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Swimming will only take you so far

[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.

First of all, the so-called "lactate threshold" -- the running speed at which the amount of lactic acid in your blood rises above a certain baseline value -- is an excellent predictor of running performance at distances of 5 kilometers to the marathon (Tanaka et al., Medicine and Science in Sports and Exercise 16: 278-82, 1984; Coyle, Exercise and Sport Sciences Reviews 23: 25-63, 1995). Second, the ingestion of buffers that counteract the acidity of lactic acid can delay fatigue, at least in some types of exercise (McNaughton et al., European Journal of Applied Physiology 80: 64-9, 1999). Third, muscles' ability to generate ATP -- their main source of chemical energy -- is impaired by acidic conditions (Paganini et al., American Journal of Physiology 272: C501-10, 1997; Walter et al., American Journal of Physiology 272: C525-34, 1997; Conley et al., Journal of Experimental Biology 204: 3189-94, 2001). Collectively, this evidence suggests that lactic acid can indeed cause fatigue.

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.

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