Is the penis a muscle? Plus 6 more erection facts

Half the population get erections, yet most of us were never taught how they actually work.

An erection is like a reflex, one you have very little control over. Despite the easing of some taboos, most people still don't know all that much about how it happens.

Maybe it's a hangover from collective Victorian prudishness. Maybe it just wasn't on the syllabus at school. Most likely, it's because the whole process is a genuinely complex bit of physiology once you scratch the surface. Here are seven things you probably didn't know about how erections work.

1. Your penis is not a muscle

With nicknames like the ‘love muscle’, it's no wonder most of us assume the penis works the same way a bicep does: tense it, and it goes hard. It doesn't work like that.

The penis isn't a muscle, though it does contain a unique muscular structure. When you're aroused, the smooth muscles in the arteries relax, opening them up and allowing the penis to fill with blood.

2. An erection is like a reflex

We’ve used the bicep analogy already, and it’s a good one to demonstrate how unique the penis is. Tense your bicep, and it will get hard. Tense your penis? Not going to happen.

That's because an erection works like a reflex. In the same way your leg kicks involuntarily when a doctor taps below the knee, your penis becomes hard involuntarily in response to arousing stimuli, touch, sight or sound. Whatever the trigger, it activates your parasympathetic nervous system (PSNS), setting off a reflex-like response.

3. Your ‘fight or flight’ response can stop an erection dead in its tracks

Back in caveman times, the fight or flight response prepared us to run from predators. Now it's the feeling you get bumping into a large angry man at a bar, or getting called into your boss's office for a chat.

It doesn't look related to erections at first glance, but the sympathetic nervous system (SNS), which controls this response, pumps blood to the organs needed for immediate survival. The penis isn't one of them. Stress and anxiety in the bedroom can leave you struggling to get or maintain an erection.

4. Relaxation is key to a harder penis

Seems counter-intuitive, we know. Yet it definitely isn’t something that copes well with undue stress and anxiety.

While the SNS controls the fight-or-flight response, it’s the parasympathetic nervous system which brings about your erection. The PSNS is yin to the SNS’s yang: while the latter is activated when you’re anxious, the former kicks in while you’re relaxed, dilating the arteries in the penis and encouraging blood flow in.

That’s why a spot of mindfulness before sex or a Spotify ‘Slow Jamz’ playlist might just help get you in the mood.

5. There’s a molecule whose whole job is to help your erection run smoothly

When your penis becomes erect, it's thanks to a signalling molecule called cGMP. It instructs the muscle proteins in the penis to relax, allowing blood in. Think of it as the doorman telling the crowd to step back so blood can get through.

6. Your penis needs to trap blood to stay erect

When an erection subsides, cGMP is broken down by an enzyme called PDE5. Blood leaves the penis, and it goes soft again.

This is where ED medication like sildenafil comes in. It inhibits PDE5 before it has time to do its job, helping you stay harder for longer.

7. Testosterone helps erections - but no one knows why

Studies show low testosterone plays a role in erectile dysfunction. There are theories, some point to its role in nerve growth and function, others to its part in nitric oxide production and PDE5 activity, but no confirmed answer yet.

Either way, it's worth keeping testosterone at healthy levels. Beyond libido, it regulates sperm production, muscle size and strength, and bone growth.¹

The numan take

An erection works like a reflex: an involuntary response to a gentle touch, a sweet nothing, or the power of your imagination.

If you're struggling to get or maintain one, it's worth finding out why. A blood test can help you get to the bottom of it.

References

^{1 Harvard Health Publishing. Testosterone: what it is and how it affects your health [Internet]. Boston: Harvard Medical School; 2015 Jul 16 [updated 2023 Jun 22; cited 2026 Jul 29]. Available from: https://www.health.harvard.edu/healthy-aging-and-longevity/testosterone--what-it-does-and-doesnt-do}