Clinical Science (1973) 44,479491.
METABOLISM O F E X E R C I S I N G A N D R E S T I N G
H U M A N SKELETAL MUSCLE, I N T H E
POST-PRANDIAL A N D F A S T I N G STATES
P A T R I C I A G. B. B A K E R
AND
R . F. M O T T R A M
Department of Physiology, University College, Cardiy
(Received 17 July 1972)
SUMMARY
1. Methods are described for study of metabolism of human skeletal muscle in situ,
at rest and during mild sustained contraction in the fed and fasted states.
2. At rest the average oxygen uptake was 0.29 ml min-l 100 ml of muscle-' and the
carbon dioxide output was 0.22 ml. Glucose uptake was 0.49 mg min-l 100 ml of
muscle-'. The respiratory quotient was 0.75, indicating that most of the glucose was
being stored.
3. When subjects made hand-grips of 5% of their maximal voluntary contraction
force (5% MVC) the oxygen and carbon dioxide exchanges both increased by six times
while the glucose uptake increased by 70% of the resting value.
4. A 7 h fast before the observations were made severely decreased both resting and
exercising glucose uptake but produced no other alteration in the metabolism of the
muscle.
Key words : skeletal muscle, respiratory quotient, oxygen uptake, carbon dioxide
production, glucose uptake.
Metabolism of exercising skeletal muscle has been extensively studied in excised nervemuscle preparations, chiefly obtained from frogs and, at the other extreme, by studies of
respiratory gas exchange in man. This paper is concerned with studies performed on human
muscle in situ. The technique used is a modification of that originally designed by Chauveau &
Kaufmann in 1887, in which the products of blood flow and arteriovenous differences give the
metabolic uptakes or productions of the substances studied.
It was intended that studies should be made during exercise as well as at rest. During exercise
it was essential to ensure that blood flow measurements and samples were obtainedunder
steady-state conditions when metabolic activity and the exchange of materials between tissue
and blood are in equilibrium.
With the isometric hand-grip ergometer described by Clarke, Hellon & Lind (1958) mild
Correspondence: Dr R. F. Mottram, Physiology Department, University College, Cardiff CFl 1XL.
479
480
Patricia G. B. Baker and R. F. Mottram
(5-10% maximal voluntary effort) contractions of the flexor muscles in the forearm can be
maintained for several minutes while a new equilibrium state is achieved with respect to both
metabolic rate and blood flow. The criteria for this state have been described (Mottram, 1973).
Under these conditions blood samples were obtained from the deep forearm veins and their
O,, CO,, glucose and haemoglobin concentrations determined and compared with those of
arterial blood. Brief reports of some of these results have already been published (Mottram,
1971; Baker & Mottram, 1968a, b). Similar studies, but using intermittent contractions for a
longer time, have been reported by Whichelow, Butterfield, Abrams, Sterkey & Garratt (I 968),
by Zierler, Maseri, Klassen, Rabinowitz & Burgess (1968) and Wahren & Hagenfeldt (1968).
Since Kontos, Richardson & Patterson (1966) did not achieve an equilibrium state in their
studies with sustained hand-grips, their results also are not directly comparable with those
reported here.
METHODS
The use of the ergometer and the plethysmographic method of blood flow recording are
described in the accompanying paper (Mottram, 1973). The only alterations in the techniques
were to occlude the veins for 6 s periods at 20 s intervals thus obtaining three records of blood
flow each minute, and to obtain blood samples as described below. Blood flow was usually
recorded for 1 min before, 4 min during and for 3 min after each period of exercise. Since the
venepunctures and catheterization of deep forearm veins carry a risk (< I %) of either cutaneous
sepsis or local vein thrombosis risk, we ensured that all subjects volunteered after they had
understood the nature, purpose of, and risks involved in the proposed studies.
Venous samples were withdrawn through an 18 gauge nylon catheter threaded into a deep
forearm vein via one of the veins connecting the deep and superficial veins in the antecubital
fossa. Samples were withdrawn into paraffinized all-glass syringes, with dead spaces filled
with potassium fluoride solution.
