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The way that contractility varies between beats--the force-frequency relationship--was determined for rabbit papillary muscles held at different lengths. When the maximum rate of rise of tension in a contraction, F max , was used as a measure of contractility, the way contractility changed between contractions was independent of length. That is, when the values of F max obtained at one length were multiplied by an appropriate scaling factor, the force-frequency relationship determined at that length was indistinguishable from the force-frequency relationship determined at another length. If, however, the peak tension in a contraction was used as a measure of contractility, the Force-frequency relationship generally was not independent of muscle length. Therefore, it is proposed that the ratio of two values of F max obtained by perturbing the rate of stimulation at any given length should be tried as a length independent index of the inotropic state of the muscle.
Anderson et al. (Sat,) studied this question.