Tension, stiffness, unloaded shortening speed and potentiation of frog muscle fibres at sarcomere lengths below optimum.

1. Unloaded shortening velocity, stiffness, and the effects of potentiators were studied to understand the basis for the shallow ascending limb (1.65-2.0 micrometers sarcomere length) of the sarcomere length-tension diagram of from single fibres. 2. The velocity of externally unloaded shortening was found to be constant over most of the range. It is therefore unlikely that this part of the sarcomere length-tension diagram results from an internal force opposing shortening. 3. Stiffness was found not to vary in proportion with tension between sarcomere lengths 1.65 and 2.0 micrometers, nor to be constant between 2.0 and 2.2 micrometers, where tension is constant. By assuming a small filament compliance, the observations could be adequately modelled on the hypothesis that the variation in tension in the range of sarcomere lengths 1.65-20 micrometers was caused by variations in the number of attached cross-bridges. 4. The twitch potentiators Zn2+, tetraethylammonium (TEA), nitrate and caffeine were found not to change the shape of the sarcomere length-tension diagram. Potentiation in a tetanus was less than 3% in all experiments. 5. Contractures induced by raised [K+] in the bathing solution were found to produce more tension than a tetanus beyond optimum length, insignificantly different tension near optimum length, and less tension at sarcomere lengths near 1.7 micrometer. An explanation is proposed for these results in terms of inhomogeneous activation and internal motion. 6. It is concluded that there is no evidence from this work that a tetanized fibre is other than maximally activated over the range of sarcomere lengths spanned by the shallow ascending limb.