The samples were withdrawn only in the last 9 of the 14 s of rest between successive 6 s
venous occlusion periods. This timing prevents the contamination of deep with superficial
forearm vein blood which can occur, due to diversion of skin vein blood into the deep veins
during venous occlusion (Roddie, Shepherd & Whelan, 1956; Coles, Cooper, Mottram &
Occleshaw, 1958).
As soon as the syringes were filled, they were sealed with mercury-filled caps and placed in
ice-water mixture for subsequent analysis of 0,, CO,, glucose and haemoglobin.
Oxygen and carbon dioxide contents were measured on the Van Slyke manometric bloodgas apparatus. Blood glucose analyses were performed on the Tecnicon AutoAnalyzer, using
the K,Fe(CN)6 reduction method. Because this method is not specific for,glucose the word
‘sugar’ has been used when referring to blood concentrations. When arteriovenous differences
or metabolic uptake are considered it is assumed that the non-glucose reducing substances
were equally present in arterial and venous blood and that A-V differences thus represent only
glucose. Haemoglobin was measured by the cyanmethaemoglobin method on a Unicam SP.600
spectrophotometer.
It was necessary in these studies to obtain samples of arterial blood for analysis, so that
arteriovenous differences could be calculated. Arterialized venous samples were obtained by
the method of Goldschmidt & Light (1925), previously tested by Mottram & Brown (1963).
Metabolism of human muscle
481
In some later studies the 45°C water bath used to heat the hand was replaced by an electric
heating pad, as the studies of Brooks & Wynn (1959) indicated that this milder heatingprovoked
sufficient vasodilation to ensure that the dorsal wrist veins contained arterial blood.
Venous blood samples were taken in the minute before each period of contraction, during
the third and fourth minute of the contraction period and in the second and third minute of
the recovery period (Fig. 1). Arterialized samples were taken before and after each of these
8 min periods of blood flow recording and venous blood sampling.
Work
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FIG.1. The timing of vascular occlusions, blood sampling and exercise during a single period of
determining muscle metabolism. Three replicate periods were studied in each subject at 25 min
intervals.
In a single study three replicate contractions were usually performed, at about 25 min
intervals. The studies were usually performed 2-3 h after the normal midday meal. The subjects
were encouraged to eat well before coming to the laboratory. In this way we hoped to perform
the studies during the period of active and reasonably steady glucose entry into muscle previously found by Mottram & Brown (1963). Six of the subjects were willing to submit to a
second study on a day when they had not eaten at midday; these repeat experiments were thus
performed after 6-7 h of fasting.
Forearm blood flow measured at rest was corrected using the observations on skin blood
flow and the composition of forearms reported in Cooper, Edholm & Mottram (1955). Flow
through the exercising muscle was determined from the whole limb blood flow during exercise
as described in the previous paper (Mottram, 1973).
The flow through resting muscle when the forearm is at rest may be determined from the
whole forearm blood flow by the method of Cooper et al. (1955). Humphreys & Lind (1963)
showed that during the exercise performed in these studies about half of the forearm muscle
remains at rest. Since the deep venous blood sampled during exercise contains contributions
from both resting and active muscle, the observed arteriovenous differences ( y ) were corrected
by the following formula to obtain the arteriovenous difference (z) across exercising muscle:
z=
+
( 0 . 5 ~ b)y -0 . 5 ~ ~
b
Patricia G . B. Baker and R. F. Mottram
482
where a is the flow through resting muscle when the forearm is at rest, b is the flow through
exercising muscle and x is the resting A-V difference.
In the calculation of metabolic exchanges during exercise, if the second determination of
0, uptake made during the fourth minute of contraction differed by more than 10% from the
Lrst determination made 1 min earlier, it was cclfisiaerea tnat equlllarluw nau Been acnleveu.
This value of 10% was derived from Mottram (i955). This method would not detect disequilibrium if blood flow and A-V difference changed in opposite directions, but such changes never
occurred.
