An enzymatic approach to lactate
production in human skeletal muscle
LAWRENCE L. SPRIET, RICHARD A. HOWLETT, and GEORGE J. F. HEIGENHAUSER
Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1 CANADA; and
Department of Medicine, McMaster University, Hamilton, Ontario L8N 35 CANADA
SPRIET, L. L., R. A. HOWLETT, and G. J. F. HEIGENHAUSER. An enzymatic approach to lactate production in human skeletal
muscle during exercise. Med. Sci. Sports Exerc., Vol. 32, No. 4, pp. 756–763, 2000. Purpose: This paper examines the production of
lactate in human skeletal muscle over a range of power outputs (35–250% V˙O2max) from an enzymatic flux point of view. The
conversion of pyruvate and NADH to lactate and NAD in the cytoplasm of muscle cells is catalyzed by the near-equilibrium enzyme
lactate dehydrogenase (LDH). As flux through LDH is increased by its substrates, pyruvate and NADH, the factors governing the
production of these substrates will largely dictate how much lactate is produced at any exercise power output. In an attempt to
understand lactate production, flux rates through the enzymes that regulate glycogenolysis/glycolysis, the transfer of cytoplasmic
reducing equivalents into the mitochondria, and the various fates of pyruvate have been measured or estimated. Results: At low power
outputs, the rates of pyruvate and NADH production in the cytoplasm are low, and pyruvate dehydrogenase (PDH) and the shuttle
system enzymes (SS) metabolize the majority of these substrates, resulting in little or no lactate production. At higher power outputs
(65, 90, and 250% V˙O2max), the mismatch between the ATP demand and aerobic ATP provision at the onset of exercise increases as
a function of intensity, resulting in increasing accumulations of the glycogenolytic/glycolytic activators (free ADP, AMP, and Pi). The
resulting glycolytic flux, and NADH and pyruvate production, is progressively greater than can be handled by the SS and PDH, and
lactate is produced at increasing rates. Lactate production during the onset of exercise and 10 min of sustained aerobic exercise may
be a function of adjustments in the delivery of O2to the muscles, adjustments in the activation of the aerobic ATP producing metabolic
pathways and/or substantial glycogenolytic/glycolytic flux through a mass action effect. Key Words: OXIDATIVE PHOSPHORY-
LATION, MUSCLE ENERGY STATE, EXERCISE INTENSITY, GLYCOGENOLYSIS/GLYCOLYSIS, PYRUVATE, LACTATE
DEHYDROGENASE, PYRUVATE DEHYDROGENASE, NADH
CHO fuel is provided by the uptake of glucose from the
blood and from glycogen stored inside the muscle. Once
produced, the pyruvate can be further metabolized in the
cytoplasm or transported across the inner mitochondrial
membrane and metabolized inside the mitochondria. The
most important mitochondrial pathway of pyruvate metab-
olism is conversion to acetyl-coenzyme A (acetyl-CoA)
with the reduction of NAD to NADH in a reaction catalyzed
by the pyruvate dehydrogenase (PDH) complex. The acetyl-
CoA is then available to enter the tricarboxylic acid (TCA)
cycle where reducing equivalents are produced for use in the
electron transport chain to generate ATP in the process of
oxidative phosphorylation. The oxidative use of 1 mmol
glucose or glucosyl unit results in the production of ?39
uring exercise, carbohydrate (CHO) is metabolized
in the cytoplasm of skeletal muscle cells to produce
pyruvate in the glycolytic pathway (Fig. 1). The
mmol ATP. Conversely, the most important cytoplasmic
pathway of pyruvate metabolism is conversion to lactate
with the oxidation of NADH to NAD in the lactate dehy-
drogenase (LDH) reaction. When 1 mmol glucosyl unit is
metabolized to lactate, it provides either 2 mmol (exogenous
glucose) or 3 mmol (muscle glycogen) ATP.
