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Gender and endurance performance

[This article originally appeared in the August 2001 issue of Northwest Runner magazine.]

Some people may think it rather obvious that, on average, men have an advantage over women when it comes to distance running. However, a 1992 analysis of track and field records (Whipp & Ward, Nature 355: 25, 1992) suggested the possibility that the fastest women in the world might catch up to the fastest men in the not-too-distant future. Such a scenario would imply that women have the same athletic potential as men. So, is that really true? Or, alternatively, do men have certain biological advantages that they are unlikely to relinquish within the next century?

I'm not going to attempt a comparison of male and female sprinters. For distance runners, however, we can compare men and women using the endurance performance model described in my February column. You may recall that the model includes three components -- maximal oxygen consumption (VO2max), the lactate threshold, and running economy -- each of which is considered an important determinant of success in races ranging from 3 to 30 miles.

Looking for biological differences

A natural question to ask is: do men and women differ with respect to these three traits? For VO2max, the answer is yes. The average VO2max is about 33 milliliters of oxygen per kilogram of body mass per minute for sedentary young women and around 42 ml/kg/min for sedentary young men (Bouchard et al., Medicine and Science in Sports and Exercise 30: 252-8, 1998). Elite female distance runners can sometimes reach VO2max readings of 70+ ml/kg/min (Pate et al., International Journal of Sports Medicine 8 (Suppl.): 91-5, 1987), whereas elite men can attain values in the 80s (Pollock, Annals of the New York Academy of Sciences 301: 310-22, 1977).

The lactate threshold -- the percentage of VO2max at which lactic acid begins to accumulate in the blood -- has not been extensively studied in female runners. Nonetheless, the available data indicate that elite women can run marathons at about 75-85% of VO2max, essentially the same as for elite men (Davies & Thompson, European Journal of Applied Physiology 41: 233-45, 1979; Iwaoka et al., International Journal of Sports Medicine 9: 306-9, 1988).

Whether men and women differ in their running economy -- a measure of how effectively the body converts oxygen consumption into forward motion -- is controversial. Of the 17 studies I have seen on this topic, six found men to be significantly more economical than women (Howley & Glover, Medicine and Science in Sports 6: 235-7, 1974; Bransford & Howley, Medicine and Science in Sports 9: 41-4, 1977; Bhambhani & Singh, Medicine and Science in Sports and Exercise 17: 131-7, 1985; Helgerud et al., European Journal of Applied Physiology 61: 433-9, 1990; Bergh et al., Medicine and Science in Sports and Exercise 23: 205-11, 1991; Daniels & Daniels, Medicine and Science in Sports and Exercise 24: 483-9, 1992); the other eleven did not (Davies & Thompson, European Journal of Applied Physiology 41: 233-45, 1979; Mayhew et al., Journal of Sports Medicine 19: 39-44, 1979; Hagan et al., Journal of Applied Physiology 49: 571-5, 1980; Cureton & Sparling, Medicine and Science in Sports and Exercise 12: 288-94, 1980; Maughan & Leiper, European Journal of Applied Physiology 52: 80-7, 1983; Sparling & Cureton, Medicine and Science in Sports and Exercise 15: 218-23, 1983; Bunc & Heller, European Journal of Applied Physiology 59: 178-83, 1989; Ramsbottom et al, Journal of Sports Sciences 7: 9-20, 1989; Padilla et al., European Journal of Applied Physiology 65: 561-6, 1992; Ramsbottom et al., Journal of Sports Sciences 10: 119-29, 1992; Ariens et al., European Journal of Applied Physiology 76: 214-20, 1997). Among the studies on elite runners, the two largest ones (Bergh et al., 1991; Daniels & Daniels, 1992) reported that men have a better running economy than women, but the other three (Davies & Thompson, 1979; Bunc & Heller, 1989; Padilla et al., 1992) found no difference between the sexes. Thus, if men do hold an edge in running economy, it must be a subtle one.

