found, for a cycle of contraction and relaxation, by adding any net other factors enter into the calculation: (1) the 'internal' work, that is

Transcription

1 J. Phy8iol. (1963), 166, pp With 4 text-figures Printed in Great Britain HEAT PRODUCTION AND ENERGY LIBERATION IN THE EARLY PART OF A MUSCULAR CONTRACTION BY R. C. WOLEDGE From the Department of Physiology, University College London (Received 8 October 1962) When a muscle contracts it converts chemical energy into heat and mechanical work. The total amount of this energy 'liberation' can be found, for a cycle of contraction and relaxation, by adding any net external work done to the heat produced. Both these quantities can be directly determined. To find the time course of the energy liberation during a contraction is much harder because not only is the recorded heat production, particularly during relaxation, greatly affected by any inequalities in the muscle (Hill & Howarth, 1957; Hill, 1961), but also two other factors enter into the calculation: (1) the 'internal' work, that is work done by the contractile part of the muscle in stretching the series elastic component and any compliance in the apparatus to which it is connected; to find out how much energy is being liberated by a muscle when the tension is rising (or falling) it is necessary to add (or subtract) the internal work which is being done (or absorbed); (2) the thermoelastic heat; it was shown by Hill (1953 a) that, when the tension falls in a contracting muscle, heat is produced proportional to the fall of tension. Woledge (1961) showed this to be a reversible process, heat being absorbed when the tension rises. Although the molecular nature of the thermoelastic effect is not known it is believed to be the result of some physical process occurring in a muscle when the tension changes; that is, it is not a direct result of the chemical reactions supplying the energy for the contraction. Therefore to find the heat or energy which is being produced by these reactions when the tension is changing it is necessary to correct the observed heat production for the thermoelastic effect: adding to it the heat absorbed by the thermoelastic effect when the tension is rising, and subtracting the heat produced by a fall of tension. The heat production so corrected will be referred to, for convenience, as the 'true' heat production. Neither the internal work nor the thermoelastic heat change can be directly recorded. Both are zero over a complete cycle of contraction and relaxation. The rate of change of each is zero when the tension is constant, therefore the rate of energy liberation can be found without these complications when the tension is constant; for example, (a) during the 'plateau' 14-2

2 212 R. C. WOLEDGE of a long tetanic isometric contraction (Abbott, 1951; Aubert, 1956), (b) during isotonic contractions (Abbott, 1951), (c) during the few milliseconds at the start of a contraction when the muscle produces heat but not tension (Hill, 1949d; 1953 b). Butwhen the tension in the muscle is changing, a calculated correction must be applied to the observed heat production to obtain the 'true' heat production, and, to find the total energy liberation, the internal work done, positive or negative, must also be calculated and allowed for. The calculation of the internal work correction is fairly certain, since Jewell & Wilkie (1958) have shown that the compliance of the series elastic component does not change during the course of a contraction and is fairly constant from one sartorius ofa frog or toad to another. The evidence, however, on which the correction for the thermoelastic heat is based is less complete. It was shown in a previous paper (Woledge, 1961) that there is a thermoelastic heat absorption during the redevelopment of tension after a quick release. But it was not certain whether this heat absorption would occur in other circumstances, in particular during the development of tension at the start of a contraction. It had not been shown whether the thermoelastic heat: tension ratio was a constant during the whole course of a contraction. It was the object of the present investigation to discover whether the thermoelastic effect occurs whenever the tension changes, even right at the start of a contraction. The procedure adopted was to compare the heat produced in an isometric contraction with that in an afterloaded isotonic contraction. Figure LA and B shows the tension and length changes which occur during the early stages of such contractions. As soon as the muscle in the isotonic contraction has developed enough tension to start raising the load, two differences occur between the isometric and isotonic contractions. One is that the muscle shortens more when isotonic than when isometric, the other is that the tension continues to rise during the isometric contraction. In order to find the difference of shortening it is necessary to subtract from the external isotonic shortening (Fig. 1 B, a) the internal shortening which occurs when the muscle is isometric (Fig. 1 B, b). The result of this calculation (Fig. 1 B, c) shows that the difference of shortening does not start abruptly as the externally recorded shortening does, but starts gradually, tending towards a constant rate as the isometric tension nears its maximum. The difference of tension, on the other hand (Fig. I A, c) obtained by subtracting the constant isotonic tension from the isometric tension, starts abruptly; the rate of increase is maximum at the start and then decreases. A comparison of the heat production in isometric and afterloaded isotonic contractions could, therefore, reveal whether the difference of heat production is due to the difference of shortening or to the difference of tension, or to both. Both expected effects are in the same direction,