Pyruvate can also combine with glutamate to form 2-oxo-
glutarate and alanine in the alanine aminotransferase (AAT)
reaction in the cytoplasm, and to a minor extent in the
mitochondria (Fig. 1). This reaction appears to be important
in the early stages of exercise to increase the content of the
TCA cycle intermediates but only accounts for ?2–5% of
pyruvate disposal (12,13). Other reactions catalyzed by
pyruvate carboxylase and malic enzyme also compete for
pyruvate in the cytoplasm but do not appear to be quanti-
tatively important (25), although they have not been studied
in human muscle. NADH, the other substrate for LDH, can
also be reconverted to NAD in the cytoplasm via the near-
equilibrium malate-aspartate and alpha-glycerophosphate
shuttle systems (SS), which transfer reducing equivalents to
the mitochondria (Fig. 1). The malate-aspartate shuttle ap-
pears to be the quantitatively important system (26,32,33).
MEDICINE & SCIENCE IN SPORTS & EXERCISE®
Copyright © 2000 by the American College of Sports Medicine
Submitted for publication December 1998.
Accepted for publication December 1998.
Determining the exact mechanisms of lactate production
in skeletal muscle during all exercise conditions has been
difficult and will not be resolved in this paper. Readers are
urged to consult the numerous research papers and reviews
that have been published on this topic (5–7,14,22). The
purpose of this paper is to use an enzymatic approach to
examine lactate production over a wide range of exercise
power outputs in human skeletal muscle. This approach
examines the flux through the key enzymes of glycogenol-
ysis/glycolysis, the SS, and pyruvate metabolism at 35, 65,
90, and 250% V˙O2max. The first 10 min of cycle exercise at
each of the aerobic power outputs and 30 s of cycle sprinting
at ?250% V˙O2maxare examined. Because the measure-
ments and estimates of enzyme and pathway fluxes are
derived from needle muscle biopsy samples, this approach
only provides average responses for the fiber type popula-
tions that exist in the sampled vastus lateralis muscles. Also,
the subjects who volunteered for these studies were active,
but not well-trained aerobically.
The rate of lactate production in skeletal muscle at any
power output will depend mainly on the flux of substrate
through the glycogenolytic/glycolytic pathway and the rel-
ative activities of the SS and the PDH and LDH pathways of
pyruvate metabolism. Generally, the higher the demand for
ATP, the greater the activation of the enzymes that regulate
the glycogenolytic/glycolytic flux. These enzymes include
glycogen phosphorylase (PHOS) and phosphofructokinase
(PFK), and to a much smaller extent, hexokinase (HK) (Fig.
1). They are nonequilibrium enzymes that are covalently
and allosterically regulated by factors related to the intensity
of the muscle contraction (hormones and Ca2?) and the
severity of the demand for ATP. Therefore, the regulators
associated with the energy status of the cell, or the muscle
contents of ATP, and free ADP, AMP, inorganic phosphate
(Pi), and H?, are important in signaling the intensity of the
contraction and demand for ATP. In sprint exercise, ammo-
nia and inosine monophosphate may also be important.
Evidence suggests that skeletal muscle glycogen PHOS
activity is regulated by a two-stage process at the onset of
exercise (2,10,31). The first stage is transformation from the
less active form of PHOS b to the more active a form,
mediated mainly by Ca2?and to a lesser extent by epineph-
rine. This stage may be regarded as a gross control mech-
anism that determines the potential upper limit for glyco-
genolytic flux. The second stage is posttransformational
control of PHOS a (and to a lesser extent, PHOS b) by
substrate availability (free Pi) and allosteric modulation
(free AMP). This level of control fine tunes glycogenolytic
flux to the ATP demand via the regulators linked to the
energy state of the cell.
The nonequilibrium enzyme PFK is also controlled by
regulators linked to the energy status of the cell, but in an
entirely different manner than PHOS (for review, see 37).
The major regulator of PFK activity is ATP, which not only
binds to its active site (as a reaction substrate) but also to an
allosteric or regulatory site. At rest, when ATP levels are
high and positive modulators are low, ATP inhibits PFK.
Citrate and H?potentiate the binding of ATP to the allo-
steric site and inhibition of the enzyme. However, during
exercise, free ADP, AMP, and Piaccumulate and decrease
the binding of ATP at the regulatory site, thereby releasing
PFK inhibition. These positive modulators are dominant
during exercise as PFK activity increases despite accumu-
lations of citrate and H?.
