Posted: April 22, 2005

Science of Sport: The Gender Gap

By Owen Anderson, Ph. D. (copyright © 2003-2005)

Male and female endurance athletes respond to training in exactly the same way. When interval workouts are conducted at vVO2max, vVO2max improves, regardless of gender. Similarly, lactate-stacker sessions improve the lactate-threshold running speeds of male and female athletes to the same extent, running-specific strength training enhances economy similarly, and hill training boosts fatigue-resistance in the same manner.

In spite of these parallel responses, however, top male endurance athletes achieve better performance times than similarly trained females. This is true in a variety of different sports, from bicycling and cross-country skiing to swimming, running, and rowing. In general, elite men are about 10 to 11 percent faster than elite women.

The reasons for this disparity have been hotly debated, and some exercise physiologists have suggested that the mechanisms underlying the performance gap may vary according to sport. In endurance rowing, for example, a sport in which the "10-percent rule" holds true (1), large body size, or at least an expanded lean body mass, is believed to be a key advantage which male rowers have over female rowing athletes (2 & 3). In fact, internationally competitive rowers tend to weigh significantly more than club-level rowers (4), and rowing performance has been strongly and positively linked with leg-muscle size in recent research (5). In theory, augmented muscle mass should allow rowers to move the oars with greater power.

It has also been logically argued that increased height per se is beneficial for rowing performance. The reasoning is simply that greater height enhances the length of the rowing stroke (2); indeed, some research has linked long stroke lengths with higher levels of rowing performance (6).

In accordance with these observations, FISA (The Federation Internationale des Societes d'Aviron) has included official lightweight divisions in major, elite rowing championship competitions since 1985. For men, the maximum weight permitted in these lightweight competitions is 72.5 kilograms (average weight is around 70 kg); for lightweight women, the max mass allowed is 59 (mean weight is approximately 57 kg). In contrast, the weights of male and female competitors in the open classes of competition average about 94 and 79 kilograms, respectively. Generally, open-class performance times are about 2.5-percent faster in male rowers, compared with lightweight males, and open-class times are around 5-percent faster in females, compared with the lightweight clockings. It has been argued, however, that this difference (between 2.5 and 5 percent) is not "real" and simply reflects the fact that the caliber of lightweight women's competitions has not yet reached the same level achieved by males. At any rate, it is clear that expanded body size in rowing is associated with stronger performances.

In an endurance sport such as running, however, large body size can be a disadvantage. No scientific investigation has ever positively linked body stature with distance-running performance; in fact, one key problem is that bone mass tends to increase exponentially as height increases, giving larger endurance runners more "dead weight" to lug around their 10-K or marathon routes (this may be less of a disadvantage in rowing, since body weight is supported by a seat). In general, elite male runners are much more similar in size to elite female runners, compared with the size differential between elite male and female rowers, and yet the 10- to 11-percent difference in performance is present in each sport.

Some physiologists have suggested that body-size difference is a red herring, and that almost all of the difference between male and female performances can be explained by differences in relative aerobic capacities. For example, elite male rowers possess maximal aerobic capacities which are about 20-percent higher than those of elite female rowing athletes (7), not too far above the discrepancies which exist between top-of-the-world male runners and their female counterparts.

To find out exactly why the performances of male and female endurance rowers are so different, Chie Yoshiga and M. Higuchi of the University of Tokyo and the National Institute of Health and Nutrition in Japan recently studied 71 female and 120 male rowers (8). Age range of the athletes was 18 to 24 years, and all of the subjects usually rowed at least five days a week, covering from 60 to 100 weekly kilometers on the water or with a rowing ergometer. Average 2-K rowing time for the females was 498 seconds (range 437-556 seconds), while mean time for the males over the same distance was 424 seconds (range 378-484), a difference of about 15 percent.

During the study, each athlete completed an all-out, 2000-meter row on an ergometer (Concept II model C). On a separate day, all 191 athletes performed a progressive treadmill run to determine VO2max. For this effort, the inclination of the treadmill was set at 3 percent, and the initial running velocity was 140 meters per minute for females and 160 meters per minute for males. This speed increased by 20 meters per minute every two minutes until an athlete could no longer maintain the heightened velocity for the required two minutes; throughout this exertion, the oxygen content of expired gas was measured by a Respiromonitor™ so that VO2max could be estimated.

