Content uploaded by Jaap J Van Netten
Author content
All content in this area was uploaded by Jaap J Van Netten on Apr 07, 2014
Content may be subject to copyright.
Arch Sex Behav (2008) 37:279–285
DOI 10.1007/s10508-006-9112-9
ORIGINAL PAPER
8–13 Hz Fluctuations in Rectal Pressure Are an Objective Marker
of Clitorally-Induced Orgasm in Women
Jaap J. van Netten · Janniko R. Georgiadis ·
Arie Nieuwenburg · Rudie Kortekaas
Received: 20 March 2006 / Revised: 7 September 2006 / Accepted: 7 September 2006 / Published online: 21 December 2006
C
Springer Science+Business Media, LLC 2006
Abstract Orgasm is a subjective experience accompanied
by involuntary muscle contractions. We hypothesized that
orgasm in women would be distinguishable by frequency
analysis of a perineal muscle-derived signal. Rectal pressure,
an index of perineal muscle activity, was measured contin-
uously in 23 healthy women during different sexual tasks:
receiving clitoral stimulation, imitation of orgasm, and at-
tempt to reach orgasm, in which case the women were asked
to report whether orgasm had been reached (“orgasm”) or
not (“failed orgasm attempt”). We performed spectral anal-
ysis on the rectal pressure data and calculated the spectral
power in the frequency bands delta (0.5–4 Hz), theta (4–
8 Hz), alpha (8–13 Hz), and beta (13–25 Hz). The most
significant and most important difference in spectral power
between orgasm and both control motor tasks (imitation of
orgasm and failed orgasm attempt) was found in the alpha
band. An objective rule based on spectral power in the al-
pha band recognized 94% (29/31) of orgasms and correctly
labeled 69% (44/64) of all orgasm attempts as either suc-
cessful or failed. Because outbursts of alpha fluctuations in
rectal pressure only occurred during orgasm and not during
voluntary imitation of orgasm or failed attempts, we propose
J. J. van Netten ·J. R. Georgiadis · R. Kortekaas
Department of Anatomy and Embryology, University Medical
Center Groningen, University of Groningen,
Groningen, the Netherlands
A. Nieuwenburg
Department of Urology, University Medical Center Groningen,
University of Groningen,
Groningen, the Netherlands
J. R. Georgiadis (
)
Department of Anatomy, University Medical Center Groningen,
PO Box 196, Groningen, AD 9700, the Netherlands
e-mail: j.r.georgiadis@med.umcg.nl
that they represent involuntary contractions of muscles in
the rectal vicinity. This is the first objective and quantitative
measure that has a strong correspondence with the subjective
experience of orgasm.
Keywords Orgasm
.
Rectal pressure
.
Spectral analysis
.
Fast fluctuations
.
Women
Introduction
Orgasm in women is a subjective experience and although
it has been studied quite extensively (Mah & Binik, 2001),
there is no objective marker. Most physiological research
has focused on contractions of striated perineal muscles
(Mah & Binik, 2001), because the occurrence of involun-
tary (reflexive) muscular contractions is an important fea-
ture of orgasm in women (Carmichael, Warburton, Dixen,
& Davidson, 1994; Masters & Johnson, 1966; Reubens,
1982). The activity of these muscles has been studied di-
rectly through EMG (Carmichael et al., 1994) and indirectly
through pressure measurements in viscera that penetrate the
pelvic floor (Bohlen, Held, Sanderson, & Ahlgren, 1982;
Graber & Kline-Graber, 1979). Based on this, muscles pre-
sumed to contract during orgasm are the levator ani muscle
(Bohlen et al., 1982; Graber & Kline-Graber, 1979) and
anal sphincter (Carmichael et al., 1994; Masters & Johnson,
1966). Uterine contractions are assumed to occur during or-
gasm (Bohlen et al., 1982; Chayen, Tejani, Verma, & Gordon,
1979; Masters & Johnson, 1966) because oxytocin, a pow-
erful stimulant of uterine smooth muscle, is released into the
blood during orgasm (Carmichael et al., 1987, 1994).
In a search for an objective orgasm marker, three types of
orgasm in women have been described based on rectal and/or
vaginal pressure patterns (Bohlen et al., 1982; Carmichael
Springer
280 Arch Sex Behav (2008) 37:279–285
et al., 1994): one type is characterized by regular contrac-
tions, another by regular and irregular contractions, and a
third type by the lack of regular contractions. However, this
method is neither objective nor quantitative.