Skeletal muscle LDH is a tetrameric enzyme that exists in
five isoforms due to the existence of two types of subunits,
a muscle and a heart form, both of which favor the produc-
tion of lactate. As a near-equilibrium enzyme, it is not under
covalent or allosteric control but is sensitive to the concen-
trations of its substrates and products. Therefore, increases
in pyruvate and NADH increase the flux through LDH in
human skeletal muscle fiber types.
POTENTIAL MECHANISMS FOR LACTATE
During exercise at low power outputs, the demand for
ATP, the glycolytic flux, and the rate of pyruvate production
are all low, and most of the pyruvate is converted to acetyl-
Figure 1—Schematic of the glycogenolytic/glycolytic pathways high-
lighting key enzymes and major fates of pyruvate. The vertical line
indicates the muscle membrane separating blood and muscle cyto-
plasm, the rectangle depicts the mitochondrion, the ellipses indicate a
transport process, and the arrows indicate the direction of net flux. G,
glucose; L, lactate; A-CoA, acetyl-coenzyme A; PHOS, glycogen phos-
phorylase; HK, hexokinase; PFK, phosphofructokinase; SS, malate-
aspartate shuttle system; LDH, lactate dehydrogenase; AAT, alanine
amino transferase; PDH, pyruvate dehydrogenase. Flux through
PHOS, HK, and PFK involves a 6-carbon moiety and through SS,
PDH, and LDH involves a 3-carbon moiety. Therefore, the metabolism
of a 6-carbon glucosyl unit may result in up to a 2-fold higher flux
through SS, PDH, and LDH.
LACTATE PRODUCTION IN HUMAN SKELETAL MUSCLEMedicine & Science in Sports & Exercise?
CoA in the mitochondria through activation of PDH. The
NADH produced in the glycolytic pathway is also trans-
ferred across the mitochondrial membrane via the SS.
Therefore, with both substrates of the LDH reaction low, the
production of lactate is minimal.
At higher power outputs, with a higher demand for ATP,
activation of the glycolytic pathway increases. If the rates of
pyruvate and NADH production exceed the ability of PDH
to metabolize pyruvate and/or the SS to transfer reducing
equivalents into the mitochondria, lactate will be produced.
However, whereas lactate production is ultimately related to
the activities of the various enzymes in the glycolytic path-
way, the SS, and the PDH complex, events in the mitochon-
dria determine how active the glycolytic pathway needs to
be, to assist in meeting the demand for energy.
According to one prevalent hypothesis, the electron trans-
fer and oxidative phosphorylation processes in the mito-
chondria are near-equilibrium and are therefore controlled
by the level of their substrates, oxygen (O2), NADH, free
ADP, and free Pi(39). The availability of these substrates
will determine how well the demand for ATP is matched by
aerobic ATP synthesis. When exercise begins, free ADP and
Piincrease and processes that increase O2delivery and
NADH provision are activated. However, at moderate aer-
obic exercise intensities and above, aerobic ATP provision
cannot initially meet the demand for ATP and anaerobic
pathways (substrate phosphorylation) contribute to ATP
provision. In this case, ATP breakdown is greater than ATP
production and ATP levels decrease slightly, producing
significant increases in free ADP and AMP, decreases in
phosphocreatine (PCr) and increases in Pi, leading to acti-
vation of the glycolytic pathway.
It has been argued that lactate production occurs only in
response to situations where the muscle cell lacks sufficient
O2to metabolize pyruvate and fat in the mitochondria
(7,22). Others have argued that lactate production at the
onset of intense aerobic exercise (65–100% V˙O2max) is not
related to the availability of O2but slow activation of the
metabolic pathways (e.g., PDH activation, TCA cycle, beta-
oxidation) that provide substrate for aerobic ATP produc-
tion (11,18,38). An additional interpretation is that lactate
production is unrelated to O2availability and is simply a
required by-product of glycolytic flux (5,7). A portion of the
produced pyruvate would always be converted to lactate via
LDH regardless of the rate of pyruvate production in a mass
action manner. At low intensities and glycolytic flux, lactate
production would be hard to detect. At higher power outputs
with higher glycolytic flux, the lactate production would be
The most common situations where these possibilities
may lead to lactate production include the transition from
rest to exercise, the transition from one power output to a
higher power output and exercise at power outputs that are
higher than can be sustained with only oxidative phosphor-
ylation. These possibilities may also contribute to the much
lower but continued rates of lactate production during sus-
tained exercise at moderate and high aerobic power outputs.
