The relation between sarcomere length and tetanic tension was determined at 10-12 degrees C for 70-80 microns long segments of single fibres isolated from the tibialis anterior and semitendinosus muscles of the frog. Measurements of segment striation spacings were performed during fixed-end or length-clamp contractions by means of a laser light diffractometer. 2. At sarcomere lengths of around 2.10 microns tetanic tension rose promptly to a steady plateau, independent of the recording conditions. At greater sarcomere lengths under fixed-end conditions the tension rise occurred in two distinct stages: an initial rapid rise followed by a much slower creep. The tension creep was entirely abolished in length-clamp contractions. 3. The sarcomere length-tension diagram of length-clamped segments of tibialis anterior fibres exhibited a definite flat region between about 1.96 and 2.16 microns where tension varied by less than 1.5%. The highly linear descending limb reached zero tension at about 3.53 microns. The shift to the left by about 0.10 microns, with respect to the length-tension diagram of length-clamped segments of semitendinosus fibres, may be tentatively explained by assuming that thin filament lengths vary in different muscles. 4. The results are in agreement with those of a previous work by Gordon, Huxley & Julian (1966) and support the hypothesis (Huxley, 1957, 1980) that muscle tension is produced by simultaneous action of independent force generators, in proportion to the number of myosin bridges overlapped by actin filaments.
