Communications

Abstract

When muscles shorten they produce heat faster than during isometric contraction. This extra heat production is the shortening heat (Hill, 1938). In a series of closely spaced tetanic contractions of frog sartorius muscle at O0 C, the shortening heat is less in the second and subsequent contractions than in the first. Fig. 1 A shows records of the heat rate during the first and second contractions of a series at 5 sec intervals. Fig. 1B shows how the average value of the shortening heat falls during such series, to 66 + 4 % (mean+ s.E.) of the initial value by the third contraction. By the 10th contraction the shortening heat was found to be only 15 + 18 % of the initial value (3 observations). The reduction in the shortening heat is less when the interval between the tetani is greater, and is hardly evident when this interval is 40 sec.The reduction in shortening heat might be due to previous activation, metabolic activity, or shortening. If either activation or metabolic activity were the cause, an isometric contraction should also reduce the shortening heat in subsequent contractions. The results in Fig. 1B show that this is not the case; thus it can be concluded that the reduction in shortening heat is a specific consequence of previous shortening.The isometric tension developed falls during a series of contractions but the tension exerted during shortening at speeds of 10, 20 and 30 mm/sec 98P