Tension as a function of sarcomere length and velocity of shortening in single skeletal muscle fibres of the frog.

1. Simple measurements of muscle tension at fixed fibre or segment length produce a range of length-tension relationships, depending primarily on the duration of the interval between stimulation onset and tension measurement, in contradiction with the simple predictions of current models. This has been explained by non-uniformity in sarcomere lengths, leading to internal motion and, in turn, to increasing tension because the force-velocity relationship has a much greater slope for slow lengthening than for slow shortening. 2. Previous attempts to reduce the effect of internal motion have been focused on decreasing the initial extent of non-uniformity and measuring tension early in a contraction, when non-uniformities are at a minimum. An alternative approach that has not been attempted previously is to reduce the non-linearity of the force-velocity relationship by avoiding the discontinuity in slope at zero velocity. This is accomplished by imposing overall fibre shortening at velocities sufficient to ensure that all sarcomeres are shortening. 3. When the tension maintained during shortening was measured and plotted against sarcomere length for each release velocity used, linear length-tension relationships resulted that extrapolated to a common sarcomere length intercept. This was true whether the release was applied early in the tetanus or near the end of the 'creep phase' of tension rise. These observations were duplicated by computer simulation using a multisarcomere model of a muscle fibre. 4. These results provide strong support for the view that cross-bridges function as independent force generators and for the explanation of the creep phase of fibre or segment isometric tension as being due to internal motion. The results also imply that the force-velocity relationship scales with sarcomere length without changing shape. 5. Using this novel method for obtaining length-tension relationships, the sarcomere length at which active tension fell to zero was found, by extrapolation, to be 3.65 microns in semitendinosus fibres and 3.53 microns in tibialis anterior fibres from the frog (Rana temporaria).