Relationships between Force and Velocity of Shortening in Rabbit Papillary Muscle

Abstract

Abstract The instantaneous relationship between isotonic load and velocity of shortening was studied in rabbit papillary muscles using a quick‐release technique which involved critical damping of the lever. The force‐velocity curve was determined from the experimental data by means of the least squares method using a computer program. The curve exhibited a true hyperbolic shape and virtually the same values of the constants a and b in Hill's equation were obtained when the analysis was carried out during the rising or falling phase of an isometric contraction. The experimental basis of the discrepant results reported in previous studies has been scrutinized. Variability in active state intensity and contractile element length between individual force‐velocity determinations may account for the non‐hyperbolic shape of the force‐velocity curve obtained in an analysis of after loaded contractions. Quick releases performed without critical damping of the lever movements were found to have a depressant effect upon the myocardial activity resulting in a distortion of the force‐velocity curve at intermediate and small loads. The distortion of the curve was more pronounced when the analysis was carried out during relaxation than during the rising phase of the contraction. The deactivating effect is most likely attributable to the rapid fluctuations in speed of shortening of the contractile unit caused by inertial oscillations of the lever system after an underdamped quick‐release.