Is the penis a muscle or bone?

When you think about the nicknames for an erect penis (for example, “boner”) you may wonder what exactly is a penis? Is it a bone? Or is it a muscle? And if it’s a muscle, surely you could exercise it like you would a bicep and make your penis bigger… right?  

Unfortunately, it’s not that simple. If you’re wondering whether the penis is a muscle or a bone, the answer is that it is actually neither. Instead, it’s made up of a mix of tissues and glands and muscle fibers that work together to create a hard-on. Read on to dig into more about what is actually inside of a penis. 

The penis is made up of many different parts, including smooth muscle fibers around an extensive series of blood vessels within the penis as well as fibroelastic tissue and some muscles at the base that help provide erectile support and propel semen through the penis during ejaculation. 

If you’re looking for a more in-depth breakdown of the anatomy of the penis, you can read even more here. 

When you picture a “muscle”, you’ll probably think of something like a bicep, tricep, or abdominal muscle. These types of muscles are called skeletal muscles and their primary function is to help with movement and maintaining posture—without them, you’d just flop over whenever you sat down. 

The penis isn’t made up of one large muscle like a bicep, but it does have a series of smooth muscles (that are not under voluntary control) to develop an erection properly. There are some skeletal muscles outside of the corpora cavernosa, but they do not produce an erection. 

Smooth muscles are frequently found throughout the body and function primarily through contraction and expansion. During an erection, arteries (which contain smooth muscle in their walls) relax to  help move blood into the penis, while the smooth muscles located in the veins relax to collect more blood within the penis. With engorgement of the penis, this action   partially restricts blood flow moving out of the penis. This increases blood in your member, which is what causes the stiffened nature of the penis that is referred to as an erection. 

Similarly, you may have noticed that your penis gets smaller/shorter at times. The penis can even appear to pull into the body, an “inny,” in cold temperatures or with sympathetic nerve activity (think: when you’re nervous or anxious.) The shortening of the penis is because of contractions of those many muscles inside of the penis around the blood vessels. Fortunately, such “shrinkage” is temporary, resolving with relaxation or sexual stimulation (where the opposite set of nerves, called “parasympathetic” nerves, cause the muscles to relax, allowing more blood flow into the penis and enlargement of your tool).

So no, the penis is not a muscle, but it does use muscles and erectile tissue to enlarge, collect blood, and produce a penile erection.

Thankfully no, otherwise it’d be pretty difficult going about your day with a rigid bone in the front of your underwear. This can be a common misunderstanding though, especially when “boner” is one of the first schoolyard euphemisms for erection that a kid can learn. So how does the penis become stiff and hard without a bone for support? Well, it all has to do with how the penis grows during arousal to allow erectile tissue in the corpora cavernosa to stiffen as it fills with blood.

As blood rushes to the penis, fibrous elastic tissue, smooth muscle, and collagen help support the penis out of its contracted, flaccid state and allow it to harden as blood fills the spongy, erectile tissues. This is also why, despite its firmness, an erection still has some degree of flexibility and malleability. If an erection were strictly supported by a bone, just imagine how little free motion it might have or how easily any bending or unusual pressure could result in a very painful fracture or break.  

When someone with a penis is able to achieve an erection, they aren’t actually flexing their penis, but instead the muscles inside the penis are relaxing, allowing an erection to occur. During an erection, it is possible to make the penis harder by activating muscles at the base of their penis and in their pelvic floor that support the penis.

Yes and no. Because the penis isn’t a muscle, there isn’t an exercise that will result in a larger or stronger penis. If you contract your pelvic muscles, you will very briefly make the penis harder and larger, but as soon as you relax those muscles, the penis goes back to baseline size. This is most noticeable when you have an erection, and the increased hardness is sometimes referred to as “thrusting pressure.”  This increase in pressure and size only lasts for seconds. Stretching and exercising the muscles within a penis can support erections. That is thought to be why men develop erections every night, when they go through the lighter stages of sleep referred to as REM sleep.  

There are a few unproven techniques, like jelqing (a technique that involves massaging and stretching the penis), that some people claim can increase penis length. However research and studies looking into its effectiveness have yet to return any promising results.

Even though there is no bone in a penis, it can still break. This is called a “penile fracture”.

A penile fracture is actually a very serious injury and requires immediate medical intervention. The good news is that penile fractures are pretty uncommon and the likelihood of a successful recovery is very high if it’s diagnosed and treated quickly. Basically what happens is when an erect penis is bent enough, it can cause a break in the tissues on the surface of the penis, creating a “penile fracture.” 

When the fracture occurs, the tunica albuginea—the layer of fibroelastic tissue that covers both the corpora cavernosa and the corpus spongiosum—ruptures. Some patients reported hearing a pop or cracking sound followed by pain, bruising, sudden swelling, and a rapid loss of their erection. Surgery is required to properly repair the rupture and the sooner the surgery can be performed, the more likely it will be that long-term damage can be avoided.  

A 2019 study looked at 56 patients who had experienced a penile fracture and a majority (57%) of those fractures occurred during sex. In these cases, sudden, intense pressure was applied to an erect penis. Some of the situations that were described included significant bending or angling during forceful thrusting, falling and landing on their erection, and sexual positions that caused a strong, accidental thrust into their partner’s pelvic bone.

Understanding your anatomy is important and feeling informed and educated about your body can help you make better decisions about your sexual health. If you are having trouble with your sexual function (for example, issues maintaining an erection, you feel you’ve lost sensitivity in your penis or you’re struggling with size), trying penile exercises probably won’t  solve your health concerns. Instead, talk to your healthcare provider about treatment options that may be right for you.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

October 03, 2024

Written by

Steven Craig Rowe

Fact checked by

Peter Schlegel, MD and Raagini Yedidi, MD

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About the medical reviewers

Peter Schlegel, MD

Peter Schlegel, MD, is a board-certified urologist, internationally acclaimed expert in the treatment of male infertility, and Ro advisor.

Raagini Yedidi, MD

Raagini Yedidi, MD, is an internal medicine resident and medical reviewer for Ro.