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Fluid pressure is generated in muscle during normal activity (1). This “intramuscular pressure” is correlated with the development of force during muscle contraction, but it is rarely considered in models of muscle function and its implications for muscle performance in vivo are unknown. Sleboda and Roberts (2) show that intramuscular pressure is not simply an indication of muscle activity; it also directly influences contractile force. While previous studies have shown that constraints that impede lateral bulging of muscle can reduce force and work (3, 4), Sleboda and Roberts’ (2) study examines the implications of a radial constraint that actually exerts force and does work on the muscle during contraction. Sleboda and Roberts (2) report the unexpected result that intramuscular pressure modulates force in a length-dependent manner; increased intramuscular pressure decreases force at short muscle lengths but increases force at long muscle lengths. Sleboda and Roberts (2) tested a potential mechanism of transduction of this pressure using water-filled silicone cylindrical models wound with relatively inextensible helical threads. The orientation of the threads in the models mimics what has been observed previously in muscle extracellular matrix, which includes the connective tissues and especially the collagen fibers that wrap muscle fibers and groups of muscle fibers. Pressurization experiments on the models, analogous to those performed on the muscle, suggest that the forces transmitted to the extracellular matrix from intramuscular pressure may be an unrecognized, but important determinant of muscle force in the whole animal.
For their experiments, Sleboda and Roberts (2) used isolated muscles from bullfrogs. Skeletal muscles typically produce maximum force at an intermediate length, denoted L0, with lower forces produced at longer lengths and at shorter lengths (5). To allow comparison between muscles this “length–force relationship,” and thus L0, was determined for each muscle by testing …
↵1Email: billkier{at}bio.unc.edu.
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Muscle force is modulated by internal pressure
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