3 HEAT AND ENERGY IN MUSCULAR CONTRACTION 213 causing the isotonic heat record to rise above the isometric one: the extra shortening should be accompanied by extra shortening heat which would increase the heat production during the isotonic contraction; the thermoelastic effect would be expected to decrease the heat produced in the isometric contraction. It should, however, be possible to distinguish between the two effects by their time courses. The shortening heat would cause a divergence of the isometric and isotonic heat records, starting gradually and proceeding at an increasing rate. The thermoelastic effect, on the other hand, would cause an instantaneous acceleration of the heat production at the moment when the muscle became isotonic. 75 " 50 C._2 W E C.C 4 ) B Time after start of tetanus (sec) Fig. 1. Tension and shortening in isometric and isotonic afterloaded tetanic contractions of toad sartorii at 00 C. Muscles weighed 150 mg; 10 = 30 5 mm. A. Tension: (a) recorded during isometric contractions (mean of records 1, 4, 5, 8, 9, 12 in series); (b) recorded during isotonic contractions (mean of records 2, 3, 6, 7, 10, 11); (c) difference between ordinates of (a) and (b). B. Shortening: (b) mean record during isotonic contractions; (a) calculated mean shortening of contractile component during isometric contractions, from the time when the tension equals the isotonic load; (c) difference between the ordinates of (b) and (a). 0-3 METHODS Animal&. Most of the experiments were made with the sartorius muscles of English toads (Bufo bufo). The animals were kept in a cold store and were in good condition. The experiments were made between December 1961 and April Heat recording. The muscles were mounted on thermopile D2 (Abbott, Aubert & Hill, 1951). In some experiments the whole of the thermopile was used, giving an output of

4 214 R.C.WOLEDGE V/10 C, with a resistance of 58 Q; there is then a 'protecting region' (Hill 1938) of 6 mm. In other experiments only the pelvic end of the thermopile was used (877 PV/ 1 C, 37 Q2) increasing the 'protecting region' to 11-6 mm, and, when a galvanometer of 20 Q resistance was being used, reducing the sensitivity by In these experiments, which depend upon accurate comparison between isometric and isotonic contractions, it is important to avoid even a small error from an inadequate protecting region. This error is caused if, when a muscle shortens, a region of the muscle not previously in contact with the thermopile, and therefore not at the same temperature as the rest, moves on to the active part of the thermopile. At 0 C the resting heat production is very slow and will cause very little temperature difference between the part off and the part on the thermopile. But recovery heat production continues for an hour or so after a tetanic stimulus so that it is not practicable to make a long series of observations with complete recovery after each one. Also it is found that the muscle behaves more consistently if the experiment is completed in a few hours with stimulation about every 10 min. Between stimulations the muscles are soaked in Ringer's solution (mm:nacl 115-5, CaCl2 1-8, KCI 2-5 and phosphate buffer 3, ph 7 2). After a few stimulations the muscles are in a steady state with a considerable rate of recovery heat production. A protecting region is therefore essential. It must be several millimetres longer than the amount of shortening occurring because the temperature of the region of the muscle at the end of the thermopile will be affected by the adjacent part not on the thermopile. The 'tibial' stimulating electrode should not be counted as part of the protecting region because it has a different thermal conductivity from the thermopile. It is therefore difficult to know how long the protecting region should be, but it is best to make a generous allowance. This has the disadvantage that the heat is being recorded from only a part of the muscle which may not be typical of the whole. The difficulty is aggravated by the fact that it is not possible to stimulate a muscle all over when it is lying on a thermopile. Stimulation is effected through two electrodes, one at each end of the thermopile. The result is that one part of the muscle is stimulated before the rest (Hill, 1949d) and the contraction cannot therefore be uniform during the first few milliseconds, and the heat recorded from a part only of the muscle cannot be exactly comparable with the mechanical record made from the whole. In an attempt to minimize this effect the stimulus, when a single shock, was made several times maximal; but, for tetani, stimuli only just above maximal were used to reduce the disturbance caused on the heat record. The output from the thermopile was recorded by a rapid, sensitive galvanometer: 'Stylo', type A. 81 made by Kipp of Delft (full period undamped 9-8 msec, internal resistance 20 Q; external resistance for critical damping 64 Q). Its deflexions were amplified photo-electrically and displayed on an oscilloscope. The main limitation to the accuracy of the records is the mechanical disturbance of the galvanometer. The vibrations to which this laboratory is continuously subjected have greatly increased in recent years. This is shown by a comparison of records made recently with those published by Hill (1949c). In spite of all possible antivibration arrangements and in spite of working when the building is at its quietest, the present records compare very unfavourably with the earlier ones. Mechanical arrangements. The arrangement for recording length and tension is very similar to that used by Jewell & Wilkie (1960, Fig. 2). The muscles are connected by a light chain to a point half way between the fulcrum and the tip of an isotonic lever, the movements of which are recorded photo-electrically. Weights are hung on the lever near its fulcrum. The lever is attached at its tip by a short chain to an R.C.A transducer valve which acts as an afterloading stop, allowing the muscle to shorten, when the chain goes slack but not allowing the load to stretch the muscle. When the load applied to the lever is greater than that exerted by the muscle the transducer records the difference, thus giving a record of the tension changes when the muscle is isometric. Either tension or length changes can be recorded on the second beam of the oscilloscope, or both together, by using mechanical input switching at 50 c/s.

