Passive mechanical properties of the medial gastrocnemius muscle of the cat

1. This is a report on the history dependence of the passive mechanical pro perties of the medial gastrocnemius muscle of the anaesthetised cat. 2. The muscle was conditioned with an isometric contraction at the test len gth, or at 3 mm longer than the test length and then returned to the test l ength, where the level of resting tension was measured, as well as tension changes during a slow stretch. 3. The level of resting tension depended on the form of conditioning and, a t the optimum length for active tension, the history-dependent component wa s 9 % of the total passive tension. 4. During a slow stretch, tension initially rose steeply up to a yield poin t, beyond which it rose more gradually. The shape of the tension rise depen ded on the form of conditioning. The level of tension at the yield point co nsisted of a stretch-dependent component, the 'short-range tension' plus th e resting tension for that length. 5. The short-range tension increased with muscle length to peak close to th e optimum for active tension. The slope of the tension rise during a stretc h, the short-range stiffness, peaked at 2 mm beyond the optimum. 6. The short-range tension was small immediately after a conditioning contr action but grew in size as the interval was increased up to 60 s, with a ti me constant of 9.9 +/- 0.6 s. After a series of conditioning movements, it recovered more rapidly, with a time constant of 6.6 +/- 0.5 s. 7. The history-dependent changes in passive tension and the response to str etch are interpreted in terms of the presence, in sarcomeres of resting mus cle fibres, of crossbridges between actin and myosin which have very slow f ormation rates, both at rest and during movements.