8-13 Hz Fluctuations In Rectal Pressure A Good Marker For Female Orgasm?: A Journal Article I Read

8-13 Hz fluctuations in rectal pressure are an objective marker of clitorally-induced orgasm in women. Van Netten JJ1, Georgiadis JR, Nieuwenburg A, Kortekaas R.
Arch Sex Behav. 2008 Apr;37(2):279-85.

This is a 2008 article that is searching for an objective, quantitative marker that matches women's subjective claims about whether or not they orgasmed. They use particular measurements from an anal pressure probe in 23 women during clitorally induced orgasm to say that they indeed have found this marker. In particular, the researchers say that increase of the alpha band (8-13 Hz) of the anal pressure measurements is a physical indication of orgasm.

This is a well-done study. However, their 8-13 Hz marker is actually not perfect. It missed identifying 2 of 31 orgasm claims from women. I would argue that theirs is not necessarily a better marker than the marker identified in the previous studies they reference. I would also argue that these markers might miss some orgasms not because they are bad markers, but because women's claims of orgasm may not be a great starting point for physically investigating orgasm.

Here's the summary. I will add in my thoughts from time to time, but I will specifically say they are my thoughts. Otherwise, everything that is said is taken from the paper.

  • Most physiological research about orgasm focused on contractions of the pelvic muscles because reflexive/involuntary contractions of these muscles "is an important feature of orgasm in women."
  • In the past contractions have been measured directly with EMG (the electric activity of those muscles) and also indirectly through measuring the pressure inside body cavities (rectum or vagina) that are affected by those muscles
  • Based on rectal or vaginal pressure, it had been previously found that there were 3 types of orgasm in women.
  1. contractions in a regular (steady) pattern
  2. a regular pattern followed by irregular contractions
  3. lack of regular contractions.
  • This paper sites 2 previous studies to back that up. The first is a study I have written about HERE (Carmichael et al), and that one only found the 1st two types of orgasmic contraction patterns. The second is an older one I have written about HERE (Bohlen et al) and that one found the 1st two and also the 3rd. Basically the 3rd grouping (2 women) is one in which the women claimed orgasm, but their pelvic muscles did not show any sign that a specific thing had happened. As the researchers in that study discuss, this 3rd grouping could be viewed as women who did not physically orgasm. However, they decided that they would base 'orgasm' off the women's claims and not off the physical markers (presence of regular muscle contractions).
  • It points out that in the Carmichael study, subjective experience of orgasm (the women saying they orgasmsed) coincides with the the objective measure of orgasm (the rhythmic pelvic contractions). However, this wasn't the case in the Bohlen study due to the 2 women who claimed orgasm but did not have the regular pelvic contractions. It also (I would say curiously) cited the original 1966 Masters and Johnson study when it said that other studies had not found the same subjective/objective agreement, but it doesn't elaborate. [I'm not sure how they are using that citation. I would be interested to know their reasoning. M&J stated unequivocally that their research showed rhythmic pelvic muscle contractions to be the physical expression of orgasm, and claimed the amount of contractions coincided with the subjective pleasure (although other studies have not found that, including the Carmichael study)]


  • 23 healthy, heterosexual women
  • 21-55 years old
  • recruited with their partners online over a period of 3 years (the equipment involved over those years did not change)
  • 13 had never given birth, 6 had given birth vaginally, and 4 through C-section
  • none had used recreation drugs or had history of physical, mental or sexual disorder