5 HEAT AND ENERGY IN MUSCULAR CONTRACTION 215 Calcuations. Allowance for the time lag due to the thermopile and galvanometer was made by analysing the records in the usual way by successive subtractions. The equivalent half-thickness of the thermopile was taken as 22-4,u, which includes an allowance for the thickness of the epimysium. This value has recently been confirmed. In order to find the difference in shortening between isotonic and isometric contractions it is necessary to allow for the internal shortening which occurs during the isometric contractions. This can easily be calculated from the tension record if the compliance in series with the muscle is known. The compliance of the series elastic component was taken from the results given by Jewell & Wilkie (1958, Fig. 8) and the compliance of the apparatus (chain, lever, transducer) was measured by stretching it known distances with a screw adjustment and recording the tension with the transducer. These data about compliance were also used for calculating the internal work. From these relationships between tension and length the relation between tension and elastic energy in the compliance was calculated (Hill & Howarth 1959, Fig. 2). The internal work done during a contraction could then be found from the recorded tension. RESULTS Figure 2A shows the records of heat production obtained in an experiment of the type discussed in the introduction. The mechanical events are shown in Fig. 1. The lines in Fig. 2A have been constructed by measuring the records (six of isometric and six of isotonic contractions) at 10 msec intervals and adding up each group. The next stage is to subtract the total ordinates of the isometric records, at each 10 msec interval, from the corresponding total for the isotonic records. The result is shown by the points in Fig. 2B. (Only every other point is shown.) The vertical lines above and below the points represent the standard errors of these differences of the means of the two groups. The observations are smoothed as shown by the lower line in Fig. 2B, and analysed in the usual way to allow for the delay in the recording system, giving the upper line in Fig. 2B. The result of the experiment is now apparent: the difference of heat production is seen to start abruptly. As pointed out above, the extra shortening heat in the isotonic contraction can only cause a divergence of the heat records starting gradually. The abrupt start of the divergence can therefore be attributed to thermoelastic absorption of heat during the isometric contractions. Figure 3 shows in the upper curve the result of the analysis on a larger scale. (For the points on this curve see below.) The lower curve represents the difference of heat production which would have been caused by the extra shortening heat produced by the muscle when isotonic. The time course is naturally that of the difference of shortening (Fig. 1 B); it has been adjusted in size to make the final rate to which it tends equal to the steady rate of the observed difference of heat production. This corresponds to a value of a (the heat produced per cm shortening) of 15 g which is about a sixth of the maximum tension produced in a tetanus. This is slightly lower than, but comparable with, the values reported in other

