Point:Counterpoint: Lactic acid accumulation is an advantage/disadvantage during muscle activity
Point:Counterpoint: Lactic acid accumulation is an
advantage/disadvantage during muscle activity
Graham D. Lamb, D. George Stephenson, Jens Bangsbo, and Carsten Juel
J Appl Physiol 100: 1410-1412, 2006;
POINT-COUNTERPOINT
Point:Counterpoint: Lactic acid accumulation is an
advantage/disadvantage during muscle activity
Graham D. Lamb and D. George Stephenson
POINT: LACTIC ACID ACCUMULATION IS AN ADVANTAGE DURING MUSCLE
ACTIVITY
Lactic acid accumulation inside muscle fibers is not responsible for
decreased muscle performance ("muscle fatigue"). Muscle fatigue
occurs because of disturbance to any of the steps in excitation-
contraction (EC) coupling. There are several broad types of muscle
fatigue, and the contribution of each to the overall decline in
performance depends on the muscle fiber type and the intensity and
duration of the activity (1, 27)...
Further support for the proposition that lactic acid accumulation is
advantageous during muscle activity is provided by the properties of
the two major monocarboxylate transporters (MCTs), which play a major
role in the regulation of intracellular pH and lactate concentration
during intense muscle activity..
Finally, we note that a rise in blood lactate (the "lactate
threshold") can indeed be used as an indicator of exhaustion.
However, although lactate may well increase when muscle performance
declines, lactate is not the cause of the decline. Lactate rises in
the blood when the muscle cells are using ATP faster than they
resynthesize it aerobically in the mitochondria. But it is the other
changes occurring in the muscle, not the lactic acid accumulation,
which cause the fatigue. Acidity associated with lactic acid
accumulation actually helps delay the onset of muscle fatigue that
would otherwise ensue from the other effects of vigorous activity.
Counterpoint: Lactic Acid Accumulation is a Disadvantage During
Muscle Activity
Jens Bangsbo1 and Carsten Juel2
In an editorial of the journal Science, lactic acid was recently
described as "the latest performance-enhancing drug" (1). The
conclusion was based on studies of isolated rat skeletal and skinned
skeletal muscle fibers (11, 13, 14), and the authors appear to have
gone too far in the interpretations of their data. In humans, the
negative consequences of lactic acid accumulation far exceed any
positive effects....
In conclusion, there is ample evidence in both humans and animals
that accumulation of lactic acid during exercise contributes to the
development of fatigue during intense exercise. It is also clear that
lactic acid accumulation and the associated lowering of intracellular
pH are not the crucial factors and the main effect may be that the
lowered pH leads to a greater release of potassium from the
contracting muscles.
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REBUTTAL FROM DRS. LAMB AND STEPHENSON
The Counterpoint by Bangsbo and Juel does not provide evidence that
lactic acid accumulation in muscle is disadvantageous. Instead the
authors agree with us in saying that "lactic acid accumulation and
the associated lowering of intracellular pH are not the crucial
factors" in the development of fatigue. They argue that raised
extracellular [K+] is the main cause of fatigue and agree that
intracellular acidosis does have a positive effect by decreasing
chloride permeability. Given this, we must say we are puzzled that
they further argue that the results of Nielsen et al. (5) supporting
these conclusions are somehow incorrect and based
on "nonphysiological depolarization."
……In conclusion, the case for beneficial effects of lactic acid
accumulation is clear.
REBUTTAL FROM DRS. BANGSBO AND JUEL
We have read the contribution by Lamb and Stephenson with great
interest. It was, however, a challenge to find data supporting their
hypothesis. Most of the arguments seem to be based on speculation.
They refer to a few human studies, and we agree that "extracellular
K+ does rise to critical levels during normal exercise." However, the
argument that "it seems very likely that the decrease in
intracellular pH has substantial beneficial effects in exercising
humans by delaying the onset of fatigue" does not have experimental
support. In contrast, a number of studies have shown that lowered pH
leads to a greater potassium release and potassium accumulation in
muscle interstitium, as we have described in our contribution.
Furthermore, that the McArdle patients display faster onset of
fatigue due to lack of lactate accumulation is hard to follow.
Fatigue in these patients is more likely related to a high K+ efflux
and the reduced number of Na+-K+ pumps compared with control subjects
(3).
The arguments for a positive role of lactic acid are based on studies
of isolated noncontracting muscle and skinned muscle fibers. In our
review we argued that these results represented artificial
nonexercise-related conditions and it is not possible to extrapolate
to the in vivo condition……..
The capacity for lactate/H+ transport in human muscle is increased by
training and there is a positive correlation between transport
capacity (i.e., efflux) and performance (5); these findings further
suggest that lactic acid accumulation is a disadvantage.
In the 1990s, we argued that high lactate and low muscle pH is not
the primary cause of fatigue in humans (1), but it seems to
contribute during intense exercise. As it currently stands, evidence
is still lacking that this is not the case.