  • The women lay on their backs in a brain scanner (PET) and their anal pressure was measured via a probe in their anus. Their partners stood next to the women and stimulated their clitoris to orgasm. The partners were used in this way because it did not induce artifacts in the rectal pressure and because clitoral stimulation is "the most effective means of inducing orgasm in women (Lloyd 2005)"
  • The women didn't report important differences between their normal orgasms and these laboratory ones
  • Measurements were performed for 2 minutes in the following sequence with an 8 minute rest in between each measurement:
  1. Passive non-sexual resting state [as a reference so they could compare individuals with different resting states against each other]
  2. Imitating an orgasm while the clitoris is stimulated [meaning "to voluntarily contract muscles in the rectal vicinity (abdominal, 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 motor output during orgasm, because we expected the same muscles to contract, but with discernible frequency characteristics")]
  3. Imitating an orgasm while the clitoris is stimulated
  4. Stimulation of the clitoris ["the partners provided clitoral stimulation, but the women were asked not to make any movements and not to have an orgasm"]
  5. Stimulation of the clitoris
  6. Stimulation of the clitoris, trying to reach orgasm ["the partners provided clitoral stimulation with the aim of inducing orgasm. After each of these measurements, women either reported that they had not reached orgasm ("failed orgasm attempt") or that they had ("orgasm"). Failed orgasm attempts were also used as a control for the motor output during orgasm."]
  7. Stimulation of the clitoris, trying to reach orgasm
  8. Stimulation of the clitoris, trying to reach orgasm
  • "Orgasm attempts with ambiguous subjective reports were excluded from the analysis."
  • Partners started stimulation of the clit before beginning of the measurements so that the women would be already excited when the measurement started
  • "Because of the technical requirements for the PET measurements, 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
So from here it gets more confusing because it gets pretty technical about measuring rectal pressure and discerning the different frequency bands of that pressure. I am not exactly familiar with this kind of science, so I'm not going to get real specific. I'll do the best I can to give you the gist of their work using simple words ('cause that's all I understand). If anyone knows this well and wants to correct me, please do.
  • The pressure measurements were broken down into frequency bands: delta (.5-4 Hz), theta (4-8 Hz), alpha (8-13 Hz) and beta (13-25 Hz).
  • In each of those categories, the resting measurements of each of the women were used to normalize, but the normalized data did not have a normal distribution over all the participants and tasks, so some math was done to better compare the data among tasks and women (Kruskal-Wallis test, post-hoc Wilcoxen rank sum test, and a Bonferroni correction...for those who want to know).
  • So, using the women's claims of whether they came or not as the gold standard, the scientists started crunching the numbers to see which measurements most accurately predicted a woman's claim of "orgasm" vs. "failed orgasm attempt." They were basically trying to find a 1-stop-shop objective measurement that when measured would match (or match best) with a woman subjectively saying she orgasmed and when it was not measured would match (or best match) with a woman either having a failed orgasm attempt or doing some other non orgasm thing like pretending to orgasm.

  • "Of the 23 women, 17 achieved 1 or more orgasms. A total of 31 orgasms and 33 failed orgasm attempts were included for analysis."
  • "Visual inspection of the pressure vs. time graphs showed that patterns of rectal pressure during orgasm were very variable between participants."
  • "further examination suggested that, in many cases, the orgasm measurements contained more high frequency components than measurements of other tasks."
  • "Typical examples are given in Figure 1. In this figure, raw data (left panels) and power spectral density (right panels) of rectal pressure are shown. The lines at .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.'"
Figure 1 8-13 Hz fluctuations in rectal pressure are an objective marker of clitorally-induced orgasm in women
  • No significant differences were found between both control tasks;  failed attempts at orgasm and imitation of orgasm.
  • A significant difference was found between the control tasks and orgasm in the 8-13 Hz range ("alpha power").
  • "Based on this, the orgasm detection algorithm applied alpha power to classify orgasm attempts as successful or failed."
  • All orgasm attempts where the "alpha power" was over 3 times higher than non-sexual rest were classified as "orgasm" and all attempts with less were classified as "failed orgasm attempts," This maximized the overlap between the prediction of orgasm and women's claims of orgasm.
  • 69% (44/64) of orgasm attempts were identified correctly as either failed or successful
  • 94% (29/31) of orgasms were correctly recognized as orgasms

  • "The aim of this study was to identify an objective physiological correlate of orgasm in women. In particular, we hypothesized that the involuntary perineal muscular contractions that accompany orgasm could be distinguished by their frequency characteristics."
  • The power in both the 8-13 (alpha) and 13-25 (beta), but especially the 8-13 Hz frequency bands were significantly greater during orgasm than all other tasks.
  • "Imitation of orgasm and failed orgasm attempts also involved , often forceful, contraction of striated perineal musculature," but the power in the alpha and beta bands were significantly lower in those tasks than during orgasm.
  • The researchers said they were unable to determine which muscles contributed to the signal but that according to the literature, the following muscles contract during orgasm and could be responsible for the fast fluctuation in rectal pressure: parts of the levator ani muscle, the anal sphincter, and the uterus.
  • A limit of this study was that because of the PET scanning, the participants had to orgasm within designated 40-second intervals. Although, as was noted before, they didn't report that these lab orgasms were any different than their normal ones. There was also a benefit to the 40-second requirement because approximately half of all orgasm attempts were failed, which gave a lot of control tasks to compare against.
  • "Orgasms in our study were all clitorally-induced. This method was chosen because it induces no artifact in rectal pressure and is the most effective means of inducing orgasm (Lloyd, 2005). Because of this reason, rectal pressure fluctuations can not be measured during orgasms achieved through intra-vaginal stimulation. Studying the question whether orgasms achieved 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 vs. vaginal orgasms can not be supported in the literature, because there is a striking lack of reliable physiological 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."
  • These researchers ignored the variability and included all reported orgasms in their analysis. "Despite this indiscriminate approach" they still found the significantly higher power in the alpha band being characteristic of orgasm, and they believe, "This clearly demonstrates the robustness of this approach."
  • This research into rectal pressure might be useful, with further research of course, in helping women who are unable to orgasm, maybe through biofeedback, where a woman and her partner can see when the alpha power is rising to increase body awareness.
  • The researchers point out that their observations of rectal pressure starting at the onset of orgasm match the literature. They note that the fluctuations are not simply related to high arousal and point out that the participants in their personal debriefings said there was a big difference between their imitation of orgasm and their failed orgasm attempts because they were highly aroused during the failed attempt but not during the imitations. Yet, no differences were found in the rectal pressure fluctuation between the two. So, they conclude their study revealed an orgasm specific physiologic quality.
  • "Taken together, our findings indicate that 8-13 Hz fluctuations in rectal pressure constitute an objective and quantitative marker of orgasm in women that is sensitive and yet robust to interindividual variability and temporal dilution."

