JOURNAL OF APPLIED PHYSIOLOGY

Vol. 26, No. 6,June 1969. Printed in U.S.A.

Energy utilization in intermittent exercise

of supramaximal intensit

R. M ARIA, R. D. OLIVA, P. E. DI PRAMPERQ,

AND P. CERRETELLI

Debartment of Physiology, University of Milan, Milan, Italy A

MARCARIA, K., R. II. OLIVA, P. E. DI PRAMPERO, AND Pm CERRETELLI. Energy utili<ation in intermittent exercise of supa- maximal intensity. J. Appl. Physiol. 26(6) : 752-756. 1969.~In supramaximal exercise the extra energy which is not met by oxidation is drawn from splitting of high-energy phosphate, and only when this source is exhausted is energy drawn from the other anaerobic source, the splitting of acid. In strenuous intermittent exercise, no lactic acid is formed if the oxygen debt contracted during the working period can be met completely by the alactic phosphagen-splitting mechanism; the oxygen debt contracted during the working period must then be completely paid during the rest period. If these condi- tions are met, very heavy intermittent exercise can be carried out indefinitely, leading to a total amount of work much greater than would have been possible were the exercise protracted continuously until exhaustion. The payment of the alactic oxygen debt fraction is confirmed to be a fast process, the half- reaction time being about 20-25 set; the capacity of this mechanism in young fit nonathletic subjects is about 20 ml/kg body wt.

muscular exercise, intermittent; oxygen debt, alactic-lactic; lactic acid, formation in intermittent exercise

I T HAS BEEN SHOWN in previous work that when a sub- ject is involved in very strenuous exercise leading to exhaustion in about 30-40 set (running on a treadmill at

kmjhr at an incline of + 15 %) lactic acid produced only after lo-15 see from the beginning of the exercise, presumably when the anaerobic energy stores, as given by the high-energy phosphate compou (phosphagen = ATP + CP) are exhausted, or h reached a critical level (7).

It may then be reasonably assumed that if this exercis is sustained only 10 set, only an alactic oxygen debt is contracted. Since the speed of payment of this fraction of the oxygen debt is very high, the half-time being about 20--30 see: (4, 8j, a very short period of recovery may be suficient to refill the phosphagen stores to enable the subject to perform the exercise again on the same energy source. No appreciable lactic acid formation and ac- cumulation in the blood would then take place and the

exercise could be repeated indefinitely, thus summing up to a total amount of work much greater than would have been possible had the work been carried out uninter- rupted until exhaustion.

On this hypothesis the following experiments were lanned.

METHODS

Three subjects, whose vital statistics are given in Table 1, came to the laboratory a few hours after the last meal, and, after 30-40 min of rest, a sample of blood was drawn from the arm vein for lactic acid determination ( enzymatic method) ( 1) .

It was known from previous experiments that when running on a treadmill at 18 km/hr and at + 15 cline, these subjects reached exhaustion after 30-40 set, and that at the end of the run the maximal lactic acid concentration in blood amounted to 50-60 mg/ 100 ml.

The experiments consisted of a run at the rate indi- cated for only 10 set, after which the subjects were allowed a. period of t lasting, in three different series, 10, 20 and 30 set, r ectively. After the rest period the subjects ran again fo 0 see, and so on, until exhaustion or a steady state was reached.

For each series of experiments the subjects came to a stop after a predetermined number of runs, after which blood was withdrawn 3 and 5 min from the end of the exercise. The maximal lactic acid value reached at these times was assumed to represent an equilibrium condition of lactic acid concentration in the body fluids, and the total lactic acid produced could then be calculated. This was considered to be produced during the exercise, since the amount of lactic acid that disappeared in the interval between the end of the exercise and the collec- tion of blood was considered negligible; the disappear- ance of LA from the blood is in fact a very slow process

(8) . On different days the subjects performed successively

) 15, and 20 runs for the different series of experi- rnents, and at the end of each period lactic acid was deter- mined as described. With this technique the concentra- tion of lactic acid in blood or the amount of lactic acid

752

ENERGY BALANCE IN INTERMITTENT EXERCISE 753

Subj Age, yr Weight, kg Height, cm Total Vozmsx. ml/kg per min --

GR 25 70 180 57.5 AC 18 71 176 58.0 AR 22 65 179 55.0

terrupted run), 10, 20, and 30 set, respectively, is shown in Table 2 for all subjects. When the rest period was 10 set, the total running time could be increased about three times and when the rest period was 20 set, six times. In the case of the 30-set pause the exercise could be carried out indefinitely.

