patch hypothesis explains TIP dysfunction through spatial inhomogeneity in compartment compliance. Together with Volume 1, this forms a unified framework establishing both 'why' (energetic necessity) and 'when/how' (temporal dynamics) for the pressure system. The framework reconciles previously puzzling EMG patterns and inverse dynamics data while providing re-annotation protocols for existing datasets.