Anal Contractions During Orgasm

This suggestion is for both male and female characters to have anal contractions while having an orgasm. Anal contractions could result from other stimuli too, though I don't have any ideas on that front. That I found "visual documentation" (porn) didn't surprise me. Much to my surprise looking up the topic I found medical documentation on anal contractions. For science... literally!

On males:
www.ncbi.nlm.nih.gov

Pelvic muscle contractions during sexual response can be monitored conveniently by the anal probe method described. Eleven young adult male subjects were each recorded for three sessions of masturbation to orgasm. Electrical signals from an anal pressure probe were automatically digitized by computer. Orgasmic contractions were easily distinguished from voluntary contractions by the steadily increasing intervals and complete muscle relaxation between orgasmic contractions. At orgasm each subject produced a characteristic series of contractions starting abruptly at an intercontraction interval of about 0.6 seconds, and continued for 10 to 15 contractions at an increasing increment of about 0.1 second per contraction. Pressure amplitude, representing the force of contractions, increased from the beginning of the regular series to a maximum at the seventh or eighth contraction. Area under the pressure curve, reflecting muscular exertion during contraction, generally increased throughout the regular series. Each man's pattern of contractions was very similar from one session to the next and distinguished his records from others'. Individuals' patterns could be grouped into three types, based chiefly on the location of the regular contraction series within the subjective span of orgasm. The most common type was a simple series of regular contractions. It had the shortest duration and fewest contractions. The next most common pattern began with the regular series, followed by a number of irregular contractions. This type was longest in duration. One man with a third type of intermediate duration, had a number of preliminary contractions before the series of regular contractions began in midorgasm.

On females:

http://link.springer.com/article/10.1007%2FBF01541

Eleven nulliparous women manually self-stimulated to orgasm, each on three separate occasions. Pelvic contraction pressure was measured by an anal probe and a vaginal probe simultaneously. Near the perceived start of orgasm, a series of regular contractions began in nine of the women. Anal and vaginal contraction waveforms were synchronized with each other, and the same number of orgasmic contractions occurred in each lumen. Anal pressure had a higher resting baseline and greater amplitude contractions than vaginal pressure. The perceived start of orgasm did not correspond precisely with the onset of regular contractions. Mean intercontraction intervals increased linearly at an increment of about 0.1 second through the series of regular contractions. Amplitudes of contraction pressure waveforms, representing pelvic muscular force, were initially low, increased through the first half of the regular series, and then decreased. Area and net area of the pressure waveforms, reflecting pelvic muscular exertion (force × time), increased during the regular orgasmic contractions. Three of the women's orgasms consistently included only a series of regular contractions (orgasm type I). For six other women, orgasms consistently continued beyond the regular series with additional irregular contractions (orgasm type II). Types I and II had been identified previously in men. Two women had no regular contractions during reported orgasms. This pattern, type IV, had not been recorded in men. Women of different types showed marked differences in orgasm duration and number of contractions. Identification of these types in subjects is important for meaningful comparison of contraction parameters in different studies.