[This article originally appeared in the
October 2002
issue of Northwest Runner
magazine.]
Q.I have a
question concerning a cross-training article I read. Basically, it
stated that for runners, swimming will improve your running because of your
increased ability to handle lactate. The author theorized that when you run,
you don't use your arms much. When you swim, you use them a lot. Therefore, by
cross-training with swimming, you can create extra "lactate buffering
capacity" in your arms that your body can utilize while running. It sounds
good, but it sounds too good to be true. --Andy Winch, Portsmouth, VA
A. I think you're right; this idea probably is too good
to be true. Its validity depends on three important assumptions, which we will evaluate below.
ASSUMPTION #1: Lactic acid causes fatigue.
This assumption is somewhat controversial. In the old days, fatigue during
exercise was usually attributed to a buildup of lactic acid (also called
lactate, which technically is the non-acidic part of lactic acid). It is now
clear that many factors other than lactic acid can cause fatigue, and some
researchers have even suggested that lactic acid does not impair muscle
function (Nielsen et al., Journal of Physiology
536: 161-6, 2001; Stackhouse et al.,
Physical Therapy 81: 1897-1903, 2001). Nevertheless, at least three lines of evidence indicate that
an accumulation of lactic acid can be detrimental to performance.
ASSUMPTION #2: Swim-trained arm muscles handle lactic acid more effectively
than untrained arm muscles.
This assumption, like #1, is probably correct. Exercise training
increases the number of monocarboxylate transporters (MCTs) in the muscles
being
trained, which enhances the transport of lactic acid from the blood to those
muscles and vice versa (Bonen, European Journal of Applied Physiology 86: 6-11, 2001; Juel, European Journal of Applied Physiology 86: 12-6, 2001). In other words, regular swimming should enhance
your arm muscles' ability to remove lactic acid from the blood.
So far, the cross-training idea looks pretty reasonable. To summarize, when
you run, some of the lactic acid generated by your leg muscles will move
into the blood. It could then be removed efficiently from the blood by the
swim-trained arm muscles, thus permitting more lactic acid to be transported
out of the leg muscles, possibly preventing fatigue.
ASSUMPTION #3: While you run, blood flow to the arm muscles is high.
The scenario described in the previous paragraph is only possible if lots of
blood passes through the swim-trained arm muscles during running. If
blood flow to the arms is very low, an enhanced removal of lactic acid from
this tiny amount of blood will not do the body much good.
It is well-established that, during exercise, highly active muscles receive
the lion's share of blood flow, while very little flow is distributed to
resting muscles (Rowell, Physiological Reviews 54: 75-159, 1974;
Musch, Medicine and Science in Sports and Exercise 20: S104-8, 1988). During running, the leg muscles do most of the work
and thus receive most of the blood flow, whereas the arm muscles are not very
active, receive little blood flow, and consequently have little effect on the
body's lactic acid buildup even if they are well-trained.
The above reasoning certainly does not mean that swimming and
other cross-training activities are completely useless. Cross-training is
better than no training at all, and it can give your legs a break from the
pounding of the roads and introduce variety into your exercise routine.
Nevertheless, when it comes to improving your lactic acid clearance while
running, the benefits of cross-training appear to be limited.
Thanks to Arend Bonen of the University
of Waterloo (Canada), George Brooks of the University of California at
Berkeley, and Carsten Juel of the August Krogh Institute (Denmark) for sharing
their thoughts on lactic acid transport with me.