ORIGINAL RESEARCH

Using an Experimental Bicycle Seat to Reduce Perineal Numbness

Kenneth S. Taylor, MD; Allen Richburg, MD;
David Wallis, MD; Mark Bracker, MD

THE PHYSICIAN AND SPORTSMEDICINE - VOL 30 - NO. 5 - MAY 2002

ABSTRACT

BACKGROUND: Perineal numbness and erectile dysfunction are emerging as health concerns among bicyclists. Three studies indicate that between 7% and 21% of male cyclists experience genital area numbness after prolonged riding.

OBJECTIVE: To evaluate the effect of an experimental seat design on perineal numbness.

DESIGN: Fifteen experienced male cyclists exercised for 1 hour on a stationary spin cycle using either an experimental or standard bicycle seat. Several days later they repeated the trial using the other seat type. Before and after each 1-hour exercise session, perineal sensation was tested using the Weinstein Enhanced Sensory Testing (WEST)-hand esthesiometer. Cyclists were also asked to report their perception of numbness after each exercise bout.

RESULTS: Cyclists reported more numbness with the standard seat than with the experimental seat (79% vs 14%; P=0.009). Similarly, sensory testing at all perineal sites yielded greater hypoesthesia with the standard seat than with the experimental seat (P=0.05). This difference was most marked at the dorsal penis (P=0.04).

CONCLUSION: The experimental bicycle seat produced significantly less subjective and objective numbness than the standard cycle seat in 1 hour of stationary cycling. Bicycle seat design and innovation may decrease or eliminate perineal numbness.

Cycling for fitness and competition has become an extremely popular form of recreation.¹ While the benefits of cycling are similar to those of many cardiovascular activities, the sport has several hazards that are unique.^2-5 Many case reports and epidemiologic studies demonstrate an association between prolonged cycling and perineal numbness and erectile dysfunction.^2,4-16 Three studies that evaluated cyclists after prolonged cross-country, multiday cycling events revealed a prevalence of perineal numbness that ranged from 7% to 21%.^2,4,17

Perineal hypoesthesia and erectile dysfunction commonly occur together. After a 540-km race, numbness with concomitant erectile dysfunction was slightly more prevalent than numbness without erectile dysfunction.² Conversely, erectile dysfunction without numbness was much less common. According to another investigation,⁷ up to 50% of male cyclists performing long day rides experience perineal neuropathic symptoms. A more recent epidemiologic survey¹⁸ revealed the frequency of genital numbness between 58% and 70% among German cycling club members. The cyclists had twice the rate of erectile dysfunction compared with other noncyclist athletes.

Perineal hypoesthesia and the development of impotence may be two points on a continuum representing conditions with a common pathophysiology. It therefore follows that preventing numbness may also prevent the more serious condition of erectile dysfunction. The pathophysiology of perineal hypoesthesia is likely multifactorial; however, seat design and position may play a significant role.^{2,4,5,8-11,14-17} Furthermore, several authors^9,11,15 have reported that adjusting or changing the seat caused riders' symptoms to resolve. Most of the scientific information, unfortunately, is based on subjective data. Adding to the confusion is the fact that many companies are now designing and marketing untested seats in the hopes of preventing these problems. To evaluate the effect of seat design on subjective and objective perineal numbness, we performed a crossover comparison trial using a standard cycle seat and an experimental seat designed to reduce or prevent excessive perineal pressure.

Methods

Subject selection. The comparison trial was approved by the University of California, San Diego Human Subjects Program. Before participation, each subject signed an informed consent form that described the nature of the experiment. Informational surveys regarding cycling experience, medical illnesses, and history of perineal numbness or erectile dysfunction were distributed to male cyclists in San Diego county. Of those who completed and returned the surveys, the first 15 cyclists to volunteer for the testing became our sample population.

Trials, seats, and subjective assessments. Each subject performed two 1-hour exercise trials using a standard stationary spin cycle (Reebok Studio Cycle 2000C, chain drive, Studio Cycle, Santa Monica, California; www.clubreebok.com) fitted with one of two types of seats. One exercise trial employed a standard seat (figure 1A), and the other, an experimental seat (Gootter and Williams, Inc, Encinitas, California; figure 1B). Each trial was performed in a standard 1-hour session during which subjects remained seated for the entire test. Seat and handlebar heights were adjusted to each rider, while the seat for all subjects was kept parallel to the floor. After each session, the cyclists completed a questionnaire about subjective numbness. Several days separated exercise bouts to prevent residual sensory symptoms from interfering with the second trial.

