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ORIGINAL RESEARCH
http://www.physsportsmed.com/issues/2002/05_02/taylor.htm
Selected Quotes From Physsportsmed.com:
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 SPORTS
MEDICINE - VOL 30
- NO. 5 - MAY 2002
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.
"Our study shows that a cycle seat specifically designed to prevent
excessive perineal pressure significantly reduced hypoesthesia compared
with a traditional cycle seat. Our study
confirms, as other case reports have implied 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."
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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 authors9,11,15 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; http:// 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 Others2,8,15
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.
References
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- Bond RE: Distance bicycling may cause ischemic neuropathy of
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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.
http://www.physsportsmed.com/issues/2002/05_02/taylor.htm
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