Contribution of damped passive recoil to the measured shortening velocity of skinned rabbit and sheep muscle fibres.

Maximum shortening velocities of skinned fibres from rabbit psoas and sheep extensor digitorum longus muscles were measured by the slack test and by extrapolating force-velocity curves to zero load. Both overall muscle velocity and sarcomere velocity were measured with each method. Maximum sarcomere velocity measured by the slack test was not significantly different from that assessed from the force-velocity curves (p greater than 0.1). Maximum overall muscle velocity measured from the slack test was significantly (p greater than 0.001) and substantially (62% rabbit, 83% sheep) greater than maximum sarcomere velocity. The difference is attributed to damped recoil of the series elastic elements contributing to the overall muscle velocity. The extent and time course of this damped recoil in isotonic steps was assessed from comparisons of overall muscle length and sarcomere length records during isotonic steps. When the records were shifted and scaled so that they superposed during the late stages of isotonic shortening, there was a substantial difference between the early parts of the records. This difference was reduced by about half in association with the step and the remaining half declined at a diminishing rate following the step, lasting longer with lower loads. This result is explained by about half of the series elastic element behaving as a viscoelastic element and half being undamped. With steps to the lowest isotonic loads, which averaged 6.7% of isometric force in sheep and 9.5% in rabbit, the total series elastic element recoil (both damped and undamped) averaged 3.4% and 2.7% of fibre segment length, respectively, in sheep and rabbit. The rapid series elastic element recoil at zero load, assessed from the slack test, was approximately 50% higher, indicating a substantial series compliance at low forces. The contribution of an additional, longer lasting, damped series elastic element recoil to the overall muscle velocity can explain the greater maximum velocity that is frequently found with the slack test.