RESULTS
Altogether twenty-two experiments were performed in the fed state. In all studies results were
obtained from two or three replicate contractions at 5% of each subject's own maximal
voluntary contraction. Six of the studies in the fed state and two of those in the fasted state
15r
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3
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0
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2
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4
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Time (min)
'
FIG.2. The arteriovenous differences for O,, CO, and glucose and the blood flow before, during
and after 4 min contractions at 5% of maximal effort in two subjects (each contraction was performed in triplicate at 25 min intervals and results were averaged). Equilibrium is reached in the
experiment shown on the right but not in that on the left.
8
Metabolism of human muscle
483
had to be rejected because there was evidence, either from blood flow or from the venous blood
samples, of failure to reach equilibrium state. An example of a rejected study is shown in the
'
left-hand half of Fig. 2.
Blood sugar concentrations in the resting state
Arterial values. There was no significant difference between the values found in this work
and those previously reported (Mottram & Brown, 1963) for post-prandial arterial blood
glucose. During the period 80-200 min after the last meal in both series of studies the average
'arterial' sugar concentration was steady. The average values were 94.6 mg 100 ml blood-'
for the morning samples and 91.9 mg 100 m1-l for the afternoon ones. This difference was not
significant (t diff. = 1.62, P>O.lO) (Fig. 3).
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FIG.3. Arterial and venous blood sugar values, averaged in 10 min periods from 80 to 200 min
after the start of the previous meal. The blood samples were obtained from fifteen subjects after
breakfast and from twenty-one different subjects after lunch.
Venous values. During this same period the average difference between the morning (Mottram
& Brown, 1963) and the afternoon samples (present studies) was 11.7 mg 100 ml-', with the
afternoon values being higher. This difference was statistically significant (t diff. = 5.57,
Pc0.001). The resultant A-V differences were therefore smaller during the afternoon. In Fig. 3
it can be seen that there is a tendency for both morning and afternoon venous sugar concentrations to approach their corresponding arterial values as time passed. The average blood
flows in the two sets of studies were similar, so this variation in A-V differences is related to a
modification in the response by the muscles to the same elevation of arterial blood sugar
occurring at different times of day. This change in uptake would be expected from the observations of Jarrett & Keen (1969) on diurnal variations in glucose tolerance curves.
Patricia G. B. Baker and R. F. Mottram
Metabolic activity of resting tissue (fed subjects)
In the resting state, measurements (generally in triplicate) were made of glucose uptake and
0, uptake in twenty-two subjects. In twenty of these CO, production was also measured. The
glucose uptake averaged 0.49 mg 100 ml muscle-' min-l (range 04-1.14 mg). This average
was smaller than that found by Mottram & Brown (1963). The mean 0, uptake was 0.29 ml
100 ml forearm muscle-' min-l, with a range for the individual observations of 0.09-0-52 ml.
These values are very similar to many previously published (Mottram, 1971). The average CO,
production was 0.22 ml, giving an average RQ of 0-75. Again these values are close to those
already published for muscle (Andres, Cader & Zierler, 1955) of fasted humans. This average,
however, is obtained from individual subjects' RQ values ranging from 0.36 to 1.08, each of
these being the average of three separate determinations.
Comparison of metabolic exchanges in the post-prandial and the 6-7 Ji fasted states
The results of these paired experiments are compared in Table 1. The results in these six
subjects in the fed state compare well with the other seventeen experiments in the series.
Unfortunately, however, we found some inexplicably low venous blood CO, values in three
TABLE
1. Average metabolic exchanges of resting muscle in each of six subjects in whom measurements were
made in both fed and 6-7 h fasted states
Subject no.