In these situations, it is important to remember that lactate
production may result from more than one of these causes,
especially when muscle fiber type differences are considered.
There is evidence that following aerobic training (with or
without the proliferation of mitochondria), lactate produc-
tion at a given submaximal power output is reduced
(3,16,17,35). In other words, the increase in glycolytic flux
and resulting lactate production at the onset of exercise and
during sustained aerobic exercise at a given submaximal
power output is reduced. PCr degradation, glycogen utili-
zation, and the accumulations of the by-products of ATP
hydrolysis are reduced consequent to an increased ability to
oxidize fat and/or provide O2to the mitochondria after
short-term and prolonged aerobic training with proliferation
of mitochondria (3,17,35). These same metabolic changes
are present during intense aerobic exercise after acute per-
turbations (short-term training, increased fat availability,
caffeine ingestion) that appear to increase fat metabolism
and/or NADH provision, when no changes in mitochondrial
potential have occurred (4,9,10,16).
AND EXERCISE INTENSITY
It is clear that the potential for lactate production in any
exercise situation will be dependent on the flux in the
glycogenolytic/glycolytic pathway. It is therefore important
to remember that the flux in this pathway varies drastically
depending on the exercise intensity. At low power outputs,
the demand for ATP is low and can be provided at a high
enough rate through aerobic combustion of CHO and fat.
Consequently, the flux through the glycolytic pathway is
low, providing small amounts of ATP, reducing equivalents
The situation during power outputs above 100% V˙O2max
is very different. These power outputs cannot be sustained
solely by aerobic ATP production. The ATP that cannot be
provided aerobically must be generated anaerobically via
substrate phosphorylation from the degradation of PCr in
the creatine kinase reaction and in the glycolytic pathway
with the conversion of glycogen to lactate. Therefore, the
demand on the glycogenolytic/glycolytic pathway is not
only to produce pyruvate for aerobic ATP production but
also for anaerobic ATP provision. Although the amount of
ATP generated by the metabolism of each mole of glucose
from stored glycogen to lactate is low, the rate of ATP
production that can be achieved is much higher than from
CHO-derived aerobic ATP production. For this to be true,
the flux through the glycogenolytic/glycolytic pathway must
be substantially higher than during aerobic exercise. This
results in pyruvate and NADH production rates that are
much higher and can be handled in the PDH and SS reac-
tions, respectively, and lactate and NAD must be produced
by LDH. Because the sum of cytosolic [NAD] and [NADH]
is fixed, the regeneration of NAD is important for sustained
high glycolytic flux as the NAD is reutilized higher up in the
pathway (glyceraldehyde phosphate dehydrogenase reac-
tion). The produced lactate accumulates in the muscle or is
transported out of the cell.
Official Journal of the American College of Sports Medicine http://www.msse.org
Therefore, the glycogenolytic/glycolytic pathway must be
sensitive to extremes in demand, the low flux of aerobic
exercise and the very high flux of sprint exercise. The
maximal activities of PHOS and PFK measured in vitro are
very high, and flux through these enzymes approaches these
activities during maximal sprint exercise. Conversely, be-
cause the maximal activities in vitro of the enzymes in-
volved only in aerobic metabolism (SS and PDH) are much
lower, flux rates approach these activities during exercise at
high aerobic power outputs.
EXERCISE INTENSITY AND SKELETAL
MUSCLE LACTATE PRODUCTION
Lactate accumulates in human vastus lateralis skeletal
muscle as a function of increasing exercise power output
(Fig. 2). However, muscle lactate contents underestimate the
production of lactate during exercise as varying amounts of
lactate will be transported out of the muscle. Therefore,
measured rates of lactate release from other studies (20,27)
were added to the muscle measurements to estimate average
LDH flux rates (Table 1). Similarly, flux rates through
additional enzymes were estimated from skeletal muscle
biopsy measurements of fuels, metabolites and enzymes in
the relevant pathways (Table 1).
