Among 41 competitive cyclists, the 19 who rode bikes with relatively low handlebars had, on average, significantly higher vibratory thresholds in the anterior vagina, compared with riders whose handlebars were level with the bike saddle, according to Marsha K. Guess, MD, of Yale University, and colleagues.
Measurements of perineum saddle pressure also showed significant increases in the cyclists with low handlebars, the researchers reported.
"Correcting modifiable risk factors for pelvic floor damage may serve as the most important next step in enhancing riding safety in women cyclists," the researchers wrote.
Genital numbness has been a recognized complaint of male as well as female cyclists, Guess and colleagues explained. They had conducted a previous study of 48 women who rode bicycles competitively and 22 female runners, which confirmed a decrease in genital sensation in the cyclists.
Bicycle saddle (seat) design appears to contribute to the problem, but, the researchers noted, how the cyclist sit on the saddle is also a factor, and perhaps the dominant one.
An earlier cyclists study by another group had found that pressures on the pelvic floor in both sexes were related to the relative positions of the handlebars and saddles.
Consequently, the researchers reanalyzed their data from the 48-rider sample, focusing on 41 participants for whom information on their bikes was available. These data included pressures exerted by the bike saddle while riding as well as biosthesiometry measures taken at various points in the women's genital areas.
The latter were expressed as vibratory thresholds, in microns. Participants received steadily increased stimulation at each point and were told to indicate when they first felt a sensation.
Low handlebars force the cyclist to exaggerate their forward lean, decreasing the trunk angle and increasing the pelvic tilt. The researchers hypothesized that this lean forward position would put more weight over the perineum region relative to the ischial tuberosities.
Results were reported for 22 cyclsists whose handlebars were level with their saddles compared with the 19 with lower handlebar positioning.
Generally the two groups were similar, except for a strong trend toward younger age for those with low handlebar sets. The mean age was 34.5 in the low-handlebar group versus 37.8 in the control group (P=0.12). About 40% of the latter group was 40 or older, compared with 16% of those setting their handlebars low.
Mean total saddle pressures did not differ between the two groups. However, there was a trend toward greater peak total pressure with low handlebars, and mean and peak perineal pressures were significantly greater.
Peak perineal pressures, for example, averaged 60 kPA in the low-handlebar group versus 50 kPA in the control group (P<0.05).
Vibratory thresholds were significantly greater -- reflecting diminished sensation -- with low handlebars in two of eight points measured. These were the anterior vagina (mean 2.8 microns versus 2.0 microns, P=0.02) and the left labia (mean 3.0 microns versus 2.0 microns, P=0.03).
However, in a multivariate model that took the age difference between groups into account, the decrease in genital sensation was significant only in the anterior vagina (by 0.86 microns, 95% CI 0.18 to 1.53, P=0.014).
The raw data indicated that, at the remaining six locations (including the clitoris, urethra, right labia, and left and right perineum), there were no reductions in sensation with low handlebars.
Guess and colleagues were surprised by some of the findings. "We cannot explain why genital stimulation deficits were one-sided in the labia," they wrote, but they speculated that riders may have "a dominant side" on which they exert more pressure.
The researchers concluded that low handlebar positions "yield detrimental effects to the female pelvic floor," but they also conceded that higher positions may increase the riders' wind resistance and reduce their speed.
"This exemplifies the ongoing struggle between selecting a more aerodynamic bicycle position for control and speed or a more ergonomic position to reduce neurovascular compromise," Guess and colleagues wrote.
The study was supported by the National Institutes of Health and the Robert Wood Johnson Foundation.
The authors reported no potential conflicts of interest.
Primary source: Partin S, et al "The bar sinister: Does handlebar level damage the pelvic floor in female cyclists?" J Sexual Med 2012; DOI: 10.1111/j.1743-6109.2012.02680.x