Exercise and Diving: When is it a Good Thing?

by Doc Vikingo

It’s Only Common Sense To Be Physically Fit

As scuba enthusiasts know, being fit not only makes diving more relaxing and enjoyable, but also leaves them prepared to meet the demands on heart, lungs and muscle made by heavy current, long surface swims and buddy rescues.

What divers may not know is that fitness ranks high among defenses against DCS. Dive physiology experts have long expressed that fit divers may be at less risk because their robust capillary systems efficiently eliminate inert gases like nitrogen.

For the average diver, being in proper shape means a regular (i.e., at least 3 times per week) program of muscle strengthening, aerobics and stretching. In particular, the muscles of the back, abdomen, hips and legs should be strong and limber. One should be able to perform 20-30 minutes of moderate aerobic activity without losing endurance or becoming winded to the point of over-breathing a regulator. Activities that promote cardiopulmonary health, flexibility and coordination should be incorporated into the day to day routine and not reserved only for periods dedicated to work outs.

There are a number of regimens to maintain adequate conditioning and most health club trainers can design a program to meet the diver’s needs. For the do-it-yourself type, guidelines developed by the President's Council on Physical Fitness and Sports are an excellent place to start. Plus there are plenty of self-help books available at bookstores and online.

But What About Exercise In Close Proximity to Diving?

As with many things in life, timing is important.

Research since the late 1980s indicates that certain exercise the day before a dive suppresses inert gas bubble production and reduces the incidence of DCS.

A recent study reported that a single episode of high-intensity aerobic exercise 24 hours before a chamber dive decreased the number of bubbles more than fourfold and halved maximum bubble size.

Follow up studies on rats found that high-intensity aerobic exercise suppressed bubble formation and reduced DCI-related deaths when performed 20 hours prior to a simulated dive. This benefit was not noted when the exercise occurred substantially before or after 20 hours. At one-half hour prior to the dive, greater bubble formation was reported.

A just reported work, however, found no significant differences in bubble grade or score, or survival or survival time, when this type of exercise was performed 30 minutes before a dive.

Dr. Alf O. Brubakk, prominent among the authors of these studies, hypothesizes that aerobic exercise may reduce bubble formation and the incidence of DCS through the production of nitric oxide. A chemical that relaxes and dilates capillary walls, it may allow activity-induced micronuclei to pass out of circulation before they can contribute to DCS. [NOTE: A more in-depth discussion of this research can be found “More About Exercise and Diving” in the May ’04 edition of Undercurrent].

Theory and science suggest that mild aerobic activities which gently increase heart rate and redistribute blood to muscle, like easy walking, promote efficient off-gassing.

However, intensely aerobic and strenuous joint and muscle punishing actions like hard running and heavy lifting are not recommended.

Early Navy research shows that post-dive exercise can increase the incidence of DCS, and more recent findings from Duke University demonstrate that DCS can develop from diving shortly after weightlifting. While the rigorousness of the diving and exercise involved well exceeded that normally performed by recreational divers, these studies nonetheless suggest a correlation between strenuous exertion and DCS.

Pre and post-dive, exercise can court both dehydration and injuries that may be mistaken for DCS. It also increases bodily temperature and blood flow, changes that may have implications for the on and off-loading of inert gas.

Peripheral blood vessels tend to dilate as temperature rises and constrict as it falls. As such, a cool diver who jumps into warm water may on-gas more slowly than when the temperature differential is reversed. The opposite could be desirable after surfacing; off-gassing concerns may be lessened when the diver is warmer following a dive than during it.

Like Tiny Bubbles, Some Of It Is Less Obvious

Rapid or powerful movements of joint, other skeletal and muscle surfaces are possibly more worrisome than exercise-related on and off-gassing rates. Literally, such actions may sow the seeds of DCS.

By the processes of nucleation and cavitation, turbulence in fluids adjoining moving bodily surfaces results in the formation of minuscule entities termed micronuclei. Given sufficient loading, these may serve as receptacles for the diffusion of gasses passing from the dissolved to the free gas phase as the diver surfaces. If such micronuclei grow to bubbles of critical size and number, DCI can ensue.

Dr. Michael Powell, a respected NASA research scientist, is firmly convinced that micronuclei generated by the wrong sort and timing of exercise are a major contributor to increased risk of DCS.

How About Activity During A Dive Itself?

Here, too, varying activity levels appear to carry varying risk and advantage.

Work by Dr. Richard Vann, Vice President of DAN Research, indicates exercise at depth increases the risk of DCS, while exercise during decompression may reduce risk by nearly one-third.

On balance, it appears wisest to limit activity while on-gassing at depth, but to remain mildly active (e.g., moving limbs, swimming easily) while off-gassing during ascent and safety stops. But watch it getting back on the boat—climbing ladders in full gear and wrestling with heavy kits are actions to be avoided.

How Long Should I Hold Off On Strenuous Activity Before And After A Dive?

DAN’s Diving Medicine website suggests a minimum two-hour delay between exercising and diving and considers four hours safer. On his Diving Medicine Online website Dr. Ernest Campbell (aka Scubadoc) states that, “If one were to put four restful hours between exercise and diving and six between diving and exercise, a diver should be in good shape in terms of absent bubbles.”

The truth is we do not know the extent to which DCS risk is affected by reasonable exercise before or after recreational scuba, and any recommendations regarding timing are only best guesses. Considering this uncertainty, it is arguably wisest to err on the side of conservatism.

However, this is not to suggest indolence or sleep shortly after diving as these could slow nitrogen elimination to an undesirable extent.

In a recent investigation of the cardiopulmonary effects of recreational scuba, subjects were assessed pre and post-dive in either a seated or laying down posture. Both positions were associated with post-scuba venous bubbles in the heart area and impairments of cardiac output and gas exchange.

The Bottom Line

The best advice is moderation—avoid extremes of temperature and activity. Stay well hydrated and warm. Don’t perform strenuous activity for 4 and 6 hours before and after diving, respectively, longer if carrying a very large inert gas load.

Remain mildly active upon ascent, between dives and for a while after the last dive. Light exertion that is gentle on the joints and done in a warm environment should result in a favorable trade-off between inert gas elimination and micronuclei formation.

In situations where strenuous activity is unavoidable, e.g., hauling heavy gear, when you cannot remain active after scuba, dive more conservatively. Do shallower and shorter profiles, limit the number of dives per day, take a mid-week scuba break, add a deep stop where appropriate, do an extended safety stop and surface interval, and use nitrox if properly certified.

About the Author: DocVikingo is presently a regular contributor to Undercurrent, and in the past wrote a monthly column for Scuba Diving. A mental health professional in the greater Washington, D.C. area for nearly 30 years, he made his first dive with a double hose regulator and no BCD.

© Doc Vikingo 2004
Reprinted by permission of Undercurrent

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