35% V˙O2max. The transition from rest to 1 min of ex-
ercise at 35% V˙O2max(?60 W) resulted in little disturbance
to the energy status of the muscle cells. The ATP content
was unchanged, PCr decreased by less than 10 mmol?kg?1
dry muscle (dm), and free Pi, ADP, and AMP increased only
slightly (Figs. 2–4). The muscle was able to meet the
increased requirements for ATP through aerobic means.
Consequently, the flux through PHOS, PFK, and the glyco-
lytic pathway was low (Table 1) and essentially matched the
aerobic use of pyruvate as no muscle lactate accumulation
could be detected after 1 and 10 min of exercise (Fig. 2). A
significant amount of PDH was transformed to its active
form (PDH a) to convert pyruvate to acetyl-CoA (Fig. 5).
Previous studies have demonstrated that transformation to
PDH a is equivalent to PDH flux in normal exercise situa-
tions (18,29). Additional acetyl-CoA may also be derived
from the metabolism of fat. It is unlikely that the AAT
reaction was very active and resulted in significant increases
in TCA cycle intermediates, although this has not been
measured during cycling at this power output. After 8 min
of cycling at 40% V˙O2max, arterial blood lactate did increase
slightly from 0.45 to 1.08 mM, and a small but measurable
lactate efflux (0.75 mmol?min?1?exercising leg??1) was
65% V˙O2max. The energy status of the muscle cells in
the initial minute of exercise at 65%V˙O2max(?165 W) was
significantly altered, indicating a temporary mismatch be-
tween the demand for ATP and aerobic resynthesis of ATP.
Consequently, anaerobic pathways provided the balance of
the ATP, until aerobic metabolism could match the required
rate of ATP provision. Although the content of muscle ATP
was well defended, PCr decreased by ?34 mmol?kg?1dm
in the 1st minute of exercise and free Pi, ADP, and AMP
increased noticeably (Figs. 3 and 4). Higher levels of these
metabolites increased PHOS and PFK activities such that
glycolytic flux was ?5-fold higher than at 35% V˙O2max
(Table 1, Fig. 6). Muscle glycogen was quantitatively the
most important CHO source for ATP resynthesis during the
transition to moderate aerobic exercise. Estimated PDH flux
(activity of PDH a) increased rapidly in the 1st minute,
reaching ?2.7 mmol pyruvate?kg?1wet muscle (wm)?min?1
(Table 1, Fig. 5). Assuming there was sufficient O2to support
the aerobic combustion of the resulting acetyl-CoA, this rate
was not high enough to metabolize all of the produced pyru-
vate. The same may have been true for the ability of the SS to
handle the production of NADH. A small portion (2–5%) of
the produced pyruvate may have been metabolized in the AAT
reaction, resulting in an increase in TCA cycle intermediates
(12,13). The remaining pyruvate and NADH would increase
lactate production via the LDH reaction. The estimated flux
through LDH in the 1st minute at 65% V˙O2maxwas 2-fold
higher than the flux through PDH (Table 1, Fig. 5). In the final
9 min of exercise at this power output, muscle lactate content
was unchanged, but arterial lactate increased from ?1 to 3
mM, and lactate release was ?4 mmol?min?1?exercising?1leg
after 8 min of cycling (27).
Figure 2—Human muscle lactate (A) and phosphocreatine (B) con-
tents during cycling for 10 min at varying power outputs (35–90%
V˙O2max) and 30 s at 250% V˙O2max. Data are means ? SE and were
obtained from references 18, 24, 30, and 36.
LACTATE PRODUCTION IN HUMAN SKELETAL MUSCLE Medicine & Science in Sports & Exercise?
The increase in ATP demand during the transition from
rest to exercise at 65% V˙O2maxwas too great for the aerobic
processes in the muscle to immediately handle. The exact
limitation(s) that prevented a more rapid increase in aerobic
metabolism at this power output are unknown. It could be a
lack of available O2that limits ATP production rate and
ultimately determines the rates of acetyl-CoA use from both
CHO and fat. It may also be the rate at which the key
enzymes regulating CHO and fat metabolism (e.g., PDH
transformation, TCA cycle, and beta-oxidation) can be acti-
vated that limits the production of reducing equivalents and
ultimately ATP provision. This energy mismatch leads to in-
creased PCr use and free ADP, AMP, and Piaccumulations.
