Sex Differences in Diving: Fact or Folklore

The first female Navy diver made history in 1975 when graduating from the Navy’s Deep Sea Diving School after Navy divers were exclusively male for more than 100 yr. The physical differences between men and women have often been cited to discourage women from becoming divers. However, the public image of women being too frail to physically perform the same as men has been shattered despite longstanding exclusionary views within this male dominated community — opinions which remain today (see the comments following this article (25)) even though not endorsed by the Department of Defense. A recent implementation plan states that the first female enlisted Navy Sea, Air, and Land Teams (SEALs) could be assigned to units in fall 2017, and the first female SEAL officers could be in place by 2018 (25). Yet, varying views about the ability/safety of women as divers remain. The greatest potential physiological differences between male and female divers include physical performance, decompression sickness (DCS), and thermal balance. Disparate qualities of research and much supposition, though, make it difficult to weed out fact from fiction. On average, women have lower maximal aerobic capacities and produce less speed and power than men. Multiple layers of underclothing, dry suit, mask, water-filled tube suit with power supply and pump, rebreather or SCUBA tanks, and weights to counteract buoyancy can easily force a diver to carry an additional 150 lb of gear today (and 250 lb when women first became Navy divers!). Additionally, dive suits, facemasks and air tanks are designed for men, forcing women divers to adapt gear for a better anatomic fit (40). These challenges could increase physical demand before water entry and make movement underwater cumbersome. Because many combat diving roles involve light physical activity levels on the sea bottom, power and speed may not be as critical. Conversely, manual dexterity/accuracy and fine motor skills, which have been suggested to be better in women by some (16,35,36) but not others (24), may be more valuable skills. Regardless, the physical performance standards required for divers are sex-neutral, so a female diver, who can pass these standards, should not have any greater risk of reduced physical performance while diving than a man. It also is possible female divers could have better overall performance than men. Numerous diving duties require buoyancy and because women have greater subcutaneous fat stores, this may prove advantageous in some diving jobs. Furthermore, not only has the energy expenditure of surface swimming been shown to be less in woman than men even after correcting for body size (33), but women divers also expend less energy when swimming underwater using predominantly fins and scuba gear compared to men (34). This was attributed to less drag in female divers who remained more horizontal than men and suggests women are more economic divers if men do not proactively position their diving weights to adjust attitude. Consequently, if male divers consume greater amounts of oxygen (i.e., gas), this results in shorter sustainable dive times than women. Therefore, overall, because training adaptations are similar in men and women and women have some inherent advantages, it is possible that female divers could physically outperform male divers. After early work found women were four times more likely than men to get altitude DCS based on incidence in the US Air Force from 1966 to 1977(4), the concept of women being more susceptible to DCS was born; later work, though, revealed no such differences (41). These results may not be comparable to DCS in hyperbaric exposures because diving and altitude exposures result in different gas saturation profiles. A 1979 questionnaire suggested that women divers reported a 3.3-fold greater incidence of either suspected or diagnosed DCS (1), but women often self-report symptoms more readily than men (10) and numerous subsequent epidemiological studies found that DCS risk in diving was independent of sex (28,29). The Naval Diving and Salvage Training Center examined the logs of 878 dives and found no increased risk of DCS among female divers; in fact, men had a 1.3% higher risk than women (43). Similar results were found in two retrospective cohort studies of dive masters in which DCS risk was independent of sex (22,34). The exact etiology of DCS is not known, but is believed to result from an event cascade initiated by nitrogen bubble formation (15). The current model suggests that while bubbles are required for DCS, they are not sufficient for causing DCS. The few randomized controlled studies on DCS risk in female divers are based on bubble formation as outcome measures with inconsistent results. Women had fewer circulating bubbles after open-sea SCUBA dives than men (5). Others have found no difference in bubble incidence, duration, latency, and time to peak grade after prolonged dry (6) and wet (14) hyperbaric exposure compared to men. Authors suggested that any sex-based differences in DCS susceptibility occur subsequent to bubble formation in the pathogenesis of DCS. Conversely, others found that bubble grade was higher in women than in men following dry hyperbaric exposure, but this did not result in greater DCS risk; in fact, no signs of clinical DCS were present (9). It was recently confirmed, though, that bubbles are poor surrogate endpoints for DCS (11). Consequently, DCS risk as assessed by bubble formation is tenuous at best and should not be used to suggest female divers have an increased risk of DCS compared to men. Research does not support menstruation as a DCS risk factor (26). Questionnaires and self-reported problems during diving in female recreational and sport divers show that diving-related problems were not evenly distributed over the menstrual cycle and varied with oral contraceptives (OC) use (12,27,37), but no randomized controlled trials have (40) assessed the risk of DCS between menstrual cycle phases or with OC use or, more importantly, (1) directly compared any difference in risk with that of men. Also, because it always seems to be asked, there is no evidence that sharks are attracted to menstruating divers or that sharks associate the small amount of menstruation fluid with feeding opportunities. Military diving is often carried out in cold water, putting divers at risk of hypothermia but also loss of manual dexterity, which could impede mission success even with the use of thermal protective garments. Sex-related differences in thermal balance during whole-body cold exposure seem to be primarily attributed to differences in body composition and anthropometry (39). In cold environments, resting women of equal body mass to men generally have lower skin temperatures and, consequently, lower core to skin thermal conductance than men (9,13,20,23,42), likely the result of more insulating subcutaneous fat. While lower skin temperatures and the ability to better store heat in the core suggest increased survivability in the cold, it also could mean impaired manual dexterity, tissue damage risk, reduced thermal comfort, and overall colder thermal status if the diver is compromised in acral skin regions. Although this is supported by the finding, that during local cold exposure women have lower finger skin temperature and blood perfusion than men (2,21), most of this is speculation and has yet to be determined in whole-body cold exposure. Given equivalent subcutaneous fat thickness, women generally have a larger surface area to body mass ratio than men. This results in greater total heat loss for women due to higher convective heat loss and lower total heat content from less muscle mass. Because shivering sensitivity (17,18) is the same for men and women, at mildly cold temperatures shivering could compensate for this. However, the smaller muscle mass appears to limit a woman’s maximal heat generating capacity in very cold water. As a result, core temperature declines more rapidly in extreme cold (30,31). When exercise is required during cold-water dives, male and female divers have similar thermal balance, though, because women have a more favorable distribution of subcutaneous fat on the exercising limbs where blood is diverted (31). In summary, women compensate for cold exposure differently than men and thus, depending on the conditions and job at hand, could have equivalent or different thermal balance status as men. Similar to the risk of DCS, women experience differences in thermoeffector responses to cold during various phases of the menstrual cycle and with the use of OC. This includes differences in shivering sensitivity (19), finger skin blood flow and temperature (3), skin vasoconstriction sensitivity to core temperature (7), and neurotransmitter modulation of reflex vasoconstriction (38). However, like DCS risk, these differences have not been directly compared with men, and thus, menstrual cycle stage and OC use cannot be said to put female divers at a greater risk of improper thermal balance compared to male divers. As a body of research, sex differences among divers is lacking. Overall, more randomized controlled trials are needed. For example, dive tables were generated to safeguard against divers, who were male at the time, from getting DCS, so the dive tables and the research using them could all be inherently biased toward the success of one sex over another. To classify women as unable or at greater risk to be military/combat divers is not warranted given the current literature, and to afford female military divers the same opportunities as male divers, we must include them not only in military positions but also include them more in future research.