Furthermore, the correspondence between these con-
tractions and the subjective orgasmic experience (received
through self-report of the women) differed between stud-
ies. While Carmichael and colleagues described that sub-
jective markers correspond to measured pelvic contractions,
such an objective-subjective agreement was not found by
others (Bohlen et al., 1982; Masters & Johnson, 1966).
For example, the three patterns described by Bohlen et al.
(1982) did not correspond with three discernible subjective
experiences.
In their review, Mah and Binik (2001) have proposed that
the frequency content of muscle contractions during orgasm
may be a good marker for the subjectiveorgasmic experience.
Therefore, we aimed to distinguish orgasm-specific muscle
contractions by their frequency characteristics. We measured
rectal pressure as an index of perineal muscle activity while
women attempted to reach orgasm through clitoral stimula-
tion. Spectral analysis was used to fractionate the signal into
different frequency bands.
Method
Participants
A total of 23 healthy heterosexual female participants (M
age, 33.5 years; range, 21 to 51) participated in the study to-
gether with their partner after giving written informed con-
sent according to the Declaration of Helsinki. Participants
were recruited via an online casting bureau. The enrollment
period was three years, but over this period the procedure
and equipment remained unchanged. The procedures were
approved by the Medical Ethics Committee of the University
Medical Center Groningen. Thirteen participants were nul-
liparrous. Ten participants had one or more children, four of
which were born by caesarian delivery. Twenty-one partic-
ipants were Caucasian and two were from African descent.
Educational level of the participants was variable. None of
the participants used recreational drugs or had a history of
physical, psychiatric or sexual disorders.
Procedure
Rectal pressure was measured during a positron emission
tomography (PET) study on brain activation related to sexual
stimulation and orgasm. This meant that women lay supine in
a brain scanner while being measured. The partner stood next
to the participant and manually stimulated her clitoris. This
type of stimulation was chosen because it induces no artifacts
in rectal pressure and is the most effective means of inducing
orgasm in women (Lloyd, 2005). During the debriefing, the
participants did not report important differences between
their “laboratory orgasms” and those reached under normal
circumstances.
Measures
Rectal pressure measurements (in cm H
2
O) were made with
a rectal probe (B. Braun Melsungen AG
c
, type 501002/7;
Melsungen, Germany) containing a microtip transducer. The
signal was sampled via the MMS system (Medical Measure-
ments Systems B.V.; Enschede, The Netherlands) and 50 Hz
data were stored for analysis.
Measurements were performed for two minutes in the
following sequence (see also Table 1). At first, rectal pres-
sure during non-sexual rest was measured as a reference task
to make inter-individual comparison possible. For measure-
ments 2 and 3, women were asked to imitate orgasm, i.e., to
Table 1 Experimental tasks
Time (minutes) Measurement Instruction
0–2 1 Passive non-sexual resting state
2–10 No measurements
10–12 2 Imitating an orgasm while the clitoris was stimulated
12–20 No measurements
20–22 3 Imitating an orgasm while the clitoris was stimulated
22–30 No measurements
30–32 4 Stimulation of the clitoris
32–40 No measurements
40–42 5 Stimulation of the clitoris
42–50 No measurements
50–52 6 Stimulation of the clitoris, trying to reach orgasm
52–60 No measurements
60–62 7 Stimulation of the clitoris, trying to reach orgasm
62–70 No measurements
70–72 8 Stimulation of the clitoris, trying to reach orgasm
Springer
Arch Sex Behav (2008) 37:279–285 281
voluntarily contract muscles in the rectal vicinity (abdomi-
nal, hip, thigh, and perineal muscles) in a rhythmic fashion,
while their clitoris was being stimulated by their partner.
This task served as a control for the motor output during
orgasm, because we expected the same muscles to contract,
but with discernible frequency characteristics. During mea-
surements 4 and 5, the partner provided clitoral stimulation
but the women were asked not to make any movements and
not to have an orgasm. At measurements 6, 7, and 8, the
partner provided clitoral stimulation with the aim of induc-
ing an orgasm. After each of these measurements, women
either reported that they had not reached orgasm (“failed or-
gasm attempt”) or that they had (“orgasm”). Failed orgasm
attempts were also used as a control for the motor output
during orgasm.
Orgasm attempts with ambiguous subjective reports were
excluded from the analysis. Stimulation of the clitoris started
before the beginning of the measurements to create an exist-
ing sexually salient context from the start of each experiment.
Because of technical requirements for the PET measure-
ments, participants were asked to attempt to reach orgasm in
a specified 40-second interval. For all measurements, only
data of this interval were analyzed.
