Exercise and Diving

In the March and May 2004 issues of Undercurrent, I examined the estimated risk of decompression sickness (DCS) posed by exercise before, during and after scuba diving. My conclusion: Based on the research at hand, divers should avoid strenuous exercise four hours before and six hours after diving. I also reviewed research showing that a single episode of high-intensity aerobic exercise 24 hours before a chamber dive decreased the number of circulating gas bubbles in humans more than fourfold, and reduced the maximum bubble size by a half. Studies on rats showed that strenuous exercise 20 hours before a chamber dive suppressed bubble formation and reduced decompression illness-related deaths in rats, but had no meaningful effect at 48, 10, or five hours prior. This line of inquiry suggests that vigorous exercise many hours before diving may reduce the already tiny incidence of DCS.

Since then, new findings suggest exercising much closer to diving substantially reduces circulating gas bubbles, and thus may have a protective effect against DCS. Clearly, these studies have implications for our prior recommendations regarding wait time between exercise and diving.

For example, in July 2005, researchers in France reported in the journal Aviation, Space and Environmental Medicine on 16 divers compressed in a hyperbaric chamber following 45 minutes of running two hours before their chamber ride. Each diver performed two dives three days apart, one without exercise and the other with exercise before the dive. Similar to doing exercised 24 hours ahead of a dive, it was found that running two hours before a dive decreased bubble formation after diving.

Other researchers followed up with a June 2011 study published in the European Journal of Applied Physiology that examined the effect on bubble formation of a single bout of exercise one hour before a dive. Twentyfour divers did openwater dives to 100-foot depths for 30 minutes, followed by a three- minute stop at 10 feet. These divers first did a control dive without exercise beforehand, then three days later, they made a second dive after vigorous (but below-maximum effort) running on a treadmill for 45 minutes one hour before immersion. Circulating bubbles were then graded every 30 minutes for 90 minutes after surfacing. Again, the exercising divers showed significantly reduced bubble grades.

Why exercise hinders inert gas bubble formation is unknown, but there are two favored hypotheses. One is exercise increases nitric oxide, which induces relaxation and expansion of capillary walls, making their linings less sticky, so gas bubbles are off-gassed more quickly and efficiently. The second hypothesis is that, rather than altering the nitrogen elimination rate, exercise may reduce the population of gaseous nuclei from which inert gas bubbles form.

The newer findings modify my earlier advice about minimizing strenuous exertion before a dive. Specifically, semi-vigorous exercise up to one hour before may have a protective effect against DCS. In any event, such exercise appears unlikely to be harmful, as micronuclei formed in the tissues as a result of predive exercise are thought to be compressed and squeezed back into solution, making them less of a concern.

As regards high-intensity activity during and after a dive, my advice has not changed. Microbubbles present at those times can only be problematic, i.e., they expand upon ascent, and in sufficient number and/or size, they can precipitate DCS.

In closing, don't forget that theise guidance only refers to exercise at the start of each new dive day. Once a dive has been made, exercise before, during or after that dive would be exercise following a previous dive, and not recommended. Also, this guidance does not advocate the avoidance of post-dive activity of all kind. Vigorous, joint-jarring exercise decidedly is a bad idea, and so is taking a nap. Your best protection is mild, gentle-on-the-joints activity, such as an easy swim or walk, following a dive.

- - Doc Vikingo