Lactic acid formation. The concentration of lactic acid in blood increases at a high rate in the first few runs in all subjects and for all series of experiments to attain a steady rate after about the fifth run (Fig. 2). The lactic acid formation in milligrams per 100 ml per run or its 02 equivalent (44 ml OS/g lactic acid produced (6)), is given for all subjects in Fig. 3. From this graph it appears that at steady state u) no lactic acid production takes place if the period of rest is 30 set; and b) the increase of lactic acid concentration in blood due to a lo-set run period increases with decreasing the duration of the rest period, from 3 mg/lOO ml for a 20-set rest to 11 mg/lOO ml for a lo-set rest period.

With a plot of the lactic acid produced (or the lactacid oxygen debt built) as a function of the length of the rest period, the minimal time of rest at which no lactic acid formation takes place can be appreciated graphically by extrapolation of this function to LA = 0 (Fig. 4). This appears to be on the average about 25 set, which is,

TABLE 2. Total time of performance of a supramaximal intermittent work of 10 set duration (running on a + 15 9% incline at 18 km/hr) with change in duration of rest periods as indicated

FIG. 1. Record of PCOZ at the mouth, VE and PEO% in expired air measured during the intermittent work. Working period is indi- cated on time line.

formed as an effect of work could be related with the duration of the intermittent exercise, or with the number of runs.

In some experiments, the oxygen consumption was measured continuously during the whole period of inter- mittent exercise. The expired air was sampled from a mixing chamber placed on the expiratory line and ana- lyzed by a fast-response 02 electrode (90 % of the re- sponse in 0.5 set), whereas the ventilation was measured on a breath-by-breath basis from a spirograph tracing. A correction was made for the time lag in the response of the Polarograph depending on the gas flow and on the dead space of the system, in order to align the VE with the PEAR. A typical record is shown in Fig. 1. In all other experiments the 02 uptake of the subjects was measured by collecting in a 150-liter Tissot spirometer the air expired in a given number of cycles, not including the first five, and by analyzing it for 02 and CO2 by means of a Scholander apparatus (“steady-state 02 consump- tion”).

Subj Uninterrupted, IO&x Pause, 20&c Pause, 30.Set Pause, xc set set set

GR 38 100 200 AC 32 90 210 Indefinite AR 33 200

Time for the 20-see pause series of experiments is actually a little higher then indicated as the subjects were nearly, but not completely, exhausted at the end of the experiment.

r L. A. b mg%

100

10-10 cy /I

Subj. A.C.

I I I I I

'O-lo " Subj. A.C.

1

c

runs

RESULTS AND DISCUSSION

The total time of the run leading to exhaustion when the interval between the running periods were 0 (unin-

FIG. 2. Blood LA concentration above resting level in inter- mittent work (10 set) as a function of number of runs for three series of performances, with resting periods of 10, 20, and 30 set, respectively (subject AC).

7 734 MARGARIA, OLIVA, DI PRAMPERO, ,4ND CERRETELLI

therefore, the minimal time necessary to pay the oxygen debt contracted during the lO-set exercise on pure alactacid ( phosphagen) sources only, without involving the glycolytic mechanism. In this case, at the end of the 1 0-set exercise period the alactic energy sources must be reduced to a minimum if the assumption that LA is

built only when the alactic mechanism is exhausted, or when it has reached a critical level, is valid.