So the interesting part about this study is that it's not looking at the overall muscle contractions (the number, strength, and interval) as a marker for orgasm because, I believe, the researchers in this study see it as failing to match what they call the "elusive subjective orgasm experience" in women (i.e. sometimes women say they orgasm even when that marker is not exhibited). Those contractions, however, through a variety of recording/observation methods, have been indicated as marker of orgasm since back in 1966 when Masters and Johnson did their groundbreaking study on the physiology of male and female orgasm and arousal. And, judging from the research so far, most women and men do exhibit this unique physical marker at orgasm.

However, the researchers in this study often reference the Bohlen study, in which 2 of the 11 women claimed orgasm but did not exhibit the distinctive contraction pattern of orgasm. So, I think they were taking the Bohlen study as proof that the distinctive pelvic muscular contractions could not possibly be the physiologic marker of orgasm and thus were looking towards finding a marker that would include all woman's subjective claims of orgasm - including women like the 2 from the Bohlen study.

Now, this study still focuses on the pelvic muscle contractions at orgasm, but instead of contraction intervals, number, or overall strength, it looks at the frequency characteristics of the contractions, and that's where it finds that the strength of the contractions in the 8-13 Hz alpha band is a fairly reliable marker for orgasm. The thing is, though, the 8-13 Hz increase seems to coincide with the distinctive contractions in the example they give (figure 1 above), but does it in every case? What if there were women like the 2 in the Bohlen study? Would their orgasms be identified with the 8-13 Hz method even if it wasn't identified with the regular contraction interval method? This wasn't discussed.

Which brings me to the fact that this study didn't show that the  8-13 Hz method is the orgasm marker either. Only 69% (44/64) of orgasm attempts could be identified correctly as either failed or successful in this model, and there were 2 out of  31 claims of orgasms that could not be physically recognized as orgasms. The study doesn't go into detail about what made the 2 orgasm claims physically unidentifiable as orgasms, but might it be the same problem the Bohlen study faced? Were there no distinctive pelvic contractions in those? The researchers had the data to tell us this, but didn't.

Maybe, much like the long-standing use of regular pelvic muscle contractions as a marker, the 8-13 Hz method fails to a degree because study of orgasm begins with women's use of the word instead of physical data. I mean, I'm not saying we need to disregard women's own admission about whether they orgasmed or not, but why base physical investigations on that instead of just including that as another element to help increase understanding.  Maybe the word 'orgasm' is just so loaded and confusing and varied in our culture, especially for women, that it's use shouldn't necessarily be trusted in scientific investigation.

What if those women in the Bohlen study (and maybe even some in this study) just said they had an orgasm even though they physically didn't? Or (since I know researchers are probably incredibly wary about asserting that they know better than a woman about whether she orgasmed or not), maybe these women had some other type of physical thing that can be described as an 'orgasm' but exhibits itself in a different and yet unknown physical way than the whole pelvic muscle contraction and/or 8-13 Hz thing. Either way, I think the phenomenon of claiming orgasm and having the discernible contractions vs. the phenomenon of claiming orgasm and not having those contractions each deserves some specific and separate investigation instead of being lumped in together simply because we use the same word to describe them.

So to me, lumping this less understood phenomenon in which women don't exhibit this discernible physical marker when they claim orgasm into the data seems incredibly messy and problematic. Maybe there needs to be studies that simply investigate what is happening physically, see where it does and does not match to when people say they orgasm, and point out further studies that need to be done to understand more clearly what is happening physically in the minority of cases where claim of orgasm is not paired with a discernible physical marker - because as of now, we just don't know.

All in all, I liked this study. It was well done, but I think it falls prey to the larger problem in female orgasm studies...It uses women's claims of orgasm as the gold standard and approaches the analysis as if each woman were surely experiencing the same physical things as every other woman when she claims orgasm.

Find more of these Lady-gasm Journal Reviews HERE

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