State
1
Fed
Fasted
Fed
Fasted
Fed
Fasted
Fed
Fasted
Fed
Fasted
Fed
Fasted
2
3
4
5
6
0,uptake
(ml100 ml
muscle- ' min-')
0.24
0.20
0.28
0.21
0.26
0.41
0.34
0.22
0.42
0.25
0.22
0.20
CO, production
(ml100 ml
muscle-' min-')
RQ
0.21
0.88
0.16
0.57
0.17
065
0.88
0.36
0.15
0.31
0.19
0.14
068
0.74
066
064
Glucose uptake
(mg 100 ml
muscle- ' min- ')
0.42
0.06
0.82
0
0.36
0
0.73
0.13
0.11
0
0.71
0
of the studies when fasting. The arterial blood sugar during fasting was 65-75 mg 100 ml-',
much lower than that of fed subjects. Glucose arteriovenous differences were undetectable in
four subjects and decreased in the remaining two. As can be seen in Table I, there is not always
good agreement between results of the two studies performed in a single subject. This has been
noted before for 0, uptake (Mottram, 1958), and most probably is due to errors in estimating
muscle blood flow from observed total forearm blood flow. It is difficult, from these results,
to say with certainty that a 7 h fast has had any effect on resting muscle metabolism other than
to decrease the glucose uptake from blood.
Metabolism of human muscle
485
Changes in arteriovenous diferences caused by exercise
The average value and ranges of arteriovenous differences for 0,, CO, and glucose are
shown in Table 2. The A-V differences for O2 and CO, rose in all fifteen studies during the
first 2 min of exercise and that for glucose fell. (The table also shows the values obtained for the
TABLE
2. The A-V differences of oxygen, carbon dioxide and of glucose of fifteen fed subjects observed
before, during the third and the fourth minutes of exerciseand in the third minute of the recovery period
Rest
Exercise
Third minute
Fourth minute
Recovery
Third minute
Oxygen
(ml 100 ml blood-')
Carbon dioxide
(ml 100 ml blood-')
Glucose
(mg 100 ml blood-')
Mean
Range
Mean
Range
Mean
Range
7.1
5.3-8.7
5.0
2.0-7.6
13
2-3 5
9.7
10.0
7'3-11.3
7.5-11.6
7.4
7.7
54-10.3
5.1-10.1
6
6
1-18
2-1 8
7.6
4.9-10.4
5.1
2'9-7.1
10
1-26
A-V differences were always positive for oxygen and glucose and negative for carbon dioxide.
third minute in the recovery period. It is not considered worthwhile to study these further, for
these recovery samples were taken in a period of disequilibrium in metabolism and blood
flow and therefore it is not possible to relate these A-V difference values to blood flow and
obtain quantitative measures of metabolic activity, but it appears, from these values, that the
muscle had recovered from the effects of the exercise.)
The average arterial 0, content of the blood in these subjects was 17.6 vol %. The average
saturation of the venous blood draining the muscles was therefore 60% at rest and 43%
during exercise. At a constant Pco, of 40 mmHg these saturations would correspond to Po,
values of 32 and 23 mmHg. However, the raised CO, content of the venous blood will raise
the Pco, of the blood by 10 mmHg at rest and by 15 mmHg in exercise. This will raise the
venous Po, values from 32 (rest) and 23 (exercise) mmHg to 36 and 28 mmHg respectively.
Changes in blood flow and metabolism produced by exercise
Because of a 10% rise in 0,uptake between third and fourth minute determinations of
metabolic activity, six of the twenty-two studies on fed subjects and two of the six on fasted
subjects were rejected. It should be noted that the differences in the third and fourth minute
determinations were consistent in each of the rejected studies, appearing in all three replicate
work periods.
Bloodflow response to exercise. The responses of muscle blood flow in the sixteen studies on
fed subjects in which equilibrium was achieved, together with the remaining six in which no
steady-state was reached, are shown in Table 3.
Although the resting blood flow was higher in the six non-equilibrated studies the response
486
Patricia G. B. Baker and R. F. Mottram
by the third minute of contraction was greater, as was the extraction of 0, from the blood at
this time (10.7 vol%, compared with 9.7 for the sixteen subjects in whom the equilibrium state
was achieved). Yet this greater increase in metabolic activity was associated with failure to
reach equilibrium, the blood flow and/or extraction of 0, being even greater in the fourth
minute of contraction. In both absolute terms (kg force exerted on the ergometer) and relative
terms (% maximal effort employed in the studies) the work done by these six subjects was
comparable to that of the other sixteen. The increased metabolic response in these six subjects
cannot therefore be explained.