6 216 R. C. WOLEDGE investigations (Hill, 1938, 1949a, b; Abbott, 1951). It is evident that the two curves could not be made to coincide by taking another value of a, since they are of different shapes E E _ EC r-.;o 25 x b 0 %._ I- _,6.# C B o 0 E 5 0 t Time after start of tetanus (sec) Fig. 2. Heat production recorded simultaneously with the tension and shortening shown in Fig. 1. A. Mean unanalysed records: (a) isometric contractions, (b) isotonic contractions. B. The points represent the difference between the mean isometric ordinate and the mean isotonic ordinate. The vertical lines indicate, above and below the points, the standard error of this difference of means. The lower line is a smooth curve drawn through these points. The upper line is the result of analysing the lower line to allow for the time lag in thermopile and galvanometer. Sensitivity of heat recording: heat equivalent to 0 17 g. cm would produce 1 mm steady, analysed deflexion, neglecting heat loss. It now remains to be shown that the difference between the two curves can be accounted for by the thermoelastic effect, which would be proportional to the difference of tension between the isometric and isotonic contractions (Fig. I A, c). The points in Fig. 3 were calculated by assuming that the relation between tension rise (AP) and heat production (AQ) is given by AQ = _ AP. 0 where lo is the standard length of the muscle. The calculated difference of heat production due to thermoelastic heat absorption was added to that caused by the shortening heat giving the points shown in Fig. 3. The result is a good fit with the observed difference of heat production.

7 HEAT AND ENERGY IN MUSCULAR CONTRACTION 217 In two other similar experiments the values of the thermoelastic heat: tension ratio (that is the numerical constant in the above equation) were found to be - 0*0075 and In one other experiment in which the load was rather larger, being lifted 180 msec after the start of the tetanus, the value was E / / -o// 0~ 0 v Time after start of tetanus (sec) Fig. 3. Difference of heat production between isometric and isotonic contractions. The upper line is the same as the upper line in Fig. 2B. The lower line is the calculated dlifference of shortening heat. The points are obtained by adding to the ordinates of the lower line the calculated difference due to thermoelastic heat absorption (see text). A number of similar experiments were made with very small afterloads, or none at all, so that the muscles started to shorten about 40 msec after the first stimulus. The results were variable and hard to interpret. The analysed difference of heat production was often found to start abruptly, but it could not generally be fitted by a combination of the calculated difference of shortening heat and the thermoelastic heat. The main reason for these anomalous results must be that the muscle is not contracting uniformly all over, so soon after the stimulus, so that the heat production recorded from a part only of the muscle is not comparable with the mechanical records made from the whole muscle. Two types of nonuniformity could affect the result: (1) non-uniform distribution of tension development across the muscle (Hill, 1961), and (2) non-uniform distribution of shortening along the muscle. Non-ulniformity of the second type

8 218 R. C. WOLEDGE must be expected when a muscle is not stimulated all over. Various types of stimuli have been tried in an attempt to reduce the difficulty: tetani of various frequencies with stimuli all in the same direction, or alternating in direction, single shocks several times the strength giving the maximum response, with various time constants. None was successful. Another difficulty in these experiments is that the calculation of the internal shortening is very uncertain at low tensions because the compliance of the series elastic component is larger and varies more from one sartorius to another in this region (Jewell & Wilkie, 1958, Fig. 8). It is not therefore possible to measure the thermoelastic heat: tension ratio at very early times in the contraction, and, since the values determined at fairly early times in the contraction were much smaller than those reported in other papers (Hill, 1953a; Woledge, 1961) the question arose whether the heat: tension ratio might increase during the course of a tetanus. Since the earlier observations had been made mostly after the tension had reached its maximum in twitch or tetanus, this would explain the larger values obtained. An experiment was therefore performed in which a comparison was made between isometric and isotonic contractions for two different isotonic afterloads. The smaller load was lifted at 75 msec after the start of the contraction, the larger one at 120 msec. For both groups a good fit was obtained between predicted and observed differences of heat production with a value of the thermoelastic heat:tension ratio of In another experiment the quick release method was used to find the value of the heat:tension ratio (Woledge, 1961) at various times after the start of a tetanic contraction. The results were as follows, at 0-2 sec , at 0-5 sec , at 1b5 sec -001 and at 9-2 sec In one further experiment the thermoelastic heat: tension ratio was found, by the method of comparing isometric and isotonic contractions, to be - 0*005 with a load which was lifted at 70 msec, and to be for a load which was lifted at 180 msec; by the method of quick release a value of was found at 200 msec. It cannot be claimed that these experiments are very accurate, because only a few records were made for each observation, in order to reduce the effect on the results of any progressive change in the muscle. They do show, however, that if there is any progressive change in the thermoelastic heat: tension ratio during the course of a tetanus it is not a large one. DISCUSSION Although the main object of the present experiments has been achieved, by demonstrating that the thermoelastic effect does occur during the early part of a contraction, the small values of the thermoelastic heat:tension

