Regional tissue fluid pressure in rat calf muscle during sustained contraction or stretch.

The tissue fluid equilibration pressure in central and peripheral regions of rat calf muscles has been measured by needles with large smoothed sideholes. Short equilibration periods were normally achieved although saline was not infused. Tissue fluid pressure during rest remained constant at -2 to 0 mmHg. During sustained isometric contraction the average tissue fluid pressure rose almost linearly with increasing force of contraction. The average pressure in the central inner zone of the muscle reached 220 +/- 80 mmHg during maximal force, whereas the average pressure in the outer peripheral zone increased to 85 +/- 56 mmHg, markedly less than in the central zone. Thus, this difference in regional tissue pressure may possibly explain the greater impediment to blood flow in central than in peripheral regions during contraction shown previously by Wisnes & Kirkebø (1976). Although a corresponding regional pressure difference was observed during passive stretch of the muscle, the absolute tissue pressures were much smaller. However, the heterogeneous pattern of muscle fiber directions and relative displacement of various muscle elements during work, may induce shear forces causing focal vessel obstructions that are different during contraction and stretch.