The force produced in single fibres isolated from the anterior tibialis muscle of the frog Rana temporaria has been measured in tetani near 4 degrees C, and then in calcium-activated contractures of segments of the same fibres after chemical demembranation. All measurements were made at a sarcomere length of 2.3 microns. Force was normalized for fibre cross-section by the dry weight per unit length of the segments, which is proportional to cross-sectional area (Elzinga, Howarth, Rall, Wilson & Woledge, 1989). 2. The ratio of the force developed by the skinned segments to that produced by the intact fibres was inversely related to segment cross-section (dry weight per unit length), falling from approximately 1.0 for the thinnest segments to 0.6 for the thickest segments. 3. It is calculated that the accumulation of orthophosphate ion within contracting segments can account for a significant part of the decline in relative force in thicker segments. 4. The absolute forces in intact fibres and their derived segments were strongly correlated, but normalization by segment cross-section removed the correlation. 5. It is concluded that the sources of the approximately twofold variation in normalized force in both intact and skinned preparations are different. The existence of diffusible, force-modulating factors in intact fibres, which may be removed during skinning, is considered.