9 HEAT AND ENERGY IN MUSCULAR CONTRACTION 219 ratio obtained remain a puzzle. The average value is only about a half of that observed in earlier experiments using the quick release method (Woledge, 1961). The difference is not a seasonal one, since those experiments were made at the same time of year as the present ones. The animals in both groups of experiments were in good condition, they were obtained from the same source and treated in the same way. For various reasons the present experiments are likely to have given a less accurate result than the earlier ones: (1) A much larger 'protecting region' had to be used, and so the heat was sampled from a smaller part of the muscle. (2) To obtain the necessary speed of recording a less sensitive thermopile had to be used. The effect of mechanical disturbance of the galvanometer was therefore greater. (3) The muscle is not contracting so uniformly during the early part of the contraction studied in this paper as it is during the 'plateau' of a tetanic contraction studied in the earlier investigation. (4) In the quick-release experiments the value of the thermoelastic heat:tension ratio is based on the rapid heat production produced by the sudden fall of tension, as well as on the slower heat absorption during the gradual redevelopment of tension. In the present experiments, however, only the gradual development of tension could be studied. The thermoelastic effect is also harder to observe, during development or redevelopment of tension, because of the simultaneous production of shortening heat. There is, however, no known reason why these difficulties should produce a consistent underestimate of the thermoelastic effect. Considering the variability of the thermoelastic effect from one frog or toad sartorius to another reported here as well as in both the earlier investigations (Hill, 1953a; Woledge, 1961), it seems at least a possibility that, for an unknown reason, the muscles used in this investigation do have a smaller thermoelastic effect than those used two years ago. This possibility is a warning against applying a value determined on one muscle, or group of muscles, to correcting for the thermoelastic effect in any other muscle. The correction for thermoelastic heat absorption during the rising phase of an isometric contraction can be applied with any certainty only when the heat: tension ratio for the muscle is known. Therefore, to demonstrate the effect of this correction, it has been applied to some of the isometric heat records obtained in the present experiments. Figure 4A shows the average of the results obtained in four good experiments. The lowest line represents the heat production observed, analysed in the usual way. The middle line represents the 'true' heat production, that is the observed heat production corrected for the effects of thermoelastic heat absorption, the heat which must actually have been produced by the chemical processes occurring in the muscle. The top line represents the total energy

10 WOLEDGE liberated by the muscle both as heat and work. It is calculated by adding to the 'true' heat production the work done by the muscle in stretching the compliance in series with it. Figure 4B shows the rates of observed heat production, 'true' heat production and energy liberation during the early part of a tetanic contraction. The graphs were constructed from the data shown in Fig. 4A, with one modification: in Fig. 4A the maximum rate of heat production is at about 60 msec. The true maximum rate was probably earlier, as the much finer analysis made by Hill (1953b) showed a maximum heat rate before 30 msec. This is before the tension has started to rise and is therefore not altered by the thermoelastic effect. The present results have therefore been slightly modified to bring the maximum rate to 30 msec. It can be seen from Fig. 4B that the correction for the thermoelastic effect oa0 E 100 c 00~~~~~~~~~~~~~~~~0 C 50 0)00 1 c ~~ oo 750~ ~~~~~~~~~~~~~~~~~0 0 V* *Z E u " 0) '0 250 a)~~~~~~~~~~~~~~~~~~~~~~~ 0* Time after start of tetanus (sec) Fig. 4A. Heat production, energy liberation and tension development during the early part of an isometric tetanic contraction of toad sartorii at 00 C. Mean of the results of 4 experiments. Muscles weighed 150, 140, 59, 84 mg; lo = 30i, 29, 24k, 29mm. (a) Observed heatproduction. (b) 'True' heatproduction obtained by correcting the observed heat production for the thermoelastic effect, using the heat: tension ratio determined for each muscle. (c) Energy liberation, obtained by adding to the true heat production the work done in stretching the series compliance. (d) Tension developed. B. The rates of (a) observed heat production; (b) 'true' heat production; (c) energy liberation during the early part of an isometric tetanus. Same experimental results as in A, slightly modified (see text). b

