Fluid accounts for over 70% of muscle mass, filling intracellular, extracellular, and capillary spaces. During normal physiological activity intramuscular fluid pressures develop as muscle exerts a portion of its developed force internally. These pressures, typically ranging between 10 and 250 mmHg, have the potential to influence force and work produced during contraction. Classic Hill-type models of muscle rarely incorporate fluid into their designs. Here we test a model of muscle structure in which intramuscular pressure directly influences muscle mechanics. Using a pneumatic cuff, we pressurized isolated bullfrog muscle mid-contraction at 5 psi (~260 mmHg) and measured the effect on isometric force. We compared the response of muscle to that of a simple physical model of muscle fiber and extracellular matrix morphology. Experimentally pressurizing isolated bullfrog muscle reduced isometric force at short muscle lengths (e.g. -11.87% of P0 at 0.9 L0), increased force at long lengths (e.g. +3.08% of P0 at 1.25 L0) but had no effect at intermediate lengths ~1.10-1.15 L0. Our physical model qualitatively mimics this variable response, displaying negative, positive, or neutral responses to pressurization depending on the orientation of reinforcing fibers representing extracellular matrix collagen. Our findings show that pressurization can have immediate, significant effects on muscle contractile force and suggest that forces transmitted to the extracellular matrix via pressurized fluid may be important, but largely unacknowledged, determinants of muscle performance in vivo. The work draws parallels between muscle and the hydrostatic skeletons typical of soft-bodied animals and plants, and exemplifies the importance of emergent, multiscale mechanics in biological systems.