TABLE3. Muscle blood flow (ml 100 ml muscle-' min-')
before, during and after exercise
(a) In sixteen subjects in whom equilibrium was reached
Rest
Third minute of exercise
Fourth minute of exercise
Third minute of recovery
Mean
Range
4.3
16.5
16.4
7.0
3.0-6.4
9.5-25.8
8.5-27.0
34-12.8
'
(b) In six subjects who failed to reach equilibrium
Rest
Third minute of exercise
Fourth minute of exercise
Third minute of recovery
Mean
Range
47
19.0
22.2
94
3.46.4
14.2-25.9
16.8-31 '6
5.2-15.7
Efsects of exercise on the metabolic exchanges. This will be confined to a consideration of
the results of the studies in which equilibrium conditions were found. (There were finally
only fifteen successful experiments on fed subjects, one having been discarded because of
anomalous blood-gas analysis results.)
In the fed subjects the mean results were as follows: O2 uptake increased from 0.30 to 1.74 ml
0, 100 ml active muscle-' min-l, and CO, production increased from 0.21 to 1.39 ml. The
RQ rose from 0.74 to 0.80. The rise in glucose uptake was much smaller, proportionally, than
that of 0,. The resting uptake in the fifteen studies was0.49 mg 100ml muscle-' min-l, and the
average uptake in exercise was 0.84 mg. In all studies, as already described (Fig. 2), the glucose
A-V difference fell as the blood flow rose and in some cases these decreased A-V differences
came within the limits of resolution of the blood glucose analysis method. Therefore an
increased supply of glucose in the increased quantity of blood passing through the muscle
was never accompanied by a corresponding increase in glucose entry into the cells, so supply
is not the rate-limiting factor in the post-prandial glucose uptake by muscles.
Relationship of arterial blood 0, content and haemoglobin concentration
In twenty-one studies a comparison was made between the actual 0, content of the arterialized venous blood and the 0, capacity calculated from the haemoglobin concentrations. In
Metabolism of human muscle
487
these experiments the average actual 0, content was 17.6 vol % and that calculated from the
Hb values was 19.3 vol % giving an average saturation of the arterialized blood of 91.2%.
DISCUSSION
Metabolic activity of resting muscle
It appeared that the occasional RQ values below 0.7 were accompanied by abnormal
CO, A-V differences. Since all blood samples were withdrawn and handled similarly, it must
be assumed that either the Van Slyke analyses were inaccurate, or the delivery of CO, from
tissues into the venous blood varies from time to time, or the arterial CO, had markedly
altered from the time the arterial blood samples were obtained, 10 min before or after the
resting venous blood samples. Hyper- or hypo-ventilation by volunteer subjects undergoing an
experiment both stressful and demanding of their co-operation could well cause fluctuations
of arterial CO, sufficient to produce the occasional anomalous A-V CO, differences and thus
RQ values that we have found. Another possible cause of RQ values less than 0.7 is the incomplete oxidation of fatty acids with the loss to venous blood of acetate or of acetoacetate.
This has been observed by Wahren & Hagenfeldt (1968).
If the average RQ value (0.75) is accepted then only about one-fifth of the observed glucose
uptake can be oxidized. Glucose conversion into fatty acid or glycolysis would produce excess
CO, (by decarboxylation or displacement from bicarbonate respectively); we therefore
suggest that glucose is converted to glycogen. Storage at 0.4 mg min-l 100 ml muscle-'
would add 0.025% by weight per hour to the muscles, a modest figure when compared with
the normal muscle glycogen content ( 21%). Blood gas analyses by the Van Slyke apparatus
have about 20 times the resolving power of any blood glucose determinations, so, in spite of
the well-known criticisms of the RQ as a means of determining metabolic pathways, this
method may still be preferable to direct measurements of glucose uptake. The fact that
replicate determinations at intervals in the same subject give substantially the same result
supports the conclusions already stated above.