Sensory testing. Before and immediately after each exercise session, the subjects were tested for bilateral dorsal penile, anterior scrotal, and posterior scrotal sensory thresholds using the Weinstein Enhanced Sensory Testing esthesiometer (WEST-hand, Connecticut Bioinstruments, Inc, Danbury, CT) and a modified rapid threshold protocol.^19,20 Examiners performing sensory measurements were blinded to the seat type that the subject used. Five different forces ranging from 200 g to 70 mg were applied to each site. The subjects were asked to report "yes" if they felt the forces. Each "yes" response was assigned one point, and these site scores were summed to yield a total score. Objective numbness (hypoesthesia) was defined as a decreased site score or a decreased total score postexercise compared with preexercise. This number was designated as the hypoesthesia index for the subject. Negative scores (greater post- than preexercise score) were designated zero since higher sensitivity after exercise more likely represents a learning effect and not true exercise- or seat-induced hyperesthesia.

Statistical analysis. Subjective and objective site-specific sensory evaluations were analyzed using Fisher's Exact test. Overall hypoesthesia indices were compared using a one-tailed paired Student's t-test. The significance level was 0.05.

Results

Subject characteristics and participation. Fourteen of 15 subjects (93%) successfully completed both exercise sessions (table 1). One subject dropped out after the first exercise session due to perineal discomfort from the standard seat and did not exercise using the experimental seat. Most subjects reported having experienced perineal numbness at least once before the study. Only one of the subjects reported a history of erectile dysfunction.

TABLE 1. Characteristics of Participants in a Crossover Trial of Bicycle Seats Parameter Number Enrolled subjects Subjects completing study Average age (range) Average miles ridden/wk (range) Experienced subjective genital numbness History of erectile dysfunction 15 14 34 187 12 1 (93%) (23-56) (55-300) (80%) (7%)

Numbness measures. Subjectively, cyclists reported significantly more numbness with the standard seat than with the experimental seat (P=0.009, table 2). Using the WEST-hand esthesiometer, researchers found that sensory testing at all perineal sites combined yielded more hypoesthesia with the standard seat than with the experimental seat (P=0.05). This difference was most marked at the dorsal penis, where a significantly greater number of cyclists displayed penile hypoesthesia after using the standard seat (P=0.004). Twice as many cyclists displayed anterior scrotal numbness after using the standard seat (see figure 2); however, this difference was not statistically significant. There was no significant difference in posterior scrotal hypoesthesia.

TABLE 2. Genital-Area Numbness in Subjects Using Standard and Experimental Cycle Seats Measure Standard Seat (No.) Experimental Seat (No.) Probability Subjective perineal numbness 11 2 0.009* Objective perineal numbness:   Dorsal penis   Anterior scrotal   Posterior scrotal 11 8 9 3 4 8 0.004* 0.126 0.50 Hypoesthesia index 3.43 1.86 0.05† *Significant at the 0.05 level †Trend is toward significance, but difference is not significant

Correlation between measures. In general, cyclists' subjective perception of numbness correlated well with objective findings. Numbness was most pronounced at the dorsal penis: Subjective sensation correlated with sensory testing in 12 of 14 subjects (86%) using the standard seat and 11 of 14 (79%) using the experimental seat. In the cyclists whose subjective reports did not correspond with objective findings, 2 reported but did not display numbness, and 3 denied but demonstrated perineal hypoesthesia when sensory testing was done.

Discussion

Our study shows that a cycle seat specifically designed to prevent excessive perineal pressure significantly reduced hypoesthesia compared with a traditional cycle seat. The experimental seat design was based on the anatomic course of the pudendal and perineal neurovascular bundles and has a central open area to prevent pressure near the perineal vessels and nerves regardless of the posture of the rider (even in the forward position—the drops—often assumed in racing). Our study confirms, as other case reports have implied,^9,11,15 that seat design contributes to perineal hypoesthesia. Given that perineal numbness and erectile dysfunction often coexist, it seems plausible that seat design also may prevent cycling-associated impotence.

Anatomic testing. Force threshold testing with the Semmes-Weinstein monofilaments and Weinstein-enhanced system may be the easiest and most reliable way to diagnose neuropathy.²¹ The Weinstein-enhanced monofilaments are a reliable objective test of cutaneous sensitivity and have been verified in a variety of neuropathic models.^19,21-23 We chose to test bilaterally the dorsal penile shaft, anterior scrotum, and posterior scrotum to evaluate the areas that were reported and that we suspected to be involved. While our study clearly demonstrates a strong statistical difference between seat-mediated hypoesthesia of the dorsal penis, the difference becomes less apparent at the more posterior sites. This finding may stem from less pressure on the anterior perineal structures with the experimental seat.

The dorsal penis alone, however, is likely the most specific area with the least chance for false-positive results for several reasons. First, regions having hair such as the scrotum and perineum are more difficult to test with the WEST-hand esthesiometer and are less reliable.²⁰ Second, no external manipulation is required to test the dorsal penile site. Exposing the posterior scrotum requires manipulation that may result in confusion of stimulus. Finally, the dorsal penile area is generally free from external compression when cycling. Hypoesthesia in this region, therefore, likely represents true proximal neurapraxia rather than local skin hypoesthesia from the immediate effects of local compression.