These signals activate the glycogenolytic/glycolytic enzymes,
and the increased glycolytic flux increases pyruvate produc-
tion. Increases in pyruvate will increase flux through the near-
Figure 3—Human muscle ATP (A) and free inorganic phosphate (B)
contents during cycling for 10 min at varying power outputs (35–90%
V˙O2max) and 30 s at 250% V˙O2max. Pi, inorganic phosphate. Data are
means ? SE and were obtained from references 18, 24, 30, and 36.
Figure 4—Human muscle free ADP (A) and free AMP (B) contents
during cycling for 10 min at varying power outputs (35–90% V˙O2max)
and 30 s at 250% V˙O2max. Data are means ? SE and were obtained
from references 18, 24, 30, and 36. Free ADP and AMP calculated as
described by Dudley et al. (8).
TABLE 1. Estimated flux through the key enzymes linked to lactate production in human skeletal muscle during cycle exercise at various power outputs.
MAX in Vitro
Power Output, % V˙O2max
65% (164 W)
35% (58 W)
90% (229 W)
250% (625 W)
Flux, mmol substrate ? kg?1vastus lateralis (wet) muscle ? min?1. V˙O2max, maximal oxygen consumption; MAX, maximal enzyme activity measured in vitro; W, watts; PHOS, glycogen
phosphorylase; HK, hexokinase; PFK, phosphofructokinase; SS, malate-aspartate shuttle system; PDH, pyruvate dehydrogenase; LDH, lactate dehydrogenase. Flux through PHOS, HK,
and PFK involves a 6-carbon moiety and through SS, PDH, and LDH involves two, 3-carbon moieties. Therefore, the metabolism of a 6-carbon glucosyl unit may result in up to a 2-fold
higher flux through SS, PDH, and LDH.
Estimated maximal in vitro activities (37°C) were taken from the following references; PHOS (2,3); HK (1,15,34); PFK (1,19); SS (32,33); PDH (18,30); LDH (15,34).
LDH flux was estimated at each power output from pre and post 30 s or 1 min muscle lactate content measurements plus estimated lactate transport from muscles. It
was assumed that no lactate left the muscle at 35% V˙O2max. At 65, 90, and 250% V˙O2max, it was assumed that lactate efflux was 20%, 25%, and 10% of the muscle lactate
accumulations, respectively. PDH flux was assumed to be equivalent to PDH a measurements at each power output (18,28,30). The flux through the SS could be no higher
than PDH flux. PFK flux was estimated as that required to account for pyruvate metabolism by LDH and PDH. HK flux was estimated from glucose uptake measurements
of Katz and coworkers (21,23). Lastly, PHOS flux was the CHO required to match the PFK flux that was not accounted for by HK. At the two highest power outputs, it
also included the build up of glycolytic intermediates between the PHOS and PFK steps (18,30).
Official Journal of the American College of Sports Medicine http://www.msse.org
equilibrium reactions catalyzed by LDH and AAT, help to
transform the nonequilibrium enzyme PDH to its active form,
and provide substrate for PDH a.
The same explanation may account for the ongoing lac-
tate production in the exercise period from minute 1 to 10.
However, it is not clear why lactate production would con-
tinue when O2delivery to the working mitochondria and
metabolic activation have had time to adjust to the demands
of the power output. It may be that the adjustments to the
power output are not perfect, although the low rate of lactate
production during continued exercise represents a very
small mismatch between pyruvate production and pyruvate
disposal through PDH. It is also unclear how the lower rates
of glycogenolytic/glycolytic flux are regulated, as the sig-
nals that control the transformation of PHOS to its more
active a form and the allosteric activators of PHOS a and b
remain constant after the initial minute of exercise at 65%
V˙O2max. It may involve changes in the amount of PHOS in
the a form and/or the sensitivity of both the a and b forms
to allosteric regulators.