Since the half-reaction time for the alactic oxygen debt payment is about 25 set (4, 8>, and since 25 set is the minimal time of the interval between runs that allows the exercise to be carried out with no lactic acid forma- tion, at steady state each run must involve an alactic oxygen debt contracted which amounts to half the alactic capacity. The alactic oxygen debt pool then

20

15

d LA. d run oscillates during this kind of exercise between 0 at the

end of the run and 50 70 at the end of the 25-set recovery period as indicated schematically in Fig. 5.

0 xygen consump hon. In Fig. 6, VOW on a breath-bv- breath basis from tracings such as in Fig. 1 is shown for’a subject AC) p er orming f 10 bursts of exercise with a 20-set pause. VOW appears to increase very fast at the beginning of the exercise and in the first pause it is not appreciably different from the first running period. At steady state, van is sensibly less during the pause than

mgz ( 1 run

x)

5

0 10 . 15 + 20

runs

FZG. 3. Rate of LA production in mg/lO ml and per run, or in

its 02 equivalent in ml/kg body wt, as a function of the number of runs. 8 subject AC; l subject AR ; -i- subject GR.

10 20 30 40 pause (set)

FIG. 5. Contribution of the alactic Voz al and lactacid pool

PIG. 4. Rate of LA production at steady state (avg of all sub- ects) expressed as its O:! equivalent (ml Oz/kg body wt and per un) as a function of the duration of the pause period.

vo 2 1act in percent of the total alactic capacity to the energy re- quirement. Working period lasts 10 set, the pause 25 set as indi- cated by vertical thin lines (schematic).

5

%,

I/min

4

1

3 -

0

60 90 120

2 0 0’ 00

4 0

l- O

1

0

210 240 270 set 300 0 30

FIG. 6. Breath-by-breath 02 uptake as a function of time during 2O-set pause). Some data at about the middle of the res

intermittent exercise (18 km/hr -l- 157& lo-set exercise followed by were not collected (subject AC).

.ing periods

ENERGY BALANCE IN INTERMITTENT EXERCISE 755

TABLE 3. Average 0, consumjhon after jifth rUn (steady State)

together with LA production expressed in O2

“4 uivalents *for subj’ect AC

Type of Exp lo-Set Run, lo-Set Run, lo-Set Run, lo-Set Pause 20-Set Pause JO-Sect Pause

I

____--____ l__l-_____- - -______-__)

Net &Q, ml/kg per min ~‘o~LA, ml kg per min

during running; an average Tjoz value however can be obtained graphically without incurring in an appre- ciable error.

Since the energy cost of the exercise is known (5) and the average oxygen consumption in these experiments has been measured (see Table 3), the oxygen consumed during the run, the oxygen debt contracted, and the 02 debt paid during the pause can be calculated.

Running at 18 km/‘hr on an incline of + 15 % involves a net energy expenditure of 1.8 Cal/m per kg equivalent to an oxygen consumption of 108 ml/kg per min, or 18 ml:/kg per run (5). Since the maximal net oxygen con- sumption for these subjects was 54 ml/kg per min, the oxygen consumption during the IO-set run at steady state could not be higher than 5416 = 9 ml. This figure how- ever should be reduced, because when the pause amounts to 25 set the average oxygen consumption is appreciably lower than the maximum (see Table 3 and Fig. 6)? and the average oxygen consumption for the lo-set exercise period cannot be expected to be higher than 7 or 8 ml. If we assume that during the run 8 ml/kg of oxygen are actually used up, then about 10 ml are to be accounted for bv the alactic oxygen debt. As this is paid in 25 set, this should therefore be half the total alactic 02 debt, which is supposed to have reached its maximal value at the end of the lo-set run. The alactic capacity of the subject should therefore amount to about 20 ml oxygen kg, i.e., about 100 Cal/kg. This is a figure of the same order of magnitude as that found previously (6, 7 j.