The basis for numbness. The pathophysiology of cycling-associated perineal hypoesthesia and erectile dysfunction is not well understood. While some authors have implicated a primary neuropathic process,^6,9,11,12,14 others have favored a vascular theory.²⁴ Direct nerve compression or entrapment in the perineum is believed to occur in the ischiorectal fossa (Alcock's canal).^6,12,14 Alternatively, the forward-leaning position may pinch the nerve against the pubic arch.^9,15 Others^2,8,15 have studied whether the phenomenon arises from a primary problem of vascular insufficiency that then leads to ischemic neuropathy. In canine models of erectile response, bilateral vascular insufficiency is necessary to significantly affect penile tumescence.²⁵ Reports of unilateral return of penile rigidity preceding bilateral recovery may, therefore, favor a neurogenic rather than a purely vascular cause.^2,7

The pudendal nerve and artery course together through the perineum. Given the proximity of these structures, a cycle seat designed to prevent significant perineal pressure is likely to reduce the risk of both perineal hypoesthesia and erectile dysfunction, regardless of whether the pathogenesis of these disorders is of neurologic or vascular origin. Therefore, innovations in cycle seat design such as the experimental seat tested are likely to prevent or limit these disorders.

Preventive measures. Other measures for preventing these disorders have been described. Of primary importance is proper fit of the cycle, including frame size, handlebar height, and seat position.²⁶ Likewise, cyclists should angle the seat parallel to the ground or slightly forward.^{2,4,5,11,12,27} A wider seat or one with a central cutout, as in our study, can limit perineal pressure and distribute weight to the proper site—the ischial tuberosities. Many competitive cyclists have eschewed wider seats in favor of the smaller, narrower, and lighter-weight versions. A seat with a cut-out thus may be particularly attractive to these cyclists who, due to their riding styles and high mileage, are at high risk for these problems.

Fitness cyclists may consider a recumbent model, which puts less pressure on the vital perineal structures.²⁴ Frequent standing on the pedals at regular intervals prevents prolonged regional pressure as well. Avoiding significant time in the forward position as well as riding in higher gears shifts pressure to the legs from the perineum. Using the legs as shock absorbers when riding over bumps is important to prevent acute or cumulative trauma and is of particular importance to mountain bikers who ride over rough and varied terrain.

Potential study limitations. When designing our study, we chose a sample of individuals at high risk of perineal numbness and erectile dysfunction: avid cyclists with several years' experience. Initially, we were concerned that only a minority would experience numbness and that our sample size might be insufficient to evaluate perineal hypoesthesia. It became apparent from our sample that most of the cyclists had in fact experienced numbness previously. Some cyclists felt numb only once, while others experienced numbness regularly. While a potential for sample bias exists—those most willing to volunteer might be the ones who experience the disorder—many cyclists who were not part of the trial but completed the survey reported numbness as well. Our data suggest that the true prevalence of perineal numbness in elite cyclists may actually be underestimated in the few published descriptive studies.^2,4,5,17

Other potential limitations involve anecdotal reports from some cyclists that episodes of perineal numbness are occasionally short-lived, lasting only a few seconds to a few minutes. The sensory threshold thus might have changed during the interval between dismounting and sensory testing (about 30 to 60 seconds). The time from cycle to testing, however, was relatively constant between riders and was irrespective of seat type. Therefore, this brief time delay was an unlikely confounding variable in our study.

The Evolving Cycle Seat

While cycling clearly has well-documented cardiovascular benefits, many riders experience symptoms of concern, such as perineal hypoesthesia. The experimental seat designed to limit perineal pressure significantly reduced the incidence of perineal hypoesthesia when measured both by subjective questioning and by objective sensory testing. Proper seat design can reduce the risk of cycling-associated perineal hypoesthesia and therefore possibly reduce the risk of erectile dysfunction. Further studies are needed to evaluate the pathophysiology of these disorders to improve our efforts to prevent them.

The WEST-hand esthesiometer was graciously donated by Connecticut Bioinstruments, Inc through its grant no. 98007. No financial support was provided by the company. This project was partially funded by Gootter and Williams, Inc, the makers of the experimental seat. Many thanks also go to the University of California, San Diego Department of Recreation for providing the spin cycles and use of its fitness room.

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Dr Taylor is an associate physician in the department of Family and Preventive Medicine and codirector of the sports medicine fellowship at the University of California, San Diego (UCSD). Dr Richburg is a clinical instructor in the departments of family medicine and preventive medicine and an associate physician at the San Diego Sports Medicine and Family Health Center. Dr Wallis is a resident physician at the UCLA-Santa Monica Family Medicine residency program, and Dr Bracker is a clinical professor in the departments of family medicine and preventive medicine at UCSD and codirector of the sports medicine fellowship. Address correspondence to Kenneth S. Taylor, MD, UCSD Medical Group, 9350 Campus Point Dr, Box 0968, La Jolla, CA 92037; e-mail to kstaylor@ucsd.edu.

Disclosure information: Drs Taylor, Richburg, Wallis, and Bracker disclose no significant relationship with any manufacturer of any commercial product mentioned in this article. No drug is mentioned in this article for an unlabeled use.