Musculature

Before we get into sensor data, it's useful to get an overview of the various muscles that are correlated with arousal.

Image copyright OpenStax

The Ischiocavernous muscle (ICM) is mainly used for enhancing rigidity. Expansion of the penis during clenching is mainly the result of this muscle (Shafik 1995).

The Bulbospongiosus muscle (BCM) and anal sphincters (IAS/EAS) are the main driver of ejaculation (shafik 1998).

Behavior

During normal operation, the ICM, BCM and IAS/EAS are highly correlated. This is a reflex (Shafik 2006), although anecdotally the ICM can contract without simultaneous contraction of BCM and IAS/EAS in some cases (my own data).

The relationship between EMG measurements of IAS and penile pressure is linear (Lavoisier 1986), and the relationship between pressure and penile area/circumference is non-linear (source: chatGPT).

During orgasm, there is a 4-second clench of the ICM during which the BCM and IAS/EAS relax completely (Gerstenberg 1990). This contraction can be clearly observed using a girth sensor.

Data from Gerstenberg 1990

Afterward, rhythmic contractions follow.

In terms of amplitude, it appears that reflex activation of ICM from stimulation results in higher EMG readings than is possible voluntary, although rhythmic contractions have about the same amplitude as voluntary contractions (Gerstenberg 1990).

For BCM, the amplitude of rhytmic contractions is much higher than voluntary contractions (Gerstenberg 1990, my own data)

For IAS/EAS, the amplitude is much lower than the maximum voluntary (Bohlen 1980, my own data)

Orgasm detection

From this data it follows there are at least three ways to detect orgasm:

• Measure any of these muscles and detect the rhytmic contractions (latency: about 10 seconds after PONR).
• Measure the BCM and detect above-maximal contractions (latency: about 4 seconds after PONR).
• Measure ICM (or girth) plus BCM or IAS/EAS, and detect sudden contraction of ICM and relaxation of BCM or IAS/EAS (latency: about 1 second after PONR).

Edging detection

Literature does not have much to say about how these muscles behave in the period before orgasm.

References

Shafik 1995: response of the urethral and intracorporeal pressures to cavernosus muscle stimulation: role of the muscles in erection and ejaculation
Shafik 1998: the mechanism of ejaculation: the glansvasal and urethromuscular reflexes
Shafik 2006: The cavernoso-anal reflex: response of the anal sphincters to cavernosus muscles’ stimulation
Gerstenberg 1990: Erection and Ejaculation in Man. Assessment of the Electromyographic Activity of the Bulbocavernosus and lschiocavernosus Muscles
Lavoisier 1986: correlation between intracavernous pressure and contraction of the ischioca vernosus muscle in man
Bohlen 1980: The Male Orgasm: Pelvic Contractions Measured by Anal Probe