The effect of various shortening histories on postshortening isometric length-force characteristics of rat medial gastrocnemius (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 indicated 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.