1. The effect of sarcomere length (SL) during a fatiguing series of isometric tetani of frog muscle fibres was investigated. Tetani at 2.3 microns SL were more fatiguing than tetani at 3.2 microns SL, in that force declined twice as much as relaxation became much slower. 2. In a second set of experiments the force and heat production were measured during a series of fatiguing tetani. Heat was separated into two components: (a) crossbridge heat which is dependent on filament overlap and interaction, and (b) non-crossbridge heat which is independent of filament overlap and due to Ca2+ turnover. 3. In a series of fifty tetani, force, crossbridge heat and non-crossbridge heat each declined by 25-30% of its initial value. 4. The 25% reduction in non-crossbridge heat occurred completely during the first few tetani of the fatiguing series while force declined by less than 3%. This may be due to a reduction in Ca2+ binding to parvalbumin and to Ca2+ remaining bound during the remainder of the fatigue series. 5. After the first few tetani of the fatigue series the non-crossbridge heat hardly changed as force declined by a further 25% of its initial value. Continuing reduction of force with constant Ca2+ turnover indicates a reduction in the Ca2+ sensitivity of the filaments, and/or a reduction in the average force per attached crossbridge. 6. At the start of the fatiguing series, as force declines by about 7.5% there is a much larger decline of crossbridge heat (17%). The reason for this is unknown. Later in the series, force declined more rapidly than heat. This is probably due to a progressive accumulation of inorganic phosphate which acts by depressing force more than it depresses ATP breakdown.
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