11 HEAT AND ENERGY IN MUSCULAR CONTRACTION 221 does not alter the conclusion of Hill that the maximum rate of heat production is reached very early in the contraction and that the rate falls thereafter. Correction of the heat production for the thermoelastic effect increases the size of the initial outburst of heat and energy at the start of an isometric tetanic contraction. (It must be remembered when considering Fig. 4 that for some muscles the correction for the thermoelastic effect would be two or three times as great as it is in this case.) This is due to two distinct causes: (1) the internal shortening occurring as the tension rises causing the production of both shortening heat and work, and (2) the fall in the rate of maintenance heat which continues after the maximum tension is reached (Abbott, 1951; Aubert, 1956). According to Aubert the steady rate of maintainance heat production eventually reached in a long tetanus of toad sartorius muscle at O C it 50 g. cm/g. sec. This figure can be used to find the relative contributions of the two causes of the initial outburst of heat: during the 0 3 sec shown in Fig. 4A maintenance heat production at the final steady rate would have produced about 15 g. cm/g. The rest of the 'true' heat production up to this time (65 g. cm/g) is a combination of the shortening heat and the greater rate of maintenance heat production. The contribution of the shortening heat has been calculated and is about 25 g. cm/g. Thus the outburst of extra heat up to 0-3 sec in a contraction which was perfectly isometric would be about 2/3 of that observed when the usual amount of internal shortening is occurring. Similarly the maximum rate of heat production would be reduced to about 2/3 if shortening could be altogether prevented. The maximum rate of shortening (vo) for a toad sartorius at 0 C is about 10/2/sec (Hill, 1938). The maximum rate of shortening heat production is therefore lo a/2m g. cm/g. sec (M is the weight of the muscle); putting a = P0/4 (Hill, 1938) we have Po. 10/8M. Since Po. 10/M is about 2000, the maximum rate of shortening heat production must be about 225 g. cm/g. sec. If this is subtracted from the maximum rate of heat production, the remainder (500 g. cm/g. sec) is still ten times greater than the rate of heat production at the end of a long tetanus. Thermoelastic heat and shortening heat It has been shown in this paper that the difference in heat production between isometric and isotonic contractions is due to a combination of the heat of shortening and the thermoelastic effect. Before present knowledge of the thermoelastic properties of the muscle was attained, the whole of the difference of heat production was attributed to the heat of shortening. What was the effect, if any, on the measurements of the shortening heat, of this failure to take into account the thermoelastic effect? If the

12 222 R. C. WOLEDGE shortening heat had been obtained by a comparison of the heat production in isometric and isotonic contractions then it would have been overestimated. Fortunately this method of obtaining the heat of shortening was avoided for another reason: because it was realized that the muscle was in fact shortening during the rising phase of an isometric contraction. Hill (1938) discusses this difficulty and avoids it in his chief experiments by measuring the total extra heat produced by a given total shortening. The comparison was made between a contraction in which the muscles were isometric throughout, and one in which they were released, allowed to shorten, and then to redevelop tension. The extra heat is measured, with correction for the change in the isometric heat rate, at the end of the redevelopment of tension. The net effect both of internal shortening and of the thermoelastic heat is therefore zero. Extra heat was produced by the thermoelastic effect when the muscle was released but was reabsorbed as the tension redeveloped. The shortening heat is thus correctly estimated. Hill (1949a) again avoided any error from the thermoelastic effect. In this paper the chief quantitative comparisons are made between isotonic contractions under different loads, when, since the tension is constant, there is no thermoelastic heat change. Comparisons between isotonic contractions and isometric contractions, or isometric parts of contractions, were avoided, to escape any error caused by the internal shortening. Abbott (1951) does obtain the shortening heat by the difference between isometric and isotonic contractions. His procedure was to release a muscle during a long isometric tetanus and then to measure the extra heat which appeared as the muscle shortened. Some of this extra heat must have been the thermoelastic heat which appeared when the muscle was released. This would alter his diagram (his Fig. 5) making the line lower (by an amount perhaps 10 % of its maximum) but not altering the slope of the line. The value of a would therefore be unaffected by this correction. SUMMARY 1. Experiments have been made on toad sartorius muscles to find out whether the rise of tension at the start of an isometric contraction causes a thermoelastic heat absorption similar to that known to accompany redevelopment of tension after a quick release. Comparisons were made of the heat production during isometric and afterloaded isotonic contractions. 2. With an afterload such that the muscle (at 00 C) becomes isotonic about 80 msec after the start of the contraction, the difference of heat production between isometric and isotonic contractions can be accounted for by the thermoelastic heat absorbed in the isometric contraction, together with the extra shortening heat produced in the isotonic

