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The effect of various shortening histories on postshortening isometric length-force characteristics of rat medial gastrocnemlus (GM) was studied. Active muscle force and muscle geometry were analyzed after isotonic as well as isokinetic shortening. Active shortening significantly changed GM length-force characteristics (i.e., maximal muscle force, optimum muscle length, and active slack length). Muscle geometry did not change, which indicates that the observed changes in length-force curves are related to intracellular processes. Length-force curves valid during shortening, derived from postshortening characteristics, were very different from the fully isometric length-force curve. Their most remarkable feature was the absence of a negative slope. It was concluded that the length-force curve valid during active shortening strongly depends upon shortening characteristics (i.e., initial length and shortening speed). As a consequence, the traditional, fully isometric, length-force curve is a poor estimator of the length-force curve during dynamic contractions of muscle. Implications for muscle function are discussed.
The authors are with the Institute for Biomedical Technology, University of Twente, Enschede, The Netherlands. Peter A. Huijing is also with the Faculteit Bewegingswetenschappen, Vrije Universiteit, Amsterdam, The Netherlands. Address correspondence to Peter A. Huijing, Laboratory of Biomédical Engineering, Department of Mechanical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.