Single twitch fibres were dissected from anterior tibial muscles of the frog, Rana pipiens, and were then either chemically or mechanically skinned. Short segments of the skinned fibres were transferred to an experimental chamber and mounted between a force transducer and a stationary wire. 2. The average sarcomere length was determined from light photomicrographs of the segments obtained during activation and while relaxed. Activations were maximal, in solutions of pCa 5.49 and at 5 degrees C. Fibre segments having regions in which the striation pattern was highly non-uniform were rejected. 3. The relationship between tension and average sarcomere length was determined for sarcomere lengths between 2.1 and 3.8 micrometer. Tension always decreased when sarcomere length was increased above about 2.2 micrometer. Tension plotted against over-all average sarcomere length lay above data obtained from living fibres by Gordon, Huxley & Julian (1966a, b). 4. Good agreement with living fibre results was found when tension was plotted against the shortest average sarcomere length within a portion of the segment. 5. These findings indicate that sarcomere length non-uniformities greatly influence the shape of the sarcomere length-tension relation in skinned fibres at long lengths. In addition, no evidence was found for a length-dependent influence of calcium on tension development at long lengths during maximal activation.