13 HEAT AND ENERGY IN MUSCULAR CONTRACTION 223 contraction. The thermoelastic heat: tension ratio was about a half of that reported in the earlier investigation, in which the quick-release method was used. 3. When similar experiments were made with smaller afterloads the results were very variable. This is probably due to the uneven start of the contraction inevitable when a muscle is not stimulated all over. 4. A few experiments were made to find out whether the thermoelastic heat: tension ratio increases during the course of a contraction. No such effect was detected. 5. The reason for the small size of the thermoelastic effect is discussed. Although the method used is not very accurate there is no reason why it should consistently underestimate the thermoelastic effect. 6. The isometric heat record is corrected for the thermoelastic heat absorption and the work done is added, to obtain the total energy liberation. 7. The possibility of an error in various published measurements of the shortening heat, due to failure to take account of the thermoelastic effect, is discussed. It is concluded that it has caused no serious errors. I should like to thank Professor A. V. Hill under whose direction this work was carried out, for his constant guidance and encouragement. I am also grateful to Dr D. R. Wilkie for much valuable advice. The work was done during tenure of a Medical Research Council scholarship for training in research methods. REFERENCES ABBOTT, B. C. (1951). The heat production associated with the maintenance of a long contraction and the extra heat produced during large shortening. J. Physiol. 112, ABBOTT, B. C., AUBERT, X. & HILL, A. V. (1951). The absorption of work by a muscle stretched during a single twitch or a short tetanus. Proc. Roy. Soc. B, 139, AUBERT, X. (1956). Le couplage energetique de la contraction musculaire. These d'agr6gation de l'enseignement superieur. Universit6 catholique de Louvain. Brussels: Editions Ascia. HILL, A. V. (1938). The heat of shortening and the dynamic constants of mlscle. Proc. Roy. Soc. B, 126, HILL, A. V. (1949a). The heat of activation and the heat of shortening in a muscle twitch. Proc. Roy. Soc. B, 136, HiLL, A. V. (1949b). The energetics of relaxation in a muscle twitch. Proc. Roy. Soc. B, 136, HiLL, A. V. (1949c). Myothermic methods. Proc. Roy. Soc. B. 136, HILL, A. V. (1949d). The onset of contraction. Proc. Roy. Soc. B, 136, HiLL, A. V. (1953a). The instantaneous elasticity of active muscle. Proc. Roy. Soc. B, 141, HILL, A. V. (1953b). Chemical change and mechanical response in stimulated muscle. Proc. Roy. Soc. B, 141, HILL, A. V. (1961). The heat produced by a muscle after the last shock of a tetanus. J. Physiol. 159, HILL, A. V. & HOWARTH, J. V. (1957). Alternating relaxation heat in muscle twitches. J. Phy8iol. 139,

14 224 R. C. WOLEDGE HIL, A. V. & HOWARTH, J. V. (1959). The reversal of chemical reactions in contracting muscle during an applied stretch. Proc. Roy. Soc. B, 151, JEWELL, B. R. & WILKIE, D. R. (1958). An analysis of the mechanical components in frog's striated muscle. J. Physiol. 143, JEWELL, B. R. & WILKIE, D. R. (1960). The mechanical properties of relaxing muscle. J. Physiol. 152, WOLEDGE, R. C. (1961). The thermoelastic effect of change of tension in active muscle. J. Physiol. 155,