90% V˙O2max. The mismatch between the demand for
ATP and the ability to produce ATP aerobically in the
working muscles in the transition from rest to 1 min of
exercise at 90% V˙O2max(?230 W) was greater than at 65%
V˙O2max. Lactate increased from ?6 to 37 mmol?kg?1dm
and PCr decreased by 44 mmol?kg?1dm in the 1st minute
of exercise (Fig. 2). Accumulations of free ADP, AMP, and
Piwere greater, and glycolytic flux was 2-fold greater than
exercise at 65% V˙O2max(Figs. 3 and 4, Table 1). Muscle
glycogen was clearly the dominant CHO fuel for ATP
production during intense aerobic exercise (Table 1). PDH
flux increased rapidly to ?3.4 mmol pyruvate?kg?1
wm?min?1, but LDH activity was 3-fold higher in the 1st
minute of exercise (Fig. 6, Table 1). Again, the AAT reac-
tion metabolized ?2–5% of the produced pyruvate at this
power output (12,13).
It is also clear that a metabolic steady state was never
reached during 10 min of sustained cycling at 90% V˙O2max
as muscle lactate accumulation continued beyond 1 min
(Fig. 2). Arterial and venous blood lactate concentrations
increased from ?1 mM at rest to 4–6 and 6–8 mM at 5 and
10 min of exercise (9,20). Free Pi, ADP, and AMP contents
also continued to accumulate, PCr stores continued to de-
crease, and PDH flux was maximal at 10 min. At this power
output, it seems possible that lactate production continued
because some muscle fibers were experiencing an inade-
quate delivery of O2, thereby maintaining the signals for
high glycolytic activity. This is supported by the continuing
decrease in the PCr store, suggesting that aerobic metabo-
lism was not able to match the demand for ATP in all fibers.
Also, although the average whole body power output was
90% V˙O2max, the demand on some fibers may have ex-
ceeded their aerobic potential, thereby requiring continued
anaerobic ATP provision (PCr degradation and lactate pro-
duction). These suggestions are strong possibilities because
the subjects in the cited studies were not aerobically trained
and had not adapted to optimize O2delivery to the muscle
and the use of O2by the muscle.
250% V˙O2max. At the most intense power output stud-
ied, subjects sprinted maximally on an isokinetic cycle
ergometer for 30 s. The power output decreased from
?800–900 W in the initial seconds of exercise to ?500 W
after 30 s and averaged ?625 W (24,36). The average power
output represented ?250% of the power output required to
elicit V˙O2maxin these subjects. The demand for ATP in this
situation was so severe that it required both the aerobic and
anaerobic systems to reach maximal rates of ATP resynthe-
sis as quickly as possible. Breath-by-breath measurements
suggest that V˙O2reached ?75–80% of V˙O2maxin 30 s (30)
Figure 5—Human muscle pyruvate dehydrogenase activation (trans-
formation) to the active (a) form during cycling for 10 min at varying
power outputs (35–90% V˙O2max) and 30 s at 250% V˙O2max. Data are
means ? SE and were obtained from references 18, 28, and 30.
Figure 6—Estimations of flux through the key enzymes linked to
lactate production during the 1st minute of cycling at 65% V˙O2max.
Units forflux, mmol substrate?kg?1
(wet)?min?1. Abbreviations and symbols as in Figure 1, and see Table
1 for calculations.
LACTATE PRODUCTION IN HUMAN SKELETAL MUSCLEMedicine & Science in Sports & Exercise?
and contributed only ?20–30% of the total required ATP
during the 30-s bout (24,30). Anaerobic pathways provided
the remainder of the ATP with glycolysis contributing ?65–
70% (muscle lactate increased to ?90 mmol?kg?1dm) and
PCr degradation contributing 30–35% (PCr decreased to
?20 mmol?kg?1dm) of the total anaerobic contribution.
Therefore, the overall energy contribution for 30 s of sprint
cycling was 20–30% aerobic, 46–53% anaerobic glycolysis
and 24–27% from PCr degradation.
