Changes in contractile dynamics during the course of a twitch of a frog muscle fibre.

Single skeletal muscle fibres from the frog were stimulated to produce isometric twitches and released after a delay to shorten isotonically unloaded or against a finite load (P). When varying the delay, the velocity of the initial shortening (V) against a given non-zero load reached its maximum value earlier than the peak of the isometric tension. The velocity of unloaded shortening (V0, slack test, range: 3.7-5.6 nm ms-1 per half-sarcomere) was independent of the delay of the release. For any given delay, V was hyperbolically related to P, except for the high-load end of the P-V curve at which the velocity took lower values than extrapolated from the hyperbolic relation. The relation between V and the load in units of P1 (corresponding to V = 1 nm ms-1 per half-sarcomere) coincided in the hyperbolic range with the relations obtained at other delays of the release. P1 was basically proportional to the maximum power which also had its peak value during the rising phase of the twitch. The quick releases required to reach the non-hyperbolic range of the P-V curves were estimated to be less than 9 nm per half-sarcomere irrespective of the delay of the release. At load levels in the non-hyperbolic range V could be increased if the quick release was followed by a brief (2 ms) extra reduction in the load preceding the shortening at isotonic load. The results can be explained if the kinetic properties of the individual strongly bound crossbridges are unaffected by the changing level of activation during the course of the contraction. The time-dependence of the non-hyperbolic range of the P-V relation can be accounted for if crossbridges attached before the release remain attached after the release thus constituting an internal load. The difference in time course of isometric tension as compared to velocity of initial shortening against a given load, P1, and maximum power may arise as the result of a reduction in the level of activation caused by the release to the isotonic load level.