As you would expect, the average height and mass were smaller for the female rowers, compared with the males, and rowing performance was strongly correlated with both body height and body mass. In addition, mean fat-free mass was lower in females, compared to males (45 kilograms in females versus 62 kilograms in males), and performance was very strongly linked with fat-free mass: The greater the fat-free mass, overall mass, or height, the faster the 2-K time.

So did differences in body size and fat-free mass account for all of the difference in performance between male and female rowers? To see if this was the case, Chie and her colleague compared the performances of males and females of the same height, body mass, and fat-free mass. When males and females of the same height were compared (26 females and 25 males with height equal to 170 centimeters (67 inches)), the males were still faster, covering 2K in 433 seconds, versus 478 seconds for the ladies (about a 9-percent swing). When males and females of the same mass (62 kilograms, or 136 pounds) were compared (37 females, 57 males), males were again faster, with clockings of 436 seconds against 477 seconds for the women (a 9-percent spread again).

However, when males and females with the same fat-free mass (51 kilograms or 112 pounds) were compared from a 2-K performance standpoint; the men checked in at 446 seconds, against 466 seconds for the females (just a 4-percent differential). Similarly, when VO2max was brought into the picture, males and females with identical maximal aerobic capacities (VO2max) had just marginally different 2000-meter times; the males finished in 441 seconds, versus 460 seconds for the females (a 4-percent difference again). Although this wasn't specifically addressed in the study, it is likely that there would have been virtually no performance difference between males and females with equal lean body mass and VO2max.

How can we summarize this? For a competitive female rower, being the same size, in terms of height or weight, as a male competitor does not dramatically pare down the "gender gap; being the same size as male counterparts still leaves female athletes slower in performance by about 9 percent. However, being similar in fat-free mass or VO2max to male competitors eliminates nearly the entire gender chasm, chopping the performance difference to just 4 percent. Being similar in both areas probably takes the male performance edge away completely.

To put it another way, equalizing body size between male and female rowers takes away about half of the difference in performance times between the two sexes, but perhaps only because body size is a kind of surrogate for the quantity of lean body mass. When lean body mass is the same between males and females, the performance difference nearly vanishes, and equalizing both lean body mass and maximal aerobic capacity probably makes male and female performances indistinguishable.

A similar situation prevails in endurance running. Male runners typically have higher max aerobic capacities than female runners, and part of the reason for this is that males routinely engage in a perfectly legal, natural form of "blood doping." The key male sex hormone - testosterone - promotes the production of hemoglobin, an oxygen-carrying protein found inside red blood cells, and testosterone also increases the concentration of red cells in the blood. The key female sex hormone - estrogen - has no such effect. As a result, each liter of male blood contains about 150 to 160 grams of hemoglobin, compared to only 130 to 140 grams for females. The bottom line is that each male liter of blood can carry about 11-percent more oxygen than a similar quantity of female blood.

Of course, male world records at running distances from 800 meters all the way up to the marathon are about 10- to 11-percent faster than female world marks. Is this just a coincidence, or does the 11-percent enhancement of blood oxygen in males directly translate into an 11-percent improvement in competitive running speeds? Since oxygen is needed to furnish most of the energy required for endurance running, some scientists have suspected that the 11-percent oxygen difference is the key culprit behind male-female performance variation.

In unique research designed to gain insight into this question, Kirk Cureton and colleagues at the Human Performance Laboratory at the University of Georgia removed just under a liter of blood from each of 10 male athletes so that their blood-hemoglobin concentrations would be the same as those found in 11 female athletes. Three days after this bloodletting, both the males and females were tested for VO2max and endurance capacity (9).

The blood removal caused male hemoglobin concentrations to drop to 134 grams per liter, exactly the same as in the female-athletes' blood. In addition, male VO2max values plunged by 7 percent and became roughly equivalent to female values. However, although male endurance capability declined by about 5 percent in response to the blood draw, males continued to fare better than females during an endurance test which involved pedaling a bicycle ergometer for as long as possible against steadily increasing resistance.

