1. Single fibres from the semitendinosus muscle of Rana temporaria were stretched during fused tetanic contractions and tension and sarcomere length (laser diffraction) responses were recorded. 2. Stretch of the fibres caused proportional increases in length of the sarcomeres. The force increased to a plateau value which was maintained during stretch or increased to a plateau value which was maintained during stretch or increased slightly. 3. The plateau value of force during stretch was dependent upon the velocity of stretch, was independent of the amplitude of stretch and was not proportional to overlap of thick and thin filaments. 4. There was enhancement of force after stretch compared with that produced at the same sarcomere length during isometric tetani. This force enhancement was independent of the velocity at which the stretch had been applied. 5. At sarcomere lengths between 1.9 and 2.3 micrometer, the force enhancement after stretch declayed rapidly, was independent of amplitude of stretch above approximately 25 nm per sarcomere not associated with a shift of the force--velocity curve. At sarcomere lengths above 2.3 micrometer the force enhancement after stretch decayed very slowly and was still present after 4 sec in long tetani. 6. At sarcomere lengths above 2.3 micrometer, force enhancement after stretch increased with amplitude of stretch and increased for any given stretch amplitude with sarcomere length. The force recorded after stretch was thus not proportional to overlap of thick and thin filaments. 7. At sarcomere lengths above 2.3 micrometer, the force enhancement after stretch was associated with a shift towards higher force value of the force--velocity curve. The velocity of shortening and zero load (V max) derived by hyperbolic extrapolation of the force--velocity curve was not affected. 8. Tension enhancement during and after stretch has a stabilizing effect in preventing dispersion of sarcomere length, particularly on the descending limb of the length--tension curve.

Single fibers from the tibialis anterior muscle of Rana temporaria at 0 .8-3 .8°C were subjected to long tetani lasting up to 8 s . Stretch of the fiber early in the tetanus caused an enhancement of force above the isometric control level which decayed only slowly and stayed higher throughout the contraction .This residual enhancement was uninfluenced by velocity of stretch and occurred only on the descending limb of the length-tension curve . The absolute magnitude of the effect increased with sarcomere length to a maximum at^-2 .9 ,um and then declined . The phenomenon was further characterized by its dependence on the amplitude of stretch . The final force level reached after stretch was usually higher than the isometric force level corresponding to the starting length of the stretch . The possibility that the phenomenon was caused by nonuniformity of sarcomere length along the fiber was examined by (a) laser diffraction studies that showed sarcomere stretch at all locations and (b) studies of 9-10 segment lengths of^-0 .6-0.7 mm along the entire fiber, which all elongated during stretch . Length-clamped segments showed residual force enhancement after stretch when compared with the tetanus produced by the same segment held at the short length as well as at the long length . It is concluded that residual force enhancement after stretch is a property shown by all individual segments along the fiber.

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1. A method has been developed to discriminate between the rate of ATP hydrolysis associated with calcium uptake into the sarcoplasmic reticulum (SR) and force development of the contractile apparatus in mechanically or saponin-skinned skeletal muscle fibres. The rate of ATP hydrolysis was determined in fibres of different types from the iliofibularis muscle of Xenopus laevis by enzymatic coupling of ATP re-synthesis to the oxidation of NADH. 2. The ATPase activity was determined before and after exposure of the preparations for 30 min to a solution containing 05% Triton X-100, which effectively abolishes the SR ATPase activity. The fibres were activated in a solution containing 5 mm caffeine to ensure that calcium uptake into the SR was maximal. 3. At saturating Ca2+ concentrations the actomyosin (AM) and SR ATPase activities in fasttwitch fibres, at 4-3 0C, amounted to 1P52 + 0 07 and 0X58 + 0 10 ,umol s-(g dry wt)-, respectively (means + 5E.M.; n = 25). The SR ATPase activity was 25% of the total ATPase activity. At submaximal calcium concentrations the AM ATPase activity varied in proportion to the isometric force. 4. The calcium sensitivity of the SR ATPase was larger than that of the AM ATPase and its dependence on [Ca2+] was less steep. The AM ATPase activity was half-maximal at a pCa of 6 11 (pCa = -log [Ca2+]) whereas the SR ATPase activity was half-maximal at a pCa of 6-62. 5. In Triton X-100-treated fibres, at different 2,3-butanedione monoxime (BDM) concentrations, the AM ATPase activity and isometric force varied proportionally. The SR ATPase activity determined by extrapolation of the total ATPase activity in mechanically skinned or saponin-treated fibres to zero force, was independent of the BDM concentration in the range studied (0-20 mM). The values obtained for the SR ATPase activity in this way were similar to those obtained with Triton X-100 treatment.6. The AM ATPase activity in slow-twitch fibres amounted to 0 74 + 0-13 ,umol s-(g dry wt)', i.e. about a factor of two smaller than in fast-twitch fibres. The SR ATPase activity amounted to 0 47 + 0'07 ,umol s-' (g dry wt)-, i.e. rather similar to the value in fasttwitch fibres. The proportion of the total ATPase activity that was due to SR ATPase (40 %) was larger than in fast-twitch fibres.7. The temperature dependence of the AM and SR ATPase activities in fast-twitch fibres differed. In the temperature range 5-10°C, the relative changes in AM and SR ATPase activities for a 10°C temperature change (Qlo) were 3X9 + 0 3 and 7'2 + 1P5, respectively.In the temperature range 10-20°C, the Qlo values of the AM and SR ATPase activities were, respectively, 2-6 + 0 4 and 3-1 + 0 5. As a result the SR ATPase activity at high temperature was a larger fraction of the total ATPase activity than at low temperature.