Data analysis
Rectal pressure data (50 Hz) were analyzed with Matlab, ver-
sion 5.3 (The MathWorks, Inc.; Massachusetts). The power
spectrum was calculated by Fourier analysis. Total spectral
power in the delta (0.5–4 Hz), theta (4–8 Hz), alpha (8–
13 Hz) and beta (13–25 Hz) band was calculated by discrete
integration of the power spectrum over frequency. The spec-
tral power in each frequency band was divided by that dur-
ing non-sexual rest (intra-individual normalization). These
normalized integrals did not have a normal distribution over
tasks and participants, so analysis of variancewas done with a
Kruskal-Wallis test for task effects, and a post-hoc Wilcoxon
rank sum test was performed for between-task comparison in
each frequency band, using a Bonferroni correction for mul-
tiple comparisons (α
crit
=.0083, because of six comparisons
in every frequency band).
We developed an orgasm detecting algorithm that la-
beled orgasm attempts as either “failed orgasm attempt” or
“orgasm.” It was based on spectral analysis of rectal pres-
sure during orgasm attempts and used the frequency band
that was most characteristic for orgasm as input. This was
done by introducing a cut-off value for spectral power that
sorted orgasm attempts into one of these two categories,
i.e., all orgasm attempts with spectral power above a spe-
cific cut-off value were classified as “orgasm” and those
with spectral power under the cut-off value were classified
as “failed orgasm attempt.” The participant’s statement was
used as gold standard. The optimal cut-off value was defined
as the one that reached the highest classification accuracy,
i.e., the maximum overlap between objective and subjective
measures. Specificity, sensitivity, positive predictive value,
and negative predictive value were also calculated.
Results
Of the 23 participants, 17 achieved one or more orgasms. A
total of 31 orgasms and 33 failed orgasm attempts were in-
cluded for analysis. Visual inspection of the pressure vs. time
graphs showed that patterns of rectal pressure during orgasm
were very variable between participants. Further examina-
tion suggested that, in many cases, the orgasm measurement
contained more high frequency components than measure-
ments of the other tasks. Spectral analysis confirmed this.
Typical examples are given in Fig. 1. In this figure, raw
data (left panels) and power spectral density (right panels) of
rectal pressure are shown. The lines at 0.5, 4, 8, and 13 Hz
indicate the different frequency bands. Note the difference in
alpha and beta power between “orgasm” and “failed orgasm
attempt.”
A Kruskal-Wallis test was performed on the normalized
spectral power and showed a significant task effect in all
four bands (see Table 2). Post hoc analysis showed that nor-
malized spectral power in all bands was significantly lower
during clitoral stimulation than in all other tasks (see Fig. 2).
The bars indicate the median; upper and lower quartiles are
indicated by the whiskers.
No significant difference was found between both control
tasks, voluntary imitation of an orgasm and failed orgasm at-
tempt, in any of the investigated bands. However, normalized
spectral power during orgasm was significantly greater in the
alpha (8–13 Hz) and beta (13–25 Hz) bands than during both
control tasks. This was not the case for the delta (0.5–4 Hz)
and theta (4–8 Hz) bands (see Table 3 and Fig. 2). The most
significant difference in spectral power between orgasm and
both control tasks was in the alpha band, a measure of rectal
pressure fluctuations between 8 and 13 Hz (“alpha power”:
Table 3 and Fig. 2).
Based on this, the orgasm detection algorithm applied al-
pha power to classify orgasm attempts as successful or failed.
When the cut off value was chosen at 3.0, i.e., all orgasm
attempts with alpha power more than three times higher than
non-sexual rest, were classified as “orgasm,” those with less
were classified as “failed attempt,” total accuracy (overlap
between objective and subjective measures) was maximal:
69% (44/64) of orgasm attempts were identified correctly as
either failed or successful. For this cut off value of alpha
power, the specificity was 45% (15/33), the sensitivity 94%
(29/31), the negative predictive value was 88% (15/17), and
the positive predictive value 62% (29/47). The dependency
Springer
282 Arch Sex Behav (2008) 37:279–285
Fig. 1 Example of a rectal
pressure pattern (subject 15)
during five different tasks
of accuracy, specificity and sensitivity on alpha power, is
depicted in Fig. 3.
Discussion
The aim of this study was to identify an objective physiolog-
ical correlate of orgasm in women. In particular, we hypoth-
esized that the involuntary perineal muscular contractions
that accompany orgasm could be distinguished by their fre-
quency characteristics. Indeed, the spectral power in both the
alpha and beta bands, representing the strength of muscular
contractions in the 8–13 and the 13–25 Hz frequency bands
respectively, was significantly greater during successful or-
gasm attempts than during all other tasks. This was not the
case for the slower frequency bands delta (0.5–4 Hz) and
theta (4–8 Hz) (see Table 3 and Fig. 2). Especially, 8–13 Hz
fluctuations in rectal pressure were a hallmark of orgasm.