GENERAL DISCUSSION

Because the oxidative mechanism is rather sluggish, and it takes about 1 min to reach full level, during the first run the subject is in a state of relative anoxia (4, 9). More energy is then drawn from anaerobic mechanisms and less from the oxidative ones, and since the capacity of the alactic mechanism is limited the body has to rely on the lactacid mechanism to fulfill its metabolic reuuire- merits. Only after 1 min or more from the beginning o the exercise does the oxygen consumption reach its maximum, and less energy is therefore drawn from the anaerobic rnecha.nisms. If the rest period is longer tha 25 set the phosphagen source is then adequate and no call is made on the lactic mechanism.

Figure 5 shows schematically the energy drawn from the pools of the alactic and lactacid energy as a function of time in a performance consisting of 10 set of exercise alternated with 25 set of rest. The first run takes place

almost exclusively (90 %) at the expense of the alactic oxygen debt, and the availability of this source is reduced at the end of the run to only 10 %. During the following 25 set of rest, the alactic pool is restored to about 45 % by the oxygen debt payment. On the second run period a greater amount of energy is paid by the oxidative mecha- nism during the run, and consequently the oxygen debt contracted is less than in the first run, i.e., about 75 % of the total energy requirement. However, this amount is too high to be sustained by the alactic mechanism only, since the alactic pool at the beginning of the second run is only 45 CT0 of the initial resting value; an appreciable amount of lactic acid is then. formed. During the next 25 set the alactic oxygen debt is paid at a higher rate than in the first interval, because the alactic energy pool is practically exhausted and because the oxygen con- sumption is higher than at the end of the first run. On the third cycle the actual oxygen consumption during the running period is still higher than during the second run, and therefore a lesser oxygen debt needs to be built while a higher amount of the debt is paid in the restin period. In the following cycles a progressively higher amount of the oxygen debt is paid during the 25-set pause and a lesser amount is contracted during the running periods, until after 4-5 runs a steady state is reached.

The payment of the lactacid oxygen debt is not shown in Fig. 5 because it is negligible, the rate of lactic acid disappearance from blood in recovery being very low (8).

In the experiment in which the pause was only 10 set, the oxygen consumption during the lo-set work period is nearly maximal when a steady state is reached. It can then be assumed that in this condition, for example, from the 6th to the 10th run, 9 of the 18 ml of oxygen neces- sary to cover the cost of the lo-sec. run are actually used during the run, and 9 ml are drawn from the anaerobic energy sources. If we assume, as before, that the rate of the alactic oxygen debt payment is such that the half- reaction time is 25 set, in lo-set recovery only about 25 % of the oxygen debt is paid, corresponding to 0.25 x 20 = 5 ml oxygen/kg body wt and per run. The remaining amount (4 ml) of the energy required for the run is then to be charged on the lactacid fraction of the oxygen debt. This amounts to 20 cal, equivalent to 0.09 g lactic acid (6) which corresponds to an increase of about 12 mg/ 100 ml of the lactic acid concentration in blood per run, approximately the figure actually found and given in Fig. 1.

In the experiments in which the pause was 20 set, at the end of this time 43 %I of the alactic oxygen debt is paid, corresponding to 8.6 ml. Since in this condition when a steady state is reached, i.e., after the fifth run, the oxygen consumption during the IO-set run can be assumed to be about 8 ml (Fig. 61, only 18 - 8.6 - 8 = 1.4 ml oxygen/kg and per run arc left on charge of the glycolitic mechanism. This corresponds to an increase of lactic acid in blood of about 4 mg/lOO ml and per run, as actually found.