TABLE
4. The effects of exercise of muscle, in the fifteen fed and four fasted subjects on the metabolism of 02,
C 0 2 and glucose and on the blood flow through the active muscle. In the fed subjects the average values are
given, together with the coefficients of variation; in the fasted only the averages are given.
'
Muscle blood flow
(ml100 ml
muscle- ' min- ')
0.21
+33%
1.39
k 38%
0.49
80%
0.84
133%
+
4.3
23%
16.4
31%
0.23
1.07
0.03
0
4.2
15.4
O2 uptake
(ml100 ml
muscle- min- ')
C 0 2 production
(ml100 ml
muscle- min- ')
Fifteen fed subjects
Rest
Coeff. var.
Work
Coeff. var.
0.30
20%
1.74
k 29%
Four 7 h fasted subjects
Rest
Work
0.28
1.52
+
Glucose uptake
(mg 100 ml
muscle- min- ')
+
+
+
488
Patricia G . B. Baker and R. F. Mottram
Metabolic activity of exercising muscle
The calculated gaseous exchanges between blood and muscle show that sustained exercise
of the forearm muscles at only 5% MVC produced a sixfold increase in their metabolic
activity. This increase was accompanied by only a slight rise in RQ which might have been
produced by a small change in metabolism towards carbohydrate oxidation or by displacement
of some CO, by acid production. The present studies provide no clear indication of the source
of the extra COz. The means of the resting and exercising values for the three metabolic
exchanges studied and for blood flow are shown together with their coefficients of variation
in Table 4. There was a considerable variation about (or around) the average value for all
determinations a t rest, but the glucose uptake shows by far the largest variation from subject
to subject. The coefficients of variation rose in all four determinations in exercise, the response
to exercise being no less variable than the resting values for these parameters. The variations
at rest may be due to true inter-subject variation, being of the same order as that seen in basal
metabolic rate measurements. They may also be due to individual variations from the mean
values used for calculation of muscle blood flow from whole-limb blood flow (Cooper et al.,
1955). The additional variation in exercise might be related to the fact that no two subjects
had the same MVC or muscle mass. If the hand venous blood was not completely arterialized,
all the A-V differences and calculated metabolic exchanges would be falsely low; since, however,
the values we obtained for 0, uptake of resting muscle are similar to those previously reported,
the arterialization procedure was unlikely to have been deficient.
At rest there was no difference between the arterial and venous blood H b contents, but the
average rise of venous blood Hb at the end of the 4 min periods of contraction was 0.3 g 100 ml
blood-'. This rise was significant (t = 405, n = 21, P<O.OOI) and corresponds to a loss of 2 mi
water 100 ml blood-' as the blood passes through the exercising muscle. This net water loss
into the tissue fluid space could produce a change of about 0.2 ml 100 ml forearm-1 min-'.
As the hyperaemia developed during contraction, much larger changes (about 1-2 ml100 ml-i
min-l) in forearm volume were observed. The greater part of this change in forearm volume
is probably due to the increased capacity of the circulation in the exercising muscle. The
observed change in 'Hb concentration will lead to a slight over-estimation of the observed
venous blood 0, content during exercise, but CO, and glucose values will not be affected.
Previous studies of metabolism in exercise
Systematic study of this subject may be said to have begun with the work of Fletcher &
Hopkins (1907) who showed that, as an excised frog muscle was induced to contract repetitively
by motor nerve stimulation, the exercise was accompanied by destruction of glycogen and
accumulation of lactic acid. Poisoning with iodoacetate stopped lactic acid production and led
to a very rapid failure of contraction (Lundsgaard, 1930). This work stressed the primary
importance of glycolyis as the source of energy in normal muscular contraction.
The problem has also been studied at the opposite extreme of physiological preparations, the
conscious human being, voluntarily exercising while the changes in gaseous exchange were
studied. Margaria, Edwards & Dill (1933) found that the exchanges in man, at rest, had an RQ
of 0.8, indicating that 50% of the 0, uptake was being used to oxidize carbohydrate. Half of
this would be the 0, required by the central nervous system which normally metabolizes
489
Metabolism of human muscle
carbohydrate alone. Therefore the remaining tissues must be using fat in preference to carbohydrate. Margaria and his associates further found that running at rates of up to 8 km/h,
with an 0, uptake of 2 litres/min produced no change in this RQ and no rise in blood lactate.
