Negative developed tension in rapidly shortening whole frog muscles

High speed isovelocity shortening using a servo-controlled lever was performed on isolated whole frog sartorius muscles at long lengths to ensure substantial passive tension. The tension records of unstimulated control experiments were subtracted from the tension records of fully-tetanized experiments on the same muscles to yield the developed tension exerted by the contractile proteins alone. There are several main results: (1) the positive developed tension had the same relation with shortening speed observed by other researchers in single fibres with no passive tension present; (2) negative developed tension was always measured at velocities of shortening above Vmax, where Vmax (typically 1.5 muscle-lengths s-1 at 2 degrees C) is defined as the velocity of shortening observed to yield zero developed tension; (3) negative developed tension was roughly asymptotic to -0.05 T(o), where T(o) is the developed isometric tetanic tension for the muscle length at which the developed tension was measured during steady shortening; (4) negative developed tension diminished in magnitude at velocities of shortening above approximately 2.5 Vmax; (5) a 10 degrees C increase in temperature from 2 degrees C to 12 degrees C had no significant effect on the shape of the normalized force-velocity curve (%T(o) versus %Vmax), but did increase Vmax by a factor of 2.6 in agreement with the results of previous studies measuring Vmax in the absence of passive tension; (6) addition of curare in the saline bath did not affect the results.