The objectives of this study were to measure the active and passive force-length (F-L) relationships in type-I human single muscle fibers and to compare the results to predictions from the sliding filament model (the "standard model"). We measured isometric forces in chemically skinned fibers at different sarcomere lengths (SLs) in separate maximal activations. The experimental tolerance interval for optimal SL was calculated to be (2.37, 2.95 microm), which included the prediction by the standard model (2.64, 2.81 microm). Average passive slack length was 2.22+/-0.08 microm, and the passive F-L relationship was well described by an exponential function. Best fit lines were used to estimate the ascending and descending limbs from the active F-L data using the average SL obtained from a digital image of the fiber. The experimental descending limb was also estimated using the shortest SL to address the possible effects of sarcomere inhomogeneity (SI). The experimental slopes of the ascending and descending limbs, 0.42 F(o)/microm and -0.52 F(o)/microm (vs. -0.55 F(o)/microm with the shortest SL) respectively, F(o) being the maximal isometric force, were significantly less in magnitude than those from the standard model. These results suggested that the difference between experimental and standard models was not fully explained by SI and other factors could be important. The broader experimental F-L curve compared to the standard model implies that human muscle has functionally a wider operating length range where its force-generating capacity is not compromised.
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