Therefore an 8-fold increase in whole body oxygen uptake (equivalent to a 25-fold increase in
skeletal muscle) may be accompanied by no change in the fractions used in oxidizing fat and
carbohydrate. These conclusions, as are our own, are based solely on RQ calculations and must
be regarded, until confirmed directly, as somewhat tentative.
The last decade has seen both the extension of the Fick method and the use of needle biopsy
specimens for studying tissue metabolism in both human and animal preparations. The work
of Stainsby and his colleagues (Welch & Stainsby, 1967; Chapler & Stainsby, 1968) are one
example of the latter. These workers studied the gastrocnemius-plantaris muscle mass of
anaesthetized dogs, in which repeated motor nerve stimulation produced maximal isotonic
twitches. The range of O2uptake of resting muscle was 0-5-2.0 ml100 g-' min- little of which
was used in carbohydrate oxidation. At one twitch per second the O2 uptake rose to five times
the resting value. There was no increase in lactate production (which was seen at rest) and no
breakdown of glycogen. Glucose uptake from the blood increased by three times and carbohydrate oxidation accounted for 35% of the total 0, uptake. Higher twitch rates caused first
an increase in carbohydrate oxidation and later a rise in glycolysis.
The Fick method has been used by Wahren & Hagenfeldt (1968), Zierler et al (1968), and
Whichelow et al. (1968) as well as by the present authors. Wahren & Hagenfeldt produced in
their subjects a 12-fold increase in 0, uptake and at this rate the RQ was 0.87. (At rest
they report an RQ of 0.73.) Although free fatty acid (NEFA) was leaving the circulation in
the active muscle, only 60% of the [I4C]NEFA appeared as 14C02,and the remainder as
water-soluble compounds. It should be noted that in such incomplete oxidation of fats,
O2 uptake is not accompanied by CO, production. The RQ can thus fall below 0.7. Zierler
et al. (1968) used a milder exercise that increased forearm blood flow by 100% and 0, uptake
3-fold. The RQ was 0.72 at rest, with oleic acid oxidation accounting for 48% of the 0,
uptake. During exercise neither of these values changed, but in the recovery period oleic
acid oxidation accounted for 61% of the 0,uptake.
In their studies on glucose uptake and forearm muscle exercise, Whichelow et al. (1968)
measured only blood flow and glucose A-V differences. Forearm blood flow was doubled by
the exercise studies which must therefore have been comparable to that of Zierler et al. (1968).
Glucose uptake was increased by exercise in both the fasting state and after 50 g of glucose had
been given by mouth, but it is not possible to determine the fate of this glucose. One possibly
significant fact is that the glucose uptake increases in both the fasted state and during the oral
glucose tolerance test are comparable to the increase in blood flow and may reflect the delivery
of greater amounts of glucose to the muscles.
The results reported in this paper are essentially similar to those of Zierler et al. (1968) and
to those of Stainsby and his colleagues. A rise of 0, uptake to six times the resting level is
accompanied by a small change in RQ and any increase in glucose uptake is disproportionally
smaller than that of 0,. This effect of exercise was seen despite the high resting glucose uptake
in these fed subjects. These direct measurements then give a different picture from that inferred
from experiments performed on isolated amphibian muscle, but one similar to that found in
studies of respiratory exchange in running exercise in man.
',
490
Patricia G . B. Baker and R. I;. Mottram
ACKNOWLEDGMENTS
The authors wish to acknowledge the loyal help of Mrs Christine Shears in the studies and
subsequent analyses, and also the co-operation of those students of the University College,
Cardiff, and the Welsh National School of Medicine, who acted as subjects for the studies.
P.G.B.B. was in receipt of a Medical Research Council post-graduate student scholarship.
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