Experiments on intermittent exercise were performed

756

recently by Keul et al. (2, 3) on subjects exercising on a bicycle ergometer. The intensity of the exercise was 350 w and it lasted in two series of experiments 30 see or 1 min, the rest period being 1 min in both series. In the 30-set exercise series a very limited increase of blood lactate (about 26 mg/ 100 ml) took place at the beginning of the exercise, but no increase of lactic acid was observed when a steady-state condition was reached. In the I-min exercise series, the lactic acid increased progressively to the end of the experiment ( 10 runs) up to about 100 mg/ 100 ml. Evidently in this case a steady state in oxygen consumption could not be reached and lactic acid was continuously produced. An exercise of 350 w corre- sponds to about 5 kcal/min of mechanical work, or, if we assume an efficiency of 20 % for this type of exercise, to an energy expenditure of 25 kcal/min, or of 5 liters/ min of oxygen consumption. The maximal oxygen con- sumption of the subjects is not given, and therefore it is not possible to calculate exactly the role played by the alactic and the lactacid mechanisms. Evidently the in- tensity of the exercise did not exceed greatly the maximal aerobic power of the subjects and the oxygen debt built in the 0.5-min exercise period could be paid completely in the I-min pause, whereas the debt contracted in the 1-min period of exercise was presumably greater than the alactic fraction. We calculated the excess from the increase of lactic acid in blood at the end of this exercise. It amounted to 4 mg/lOO ml lactic acid per run, corre- sponding to 1.4 ml/kg 02 or to about 100 ml for the whole body. If we assume that the maximal alactic oxygen debt in these subjects was about 1,400 ml, the oxygen needed to fill up the 5,000 ml 02 requirement during the 1-min exercise amounts to 5,000 - 100 - 1,400 = 3,500 ml, a value which is presumably of the order of magnitude of the maximal 02 consumption for these subjects. In the 0.5-min run, the oxygen require- ment being only 2.5 liters and the oxygen actually con-

REFERENCES

1.

2.

3.

4.

5.

GERCKEN, G. Die quantitative enzymatische Dehydrierung von L (+) Lactat fur die Mikroanalyse. 2. Physiot. Chem. 320: 180- 186, 1960. KEUL, J., E. DOLL, D. KEPPLER, AND H. REINDELL. ZurBedeu- tung der Lactatbildung bei Intervallarbeit. Z. KreisZau$orsch. 56 : 823-830, 1962. KEUL, J., E. DOLL, D. KEPPLER, AND H. REINDELL. Intervall- training und anaerobe Energiebereitstellung. Spot-tar& Sportmed. 1‘2: 493-496, 1967. MARGARIA, R. Energy source for aerobic and anaerobic work. In : Exercise at Altitude, edited by R. Margaria. Amsterdam: Excerpta Medica Foundation, 1967, pp. 15-32. MARGARIA, R.,P. CERRETELLI, P. AGHEMO,AND. G. SASSI. Energy cost of running. 9. AMd Physiol. 18: 367-370, 1963.

MARGARIA, OLIVA, DI PRAMPERO, AND CERRETELLI

sumed during the run about 1.75 liters, only an additional 0.75 liter oxygen is required, and obviously this can be met completely by the alactic fraction only of the 02 debt.

In conclusion, the behavior of the lactic acid produc- tion, as observed in very heavy intermittent exercise, is in substantial agreement with the predictions made on the basis of previous findings which are therefore supported by the present experiments. Particularly, the following points are confirmed. a) In supramaximal exercise, the energy for the contraction is not drawn from the glyco- litic mechanism until the high-energy phosphate (phos- phagen) sources are exhausted or they reach a critical level. This alactic mechanism for providing energy in muscular contraction always precedes chronologically the lactacid mechanism. b) Payment of the alactic oxygen debt takes place during recovery at a very fast pace. It is an exponential process with a half-reaction time of the order of 20-25 sec. c) The capacity of this mechanism is about 20 ml oxygen/kg body wt. d) Very heavy inter- mittent exercise can be carried out indefinitely if the in- tensity of the exercise and its duration are such as to in- volve an energy expenditure, besides that sustained by the actual 02 consumption, not greater than that corre- sponding to the alactic fraction of the oxygen debt, and if the rest periods are long enough to allow the payment of the oxygen debt during the exercise period. If the recovery period is too short, and the oxygen debt con- tracted during the working period cannot be paid com- pletely, the energy balance is filled up by an energeti- cally equivalent amount of lactic acid built in the muscles.

This work has been supported by a grant from the Italian Na- tional Research Council (CNR).

R. D. Oliva is a Fellow of the World Health Organization.

Received for publication 3 December 1968.

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