The heat produced by frog muscle in a series of contractions with shortening.

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Abstract
Heat production was measured in muscles undergoing rapid shortening 1 s after the start of stimulation in a 2 s tetanus. In a series of 3 such tetani where the periods of shortening were separated by 5 s, the shortening heats in the 2nd and 3rd tetani were 111% (.+-. 4%) and 116% (.+-. 6%), respectively, of that in the 1st tetanus (mean .+-. SE of mean, 9 experiments). When a similar series was carried out after an interval of 3 min the shortening heat in the 1st tetanus of the 2nd series was 127% (.+-. 3%) of that in the 1st tetanus of the 1st series (mean .+-. SE of mean, 3 experiments). An artifact which, with certain arrangements of the apparatus, affects the measurements of shortening heat in a series of contractions was measured by 2 independent methods; the shortening heat values quoted above have been corrected accordingly. The increase in the shortening heat in repeated contractions is not a specific consequence of previous shortening; an increase of the same magnitude is observed following isometric tetani. When the muscle is shortening, the total rate of heat production, which is initially high, falls to a steady value. This value is not significantly different in the 3 tetani of the series.