In summary, the most important physiological difference between male and female distance runners is that males tend to have larger VO2maxes even when the values are expressed relative to body size. This difference is due at least in part to the fact that women generally have (a) more body fat, which consumes virtually no oxygen (Drinkwater, Exercise and Sport Sciences Reviews 12: 21-51, 1984); (b) less hemoglobin, an oxygen delivery protein in the blood (Cureton et al., European Journal of Applied Physiology 54: 656-60, 1986); and (c) smaller hearts which can't pump as much blood per unit time (Mitchell et al., Medicine and Science in Sports and Exercise 24: S258-65, 1992; George et al., Medicine and Science in Sports and Exercise 27: 1362-70, 1995; Rowland et al., Chest 117: 629-35, 2000).

Although the gender difference in VO2max is well-documented, a mystery remains (Joyner, Exercise and Sport Sciences Reviews 21: 103-33, 1993). Some champion male distance runners (past examples include Derek Clayton and Frank Shorter) have VO2maxes of "only" 70 ml/kg/min, a value occasionally exceeded by elite women, yet no woman has ever matched their race times. Why not? Again, a firm answer is not forthcoming at present. However, it is possible that "an excellent value for one of the limiting factors [V02max, lactate threshold, and running economy] is mutually exclusive with another," Joyner speculates. If this is the case, "It might be more likely for some of the males with VO2max values in the low range for [elite] men to have outstanding running economy and lactate threshold values and, therefore, faster times in competition than a female competitor with the same VO2max."

Ladies first

The above analysis is most relevant to race distances of roughly 3 to 30 miles. When considering events toward the upper end of this range and beyond, however, some biological factors that favor women may also come into play. First, women should tolerate hot and humid racing conditions better than men due to their smaller body size (Dennis & Noakes, European Journal of Applied Physiology 79: 280-4, 1999; Marino et al., Pflugers Archiv 441: 359-67, 2000). The issue here is whether heat can be removed from the body as quickly as it is produced. The rate of heat removal depends on the body's surface area (how much skin you have, more or less), while heat production is approximately proportional to the body's volume or weight. People with a high ratio of surface area to volume are therefore better able to get rid of the heat they generate. Because of the way in which various bodily dimensions scale relative to one other, it turns out that small people have higher surface-area-to-volume ratios than large people; therefore, women may be less susceptible than men to overheating during a long race in oppressive weather.

Another interesting male-female difference is the fact that women appear to burn more fat and less carbohydrate than men during endurance exercise (Tate & Holtz, Canadian Journal of Applied Physiology 23: 570-82, 1998; Carter et al., American Journal of Physiology 280: E898-907, 2001). This offers women the possibility that, in events taking two hours or more to complete, their supply of liver and muscle glycogen (a storage form of carbohydrate) will outlast that of men. The reasons for this greater reliance on fat are not fully understood but may relate to estrogen's effects on metabolism, since male rats given estrogen burn less glycogen than male control rats during prolonged treadmill exercise (Kendrick & Ellis, Journal of Applied Physiology 71: 1694-9, 1991; Rooney et al., Journal of Applied Physiology 75: 1502-6, 1993), although similar studies in humans have yielded mixed results (Tarnopolsky et al., International Journal of Sports Medicine 22: 175-80, 2001; Carter et al., Journal of Applied Physiology 90: 139-46, 2001). As a side note, estrogen may also protect muscles from exercise-induced damage under some circumstances (Tiidus, Canadian Journal of Applied Physiology 25: 274-87, 2000).

The information presented in the previous two paragraphs leads to the prediction that women might compete against men most successfully in events lasting several hours, where overheating and glycogen depletion are particularly common. The limited data we have so far provide preliminary support for this idea. It has been shown that women can sometimes finish ultramarathons in times similar to those of men who can beat them in "short" (26.2-mile) marathons (Bam et al., Medicine and Science in Sports and Exercise 29: 244-7, 1997). And when men and women with equivalent marathon times are pitted against each other in ultras, the women tend to win (Speechly et al., Medicine and Science in Sports and Exercise 28: 359-65, 1996).


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