Imitation of orgasm and failed orgasm attempts also in-
volved, often forceful, contraction of striated perineal mus-
culature. Nevertheless, spectral power in the alpha and beta
bands was significantly lower during these control motor
tasks than during orgasm. Taken together, these results sug-
gest that frequency characteristics can be used to distin-
guish the involuntary muscular contractions during orgasm,
because faster fluctuations occurred predominantly during
Table 2 Results of
Kruskal-Wallis analysis
Frequency band χ
2
df p
Delta (0.5–4 Hz) 43.03 3, 84 <.001
Theta (4–8 Hz) 55.82 3, 84 <.001
Alpha (8–13 Hz) 49.47 3, 84 <.001
Beta (13–25 Hz) 35.31 3, 84 <.001
Springer
Arch Sex Behav (2008) 37:279–285 283
Fig. 2 Task effects on normalized spectral power in four frequency-bands
orgasmandnotduring voluntaryimitationof orgasm or failed
attempts.
The origin of these fast fluctuations in rectal pressure
is likely to be muscular contractions in the vicinity of the
rectum. From the present data, we were unable to determine
which muscles contributed to the signal but, according to
the literature, the following muscles contract during orgasm
and hence could be responsible for the fast fluctuations in
rectal pressure: (parts of the) levator ani muscle (Bohlen
et al., 1982; Graber & Kline-Graber, 1979), the anal sphincter
(Carmichael et al., 1994; Masters & Johnson, 1966), and the
uterus (Bohlen et al., 1982; Chayen et al., 1979;Masters&
Johnson, 1966).
A limitation of this study was the fact that participants lay
in a scanner and were asked to reach orgasm in a specified
40-second interval. Despite this, they did not report impor-
tant differences between their “laboratory orgasms” and their
normal orgasms. A benefit of this requirement was that ap-
proximately half of all orgasm attempts failed, thus creating
an additional control task for the motor output of orgasm.
Table 3 Post hoc comparisons between orgasm and imitation of orgasm or orgasm and failed orgasm in different
bands
Frequency band Tasks z df p ES (d)
Delta (0.5–4 Hz) Orgasm vs. Imitation of orgasm 0.82 73 ns 0.05
Orgasm vs. Failed orgasm attempt 1.69 62 < .05 0.27
Theta (4–8 Hz) Orgasm vs. Imitation of orgasm 3.90 73 < .001
∗
0.66
Orgasm vs. Failed orgasm attempt 2.61 62 < .01 0.51
Alpha (8–13 Hz) Orgasm vs. Imitation of orgasm 3.73 73 < .001
∗
0.62
Orgasm vs. Failed orgasm attempt 2.98 62 < .01
∗
0.17
Beta (13–25 Hz) Orgasm vs. Imitation of orgasm 2.90 73 < .01
∗
0.52
Orgasm vs. Failed orgasm attempt 2.68 62 < .01
∗
0.27
Note.
∗
p <αcrit. Comparison was only significant when p <αcrit. αcrit =0.0083 because of post hoc Bonferroni
correction for six comparisons in every frequency band; only two of these comparisons are shown. Effect size (ES) is
calculated using Cohen’s d.
Springer
284 Arch Sex Behav (2008) 37:279–285
Fig. 3 Performance of orgasm detecting algorithm
These failed orgasm attempts were similar in their delta and
theta content of rectal pressure (contractions between 0.5
and 8 Hz), but, like voluntary imitation of orgasm, lacked
the faster contractions (esp. 8–13 Hz) that accompanied
orgasm.
Orgasms in our study were all clitorally-induced. This
method was chosen because it induces no artifacts in rec-
tal pressure and is the most effective means of inducing
orgasm (Lloyd, 2005). Because of the first reason, rectal
pressure fluctuations can not be measured during orgasms
achieved through intra-vaginal stimulation. Studying the
question whether orgasms achieved via that way will give
the same result is therefore not possible. However, we think
that there will be no difference in rectal pressure fluctuations
during orgasm when the orgasm is achieved via a different
way. The assumption that there is a different typology of
clitoral versus vaginal orgasms can not be supported in the
literature, because there is a striking lack of reliable physio-
logical data for this typology (Mah & Binik, 2001).