At this power output, the calculated fluxes through mus-
cle PHOS and PFK approached the maximal values mea-
sured in vitro and were 50- to 75-fold higher than at 35%
V˙O2max(Table 1, Fig. 7). At these high glycolytic rates,
pyruvate was metabolized mainly through LDH (flux aver-
aged 60 mmol pyruvate?kg?1wm?min?1), even though ac-
tivation of PDH was already maximal (?3.5–4 mmol
pyruvate?kg?1wm?min?1) after only 15 s of sprinting (28).
The accumulations of free ADP, AMP and Piwere ex-
tremely rapid and higher after only 30 s of exercise than
after 1 min of aerobic exercise (Figs. 3 and 4), accounting
for the large increase in glycolytic flux.
The extreme ATP demand at this power output required
that maximal activation of aerobic and anaerobic ATP pro-
ducing pathways were reached as rapidly as possible. The
aerobic systems were 75–80% activated at the end of 30 s,
but PDH was fully activated after 15 s of sprinting. Because
there was an abundance of pyruvate available, it suggests that
the delivery of O2to the mitochondria is the factor that limits
how quickly aerobic metabolism is turned on during this type
of exercise. In spite of this limitation, aerobic metabolism
contributed 20–30% of the required ATP. It should also be
wave fashion at the onset of sprinting, the aerobic ATP con-
tribution would only rise to 40% of the total required energy.
This makes it very clear how important the anaerobic systems
are for providing the balance of the energy required during
maximal sprint situations. This includes the production of
lactate and regeneration of NAD in the LDH reaction.
This paper examined the production of lactate in human
skeletal muscle over a range of power outputs (35, 65, 90, and
250% V˙O2max) from an enzymatic point of view. Flux through
the reaction substrates, NADH and pyruvate. The factors gov-
erning the production of these substrates will largely dictate
how much lactate is produced at any exercise power output.
At low power outputs, the signals that turn on glycogen-
olysis/glycolysis accumulated to a minor extent, resulting in
low rates of pyruvate and NADH production in the cyto-
plasm. PDH and the SS enzymes metabolized the majority
of these LDH substrates, resulting in little or no lactate
production. At moderate and high power outputs (65 and
90% V˙O2max), the mismatch between the ATP demand and
ATP provision at the onset of exercise increased as a func-
tion of exercise intensity, resulting in accumulations of the
glycogenolytic/glycolytic activators (free ADP, AMP, and
Pi). The resulting glycolytic flux, and NADH and pyruvate
production was greater than could be handled by the SS and
PDH reactions, and lactate production was related to the
exercise intensity. The delivery of O2to the muscle, the
activation rate of the aerobic ATP producing pathways, and
a mass action effect of increasing glycolytic flux may all
have contributed to lactate production during the transition
from rest to exercise.
At the highest power output studied (?250% V˙O2max),
the signals that stimulate both the aerobic and anaerobic
ATP producing pathways were maximal. Aerobic pathways
produced only 20–30% of the total ATP requirement, leav-
ing the remainder for the anaerobic glycolytic and PCr
systems. Consequently glycogenolytic/glycolytic flux ap-
proached the maximal activities of PHOS and PFK measured
at least 15- to 20-fold higher than conversion to acetyl-CoA.
In conclusion, this paper has described the production of
lactate at varying power outputs from an enzymatic view-
point. However, the mitochondrial events that ultimately
result in activation of the enzymes involved in the produc-
tion of lactate remain obscure.
The authors’ experiments cited in this paper were supported by
grants from the Natural Science and Engineering Research and
Medical Research Councils of Canada. R. A. Howlett was supported
by a Gatorade Sports Science Institute student research award.
G. J. F. Heigenhauser is a Career Investigator of the Heart and
Stroke Foundation of Ontario.
Address for correspondence: Lawrence L. Spriet, Ph.D., Dept. of
Human Biology & Nutritional Sciences, University of Guelph, Guelph,
Ontario, N1G 2W1 Canada. E-mail: email@example.com.
Figure 7—Estimations of flux through the key enzymes linked to
lactate production during 30 s of sprint cycling at 250% V˙O2max. Units
for flux, mmol substrate?kg?1vastus lateralis muscle (wet)?min?1. Ab-
breviations and symbols as in Figure 1 and see Table 1 for calculations.
Official Journal of the American College of Sports Medicinehttp://www.msse.org
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