As it turned out, Dr. Cureton and an amiable co-researcher named Phil Sparling from the Department of Physical Education at the Georgia Institute of Technology had actually known for several years that higher hemoglobin and its resulting elevation of blood oxygen content were not the whole story behind faster male running. Cureton and Sparling recognized that testosterone also stimulated muscle-mass development, while estrogen tended to spur on the accumulation of fat. Cureton and Sparling were aware that female runners tended to be eight to 10 percentage points fatter than their male counterparts and wondered how such enhanced corpulence might influence performance times.

So, in experimental work involving 34 male and 34 female runners who were very similarly trained (the average running distance was 25 to 30 miles per week), Sparling and Cureton tested each runner for VO2max, body fat, and running economy and then asked all 68 runners to run as far as possible in 12 minutes (10).

Males did significantly better on the test, running an average of almost 3300 meters in 12 minutes, compared with 2750 meters for females. Although the male performance was about 20-percent better, males did not run more economically than the females, and male maximal aerobic capacities (VO2maxs) were actually only slightly higher (by about 5 percent). So what caused the big difference in performance?

As it turned out, percent body fat averaged 20 percent in the females but only 11 percent in the guys (very similar to the 22-11 disparity noted by Chie in her study). When Sparling analyzed the data, he found that a whopping 74 percent of the variation between male and female performances could be accounted for by this difference in body fat, while a much smaller amount (just 20 percent) was explained by the males' higher VO2max values. The increased body fat in the female runners basically acted as dead weight, making quality running paces more strenuous.

Of course, if the enhanced presence of non-propulsive fat is the key factor separating male and female running speeds, attaching appropriate weights to the bodies of male runners should nearly equalize the performance times of the two sexes. Ingeniously, Sparling and Cureton collaborated in a study in which weight-bearing harnesses were strapped over the shoulders of 10 male distance runners (11). The idea was to make the percent excess weight lugged around by the male runners equal to the weight of the extra body fat carried along by 10 female distance runners who also participated in the study. From five to 23 pounds were added to individual male runners, depending on leanness (naturally, leaner males had more weight added).

In a test in which the athletes ran as far as possible in 12 minutes, males ran an average of 568 meters farther than females when not wearing weights but only 395 meters farther with the additional luggage. In a second test in which the athletes ran for as long as possible on a treadmill, males ran four minutes longer without weights but only 2.7 minutes longer with the added poundage. The gap between the weight-loaded males and the fat-loaded females would have been even narrower, but the males in the study happened to be more economical runners than the females. The better male economy meant that even when males and females were carrying around similar amounts of excess weight (artificial weights for the males and extra fat for the females), a particular running pace felt easier for the males. This improved economy in the male distance runners was an anomaly; many subsequent studies have found similar economies in the two sexes.

Some scientists have speculated that significant differences in muscle composition might also slow females a bit during endurance competition. However, research in this area is hardly conclusive. In one high-quality study, men and women in the same sport or event had exactly the same average fiber-type distributions (12), although male athletes were more likely to have extremely high concentrations (> 90 percent) of either fast or slow muscle fibers. In contrast, other research has found that the extremes for percentages of slow fibers were the same for male and female athletes, while the mean percentage of slow fibers was higher in men (79 percent versus 69 percent) (13 & 14). At this point, it's impossible to say with assurance that males' faster endurance performances are in any way related to muscle composition.

This all means that when just two things - percent body fat and VO2max - are equalized, the running performances of similarly trained males and females become nearly identical. As an excellent example of this, one scientific investigation found that eight male and eight female runners with equivalent VO2max values and nearly equal amounts of body fat (about 17 percent) had nearly perfectly matched finishing times in a 15-mile race (15).

At a typical weekend 10K, you can expect the average male finishing time to be about 44 minutes and the average female clocking to be close to 50 minutes, approximately one minute per mile slower. That might make females appear to be less speedy than males during distance running, but remember that the race is not properly handicapped. If an appropriate amount of fat were added to the males' bodies and a bit of male hemoglobin was drained away to equalize VO2max values between the sexes, the finishing times would be essentially identical for males and females with similar training programs.

The situation is remarkably similar in rowing. As Chie Yoshiga has discovered in her research, the difference in rowing performances between males and females can be almost completely explained by sex-related disparities in VO2max and fat-free mass. We expect that the same scenario prevails in cycling and cross-country skiing, too. We'll wait to see whether natural fatness and oxygen-processing capability account for the gender difference in swimming, a sport in which females' added fat levels might slightly improve buoyancy and thus buoy efficiency of movement. It is clear that the 10- to 11-percent rule for male-female competitive speeds also applies in elite swimming; female world records for freestyle, breaststroke, butterfly, backstroke, and even individual-medley swimming at distances ranging from 100 meters up to 1.5K are generally about 10- to 11-percent slower for women.

As a final point, it would be negligent of us not to mention that rowing is an excellent form of cross-training for endurance runners. The reason for this is that rowing can provide an outstandingly intense workout without exposing the leg muscles to impact stresses. Rowing brings almost all of the major muscle groups in the body into action, and the rate of oxygen consumption during a very intense rowing session tends to be significantly higher, compared with a high-level running exertion (16). In addition, blood-lactate levels during a strenuous rowing session can reach an incredible 30mM/liter (17), far above the concentrations associated with even the most rigorous interval workout in runners (most endurance runners will never get their blood-lactate readings above 15mM). Such high lactate levels might "teach" runners' muscle cells to get much better at removing lactate from the blood and metabolizing it internally and thus might have a very positive impact on lactate threshold. ©

References:

(1) "Rowing," In: Shephard, R. J., Astrand, P. O., Eds. Endurance in Sport, Oxford: Blackwell Scientific, pp. 836-843, 2000
(2) "The Physiology of Rowing," Journal of Sports Science, Vol. 1, pp. 23-53, 1983
(3) "Rowing Performance: A Mathematical Model Based on Analysis of Body Dimensions as Exemplified by Body Weight," European Journal of Applied Physiology, Vol. 52, pp. 88-93, 1983
(4) "Isometric Rowing Strength of Experienced and Inexperienced Oarsmen," Medicine and Science in Sports, Vol. 7, pp. 280-283, 1975
(5) "Rowing Prevents Muscle Wasting in Older Men," European Journal of Applied Physiology, Vol. 88, pp. 1-4, 2002
(6) "Determinants of 2000m Rowing Ergometer Performance in Elite Rowers," European Journal of Applied Physiology, Vol. 88, pp. 243-246, 2002
(7) "Influence of Body Mass on Maximal Oxygen Uptake: Effect of Sample Size," European Journal of Applied Physiology, Vol. 84, pp. 201-205, 2001
(8) "Rowing Performance of Female and Male Rowers," Scandinavian Journal of Medicine and Science in Sports, Vol. 13, pp. 317-321, 2003
(9) "Sex Difference in Maximal Oxygen Uptake," European Journal of Applied Physiology, Vol. 54, pp. 656-660, 1986
(10) "Biological Determinants of the Sex Difference in 12-Minute Run Performance," Medicine and Science in Sports and Exercise, Vol. 15(3), pp. 218-223, 1983
(11) "Distance Running Performance and Metabolic Responses to Running in Men and Women with Excess Weight Experimentally Equated," Medicine and Science in Sports and Exercise, Vol. 12(4), pp. 288-294, 1980
(12) Physiology of Sport and Exercise, Human Kinetics: Champaign, Illinois, p. 447, 1994
(13) "Muscle Fiber Composition and Enzyme Activities in Elite Female Distance Runners," International Journal of Sports Medicine, Vol. 8 (Supplement 2), pp. 103-106, 1987
(14) "Submaximal and Maximal Working Capacity of Elite Distance Runners. Part II. Muscle Fiber Composition and Enzyme Activities," Annals of the New York Academy of Sciences, Vol. 301, pp. 323-327, 1988
(15) "A Physiological Comparison of Performance-Matched Female and Male Distance Runners," Research Quarterly of Exercise and Sport, Vol. 56, pp. 245-250, 1985
(16) "Heart Rate Is Lower during Ergometer Rowing than during Treadmill Running," European Journal of Applied Physiology, Vol. 87, pp. 97-100, 2002
(17) "Arterial Desaturation during Exercise in Man: Implication for O2 Uptake and Work Capacity," Scandinavian Journal of Medicine and Science in Sports, Vol. 13(6), pp. 339-358, 2003

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