Orgasm-associated rectal pressure patterns are highly
variable between individuals (Carmichael et al., 1994) and
we could qualitatively confirm this for our participants. We
deliberately ignored this fact and included all reported or-
gasms in the spectral analysis. Also, we always analyzed a
fixed 40-second interval containing the orgasm, regardless
of orgasm duration. Despite this indiscriminate approach,
we found that significantly higher spectral power in the al-
pha and beta band (contraction frequencies between 8 and
25 Hz) was characteristic of orgasm (see Fig. 2C and 2D).
This clearly demonstrates the robustness of this approach.
We also developed an algorithm to recognize orgasms
on the basis of rectal pressure. It used the alpha band, the
frequency band most specific for orgasm, and classified an
orgasm attempt as successful when spectral power in the al-
pha band (alpha power) was at least three times higher than
during rest. When alpha power was less than three times
higher than during rest, the attempt was classified as failed.
This algorithm could recognize 94% (29/31) of all orgasms
and correctly labeled 69% (44/64) of all orgasm attempts
as successful or failed. This means that our algorithm dis-
played a 69% overlap with the elusive subjective orgasmic
experience.
Frequency characteristics of rectal pressure might also
be of clinical use. Orgasmic disorders are the second sex-
ual problem in women by prevalence (Laumann, Gagnon,
Michael, & Michaels, 1994). One evident orgasmic disor-
der is anorgasmia, the inability to achieve orgasm. Rectal
pressure frequency characteristics might help to diagnose or
treat anorgasmic women. One might think of a biofeedback
method that informs a patient of the alpha power in rectal
pressure, in order to increase awareness of a women and
her partner. This, however, requires further research (e.g. in
women with sexual disorders).
The observation that fluctuations in rectal pressure are
orgasm-specific is in accordance with the literature, where
has been described that muscular contractions start at the on-
set of orgasm (Bohlen et al., 1982; Carmichael et al., 1994).
This means that these muscular contractions do not corre-
spond with sexual arousal, which is a sexual phase before the
orgasm. In our study, this can be seen in fluctuations in rectal
pressure during both control tasks. During personal debrief-
ing after the experiment, the participants mentioned that there
was a big difference in sexual arousal between both tasks,
being highly sexually aroused during the failed attempts.
But no differences in rectal pressure fluctuations were found
between the imitation task and the failed orgasm attempts.
Therefore, the presented method, which is orgasm-specific,
could become a meaningful addition to photoplethysmogra-
phy, which correlates mainly with physiological arousal.
Taken together, our findings indicate that 8–13 Hz fluctu-
ations in rectal pressure constitute an objective and quanti-
tative marker of orgasm in women that is sensitive and yet
robust to interindividual variability and temporal dilution.
References
Bohlen, J. G., Held, J. P., Sanderson, M. O., & Ahlgren, A. (1982). The
female orgasm: Pelvic contractions. Archives of Sexual Behavior,
11, 367–386.
Carmichael, M. S., Humbert, R., Dixen, J., Palmisano, G., Greenleaf,
W., & Davidson, J. M. (1987). Plasma oxytocin increases in the
human sexual response. Journal of Clinical Endocrinology and
Metabolism, 64, 27–31.
Carmichael, M. S., Warburton, V. L., Dixen, J., & Davidson, J. M.
(1994). Relationships among cardiovascular, muscular, and oxy-
tocin responses during human sexual activity. Archives of Sexual
Behavior, 23, 59–79.
Chayen, B., Tejani, N., Verma, U. L., & Gordon, G. (1979). Fetal heart
rate changes and uterine activity during coitus. Journal of Clinical
Psychiatry, 40, 348–351.
Graber, B., & Kline-Graber, G. (1979). Female orgasm: Role of pubo-
coccygeus muscle. Journal of Clinical Psychiatry, 40, 348–351.
Springer
Arch Sex Behav (2008) 37:279–285 285
Laumann, E. O., Gagnon, J. H., Michael, R. T., & Michaels, S. (1994).
The social organization of sexuality:Sexual practices in theUnited
States. Chicago: University of Chicago Press.
Lloyd, E. A. (2005). The case of the female orgasm: Bias in
the science of evolution. Cambridge, MA: Harvard University
Press.
Mah, K., & Binik, Y. M. (2001). The nature of human orgasm: A critical
review of major trends. Clinical Psychology Review, 21, 823–856.
Masters, W. H., & Johnson, V. E. (1966). Human sexual response.
Boston: Little, Brown and Company.
Reubens, J. R. (1982). The physiology of normal sexual response in
females. Journal of Psychoactive